Patent Publication Number: US-6983463-B1

Title: Network independent profiling of applications for automatic partitioning and distribution in a distributed computing environment

Description:
RELATED APPLICATION DATA 
   This application claims the benefit of U.S. Provisional Application No. 60/102,815, filed Oct. 2, 1998, entitled “Automatic Partitioning and Distribution of Applications,” the disclosure of which is incorporated by reference. 

   TECHNICAL FIELD 
   The present invention relates generally to profiling an application using a structural metadata description of the application, and combining the application profile with a network profile to enable network independent automatic partitioning and distribution of the application. 
   BACKGROUND OF THE INVENTION 
   Fueled by the growing importance of the Internet, interest in the area of distributed computing environments (two or more computers connected by a communications medium) has increased in recent years. Programmers desiring to take advantage of distributed computing environments modify existing application programs to perform on distributed computing environments, or design applications for placement on distributed computing environments. 
   A distributed application is an application containing interconnected application units (“units”) that are placed on more than one computer in a distributed computing environment. By placing units on more than one computer in a distributed computing environment, a distributed application can exploit the capabilities of the distributed computing environment to share information and resources, and to increase application reliability and system extensibility. Further, a distributed application can efficiently utilize the varying resources of the computers in a distributed computing environment. 
   Various types of modular software, including software designed in an object-oriented framework, can conceivably be distributed throughout a distributed computing environment. Object-oriented programming models, such as the Microsoft Component Object Model (“COM”), define a standard structure of software objects that can be interconnected and collectively assembled into an application (which, being assembled from component objects, is herein referred to as a “component application”). The objects are hosted in an execution environment created by system services, such as the object execution environments provided by COM. This system exposes services for use by component application objects in the form of application programming interfaces (“APIs”), system-provided objects and system-defined object interfaces. Distributed object systems such as Microsoft Corporation&#39;s Distributed Component Object Model (DCOM) and the Object Management Group&#39;s Common Object Request Broker Architecture (CORBA) provide system services that support execution of distributed applications. 
   In accordance with object-oriented programming principles, the component application is a collection of object classes which each model real world or abstract items by combining data to represent the item&#39;s properties with functions to represent the item&#39;s functionality. More specifically, an object is an instance of a programmer-defined type referred to as a class, which exhibits the characteristics of data encapsulation, polymorphism and inheritance. Data encapsulation refers to the combining of data (also referred to as properties of an object) with methods that operate on the data (also referred to as member functions of an object) into a unitary software component (i.e., the object), such that the object hides its internal composition, structure and operation and exposes its functionality to client programs that utilize the object only through one or more interfaces. An interface of the object is a group of semantically related member functions of the object. In other words, the client programs do not access the object&#39;s data directly, but instead call functions on the object&#39;s interfaces to operate on the data. Polymorphism refers to the ability to view (i.e., interact with) two similar objects through a common interface, thereby eliminating the need to differentiate between two objects. Inheritance refers to the derivation of different classes of objects from a base class, where the derived classes inherit the properties and characteristics of the base class. 
   An application containing easily identifiable and separable units is more easily distributed throughout a distributed system. One way to identify separable units is to describe such units with structural metadata about the units. Metadata is data that describes other data. In this context, structural metadata is data describing the structure of application units. Further, application units are desirably location-transparent for in-process, cross-process, and cross-computer communications. In other words, it is desirable for communications between application units to abstract away location of application units. This flexibly enables the distribution of application units. 
   The partitioning and distribution of applications are problematic and complicated by many factors. 
   To partition an application for distribution, a programmer typically determines a plan for distributing units of the application based on past experience, intuition, or data gathered from a prototype application. The application&#39;s design is then tailored to the selected distribution plan. Even if the programmer selects a distribution plan that is optimal for a particular computer network, the present-day distribution plan might be rendered obsolete by changes in network topology. Moreover, assumptions used in choosing the distribution plan might later prove to be incorrect, resulting in an application poorly matched to its intended environment. 
   Generally, to distribute an application, one can work externally or internally relative to the application. External distribution mechanisms work without any modification of the application and include network file systems and remote windowing systems in a distributed computing environment. Although external distribution mechanisms are easy to use and flexible, they often engender burdensome transfers of data between nodes of the distributed computing environment, and for this reason are far from optimal. Internal distribution mechanisms typically modify the application to be distributed in various ways. Internal distribution mechanisms allow optimized application-specific distribution, but frequently entail an inordinate amount of extra programmer effort to find an improved distribution and modify the application. Further, internal systems frequently provide ad hoc, one-time results that are tied to the performance of a particular network at a particular time. 
   An automatic distributed partitioning system (ADPS) works internally relative to an application to partition application units, and works automatically or semi-automatically to save programmer effort in designing distributed applications. 
   In the 1970&#39;s, researchers postulated that the best way to create a distributed application was to use a compiler in a run time environment to partition the application, and to provide the exact same code base to each of plural distributed machines as used on a single machine to execute the distributed application. After analyzing the structure of procedures and parameters in the source code of an application, metadata describing the structure of an application was generated from the application source code. Using this metadata, these ADPSs profiled the application and generate a communication model for the application. The Interconnected Processor System (ICOPS) is an example of an ADPS designed in the 1970&#39;s. The Configurable Applications for Graphics Employing Satellites (CAGES) also supports creation of distributed applications, but does not support automatic application profiling at all. A more recent example of an ADPS is the Intelligent Dynamic Application Partitioning (IDAP) System. IDAP generates from application source code an instrumented version of the application for execution in profiling scenarios, then generates from application source code another version of the application for distributed execution. ICOPS and IDAP suffer from numerous drawbacks relating to the universality, efficiency, and automation of these systems. 
   For example, access to application source code is required in ICOPS and IDAP, which compile application source code to generate metadata or an instrumented application for profiling. Neither ICOPS nor IDAP can profile an application without access to application source code, limiting the applicability of the systems. 
   An application profile is a model of an application. The application profile can include the units of an application and/or the costs of communication between units of the application according to expected usage patterns. Communication costs can be represented through several abstractions. For instance, communication costs can be represented as the time to transmit data from one machine to another or the amount of data transmitted. The former is network-dependent and will change with network interconnection. The latter fails to consider the realities of network latencies and bandwidths, i.e., it fails to consider network characteristics. Neither ICOPS, CAGES, nor IDAP produces a network-independent profile of an application that is combined with measurements of network characteristics and analyzed to partition the application for the network. Neither ICOPS, CAGES, nor IDAP allows re-profiling of an application or a network to adjust for changes in the application or the network, or to partition on different networks. 
   Distributed Operating Systems 
   Distributed operating systems offer alternative mechanisms for utilizing resources in distributed systems. In an object-oriented system, a distributed operating system makes placement decisions for software objects. For example, many distributed operating systems place all objects locally unless specifically instructed to do otherwise. Other distributed operating systems automatically place an object on an idle machine, if possible, but provide no user or program level control over object placement. A typical distributed operating system does not attempt any form of automatic distributed partitioning to minimize application communication costs. 
   SUMMARY OF THE INVENTION 
   The present invention pertains to automatic partitioning of an application and distribution of units of the application through a distributed computing environment. The present invention works internally to an application, but entails little extra programmer effort to find an optimal distribution. Moreover, the present invention does not just provide a one-time distribution of an application on a network. Instead, it enables distribution of an application over multiple networks and changing networks. 
   An application is profiled using a structural metadata description of the application. Units of the application have strongly-typed, binary-standard interfaces, and are profiled without access or reference to the application source code—the application can be available only as an application binary. Because no access to application source code is required, the present invention is applicable to a large class of commercial applications. 
   A structural metadata description of the application includes compiled interface-level type information used to identify and measure interaction between units of the application. For example, static interface metadata such as marshaling byte codes describing the strongly-typed, binary-standard interfaces of the units of the application can be included in the structural metadata description. If a source code description of the interfaces is provided, static analysis of the source code yields the structural metadata description of the application. Alternatively, if a compiled file including type description of the interfaces is provided, analysis of the compiled file yields the structural metadata description of the application. 
   An application profile is produced by profiling the application with the help of the structural metadata description. The application profile includes description of the static relationships between the units of the application. Alternatively, the application profile includes description of the dynamic interactions between units of the application during profiling scenarios meant to track the expected usage of the application. Dynamic interaction of the units can include the number and size of messages sent between units, elapsed time of messages sent between units, or elapsed processing time for units. For example, type information in the structural metadata description can be used to parse communications sent across the interface of a unit, and measure the arguments making up the communication. 
   Generally, after analyzing the application profile, the application or execution of the application is modified based on the results. In one embodiment, the application profile combines with a network profile to create a model of how the application is expected to behave on the profiled network. The network profile can be of an idealized network or an actual physical network, in which case the network profile can include measurements of the capabilities of computers on the physical network. Alternatively, the network profile includes estimates of network characteristics such as latency and bandwidth estimated at the same time as application characteristics, or at a different time. 
   By analyzing the combination of the application and network profiles, a distribution plan for the application on the profiled network is determined. When determining a distribution plan, location constraints on the distribution of application units can also be considered. According to one embodiment of the present invention, a commodity flow algorithm is applied to a representation of relevant accumulated application and network data to determine an optimal distribution plan. The distribution plan is an association between units of the application and locations in the distributed computing environment. By combining the application profile with network profiles for different networks, different distribution plans are determined for the application as executed on different networks. 
   During execution of the application, units of the application are distributed through the distributed computing environment according to the distribution plan. The support for remote units of the application is provided by system services of the distributed computing environment, possibly in combination with services built on top of existing system services of the distributed computing environment. 
   During execution, a threshold for execution of the application in the distributed computing environment can be defined. If this threshold is exceeded during execution, the recent behavior of the application in the distributed computing environment is noted. A new distribution plan is generated that assimilates the recent behavior. Alternatively, the application and/or the network are re-profiled. 
   Additional features and advantages of the present invention will be made apparent from the following detailed description of an illustrated embodiment, which proceeds with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of a distributed computing environment in which the present invention can be implemented. 
       FIG. 2  is a block diagram of a computer system that can be used to implement the present invention. 
       FIG. 3  is a block diagram of a Microsoft Component Object Model software component that can be used to implement the present invention. 
       FIG. 4  is a block diagram of a client and the component of  FIG. 3  in a distributed computing environment. 
       FIG. 5  is a block diagram of the component of  FIG. 3  with multiple interfaces specified according to Microsoft&#39;s Component Object Model. 
       FIG. 6  is a flow chart showing the automatic partitioning of an application into application units according to the illustrated embodiment of the present invention. 
       FIG. 7  is a flow chart showing the scenario-based profiling of an application to generate a description of the run-time behavior of the application according to the illustrated embodiment of the present invention. 
       FIG. 8  is a commodity flow diagram cut by a MIN CUT MAX FLOW algorithm according to the illustrated embodiment of the present invention. 
       FIG. 9  is a listing showing a code fragment in which a component like that illustrated in  FIG. 3  is created, and types of dynamic classifiers for the component. 
       FIG. 10  is a listing containing code fragments illustrating various techniques for intercepting communications according to the illustrated embodiment of the present invention. 
       FIG. 11  is a diagram showing a graphical representation of a distribution chosen for a profiled scenario in which the user loads and previews an image in Picture It!® from a server in the COIGN system. 
       FIG. 12  is a block diagram of an object-oriented framework for partitioning and distributing application units of an application according to the COIGN system. 
       FIG. 13  is a block diagram of an object-oriented framework for partitioning and distributing application units of an application showing the pattern of intercommunication between the objects according to the COIGN system. 
       FIG. 14  is a listing containing code fragments illustrating interception and in-line redirection of communications according to the COIGN system. 
       FIG. 15  is a block diagram showing an application binary in common object file format that is statically linked according to one embodiment of the present invention. 
       FIG. 16  is a block diagram showing the application binary of  FIG. 15  reversibly static re-linked to a second set of libraries. 
       FIG. 17  is a block diagram of a series of COIGN data structures showing a component object, an interface wrapper appended to the component object, and analytical data appended to the wrapped component object. 
       FIG. 18  is a block diagram of a series of COIGN data structures showing a table of interfaces, a group of interface wrappers, and a table of instrumentation functions. 
   

   DETAILED DESCRIPTION OF AN ILLUSTRATED EMBODIMENT 
   The present invention is directed toward automatic partitioning of units of an application and distribution of those units. In the illustrated embodiment of the present invention, an application is partitioned into one or more application units for distribution in a distributed computing environment. The COIGN system is one possible refinement of the illustrated ADPS that automatically partitions and distributes applications designed according to the Component Object Model (“COM”) of Microsoft Corporation of Redmond, Wash. Briefly described, the COIGN system includes techniques for identifying COM components, measuring communication between COM components, classifying COM components, measuring network behavior, detecting component location constraints, generating optimal distribution schemes, and distributing COM components during run-time. 
     FIGS. 1 and 2  and the following discussion are intended to provide a brief, general description of a suitable computing environment in which the illustrated ADPS can be implemented. While the present invention is described in the general context of computer-executable instructions that run on computers, those skilled in the art will recognize that the present invention can be implemented as a combination of program modules, or in combination with other program modules. Generally, program modules include routines, programs, components, data structures, etc. that perform particular tasks or implement particular abstract data types. The present invention can be implemented as a distributed application, one including program modules located on different computers in a distributed computing environment. 
   Exemplary Distributed Computing Environment 
     FIG. 1  illustrates a distributed computing environment  1  in which units of an application are partitioned and distributed by the illustrated ADPS in accordance with the present invention. The distributed computing environment  1  includes two computer systems  5  connected by a connection medium  10 . The computer systems  5  can be any of several types of computer system configurations, including personal computers, hand-held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, mainframe computers, and the like. In terms of logical relation with other computer systems  5 , a computer system  5  can be a client, a server, a router, a peer device, or other common network node. Moreover, although  FIG. 1  illustrates two computer systems  5 , the present invention is equally applicable to an arbitrary, larger number of computer systems connected by the connection medium  10 . Further, the distributed computing environment  1  can contain an arbitrary number of additional computer systems  5  which do not directly involve the illustrated ADPS, connected by an arbitrary number of connection mediums  10 . The connection medium  10  can comprise any local area network (LAN), wide area network (WAN), or other computer network, including but not limited to Ethernets, enterprise-wide computer networks, intranets and the Internet. 
   The illustrated ADPS automatically partitions an application and distributes program units by locating them in more than one computer system  5  in the distributed computing environment  1 . Portions of the illustrated ADPS can be implemented in a single computer system  5 , with the application later distributed to other computer systems  5  in the distributed computing environment  1 . Portions of the illustrated ADPS can also be practiced in a distributed computing environment  1  where tasks are performed by a single computer system  5  acting as a remote processing device that is accessed through a communications network, with the distributed application later distributed to other computer systems  5  in the distributed computing environment  1 . In a networked environment, program modules of the illustrated ADPS can be located on more than one computer system  5 . 
   Exemplary Computer System 
     FIG. 2  illustrates an example of a computer system  5  that can serve as an operating environment for the illustrated ADPS. With reference to  FIG. 2 , an exemplary computer system for implementing the invention includes a computer  20  (such as a personal computer, laptop, palmtop, set-top, server, mainframe, and other varieties of computer), including a processing unit  21 , a system memory  22 , and a system bus  23  that couples various system components including the system memory to the processing unit  21 . The processing unit can be any of various commercially available processors, including Intel x86, Pentium and compatible microprocessors from Intel and others, including Cyrix, AMD and Nexgen; Alpha from Digital; MIPS from MIPS Technology, NEC, IDT, Siemens, and others; and the PowerPC from IBM and Motorola. Dual microprocessors and other multi-processor architectures also can be used as the processing unit  21 . 
   The system bus can be any of several types of bus structure including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of conventional bus architectures such as PCI, VESA, AGP, Microchannel, ISA and EISA, to name a few. The system memory includes read only memory (ROM)  24  and random access memory (RAM)  25 . A basic input/output system (BIOS), containing the basic routines that help to transfer information between elements within the computer  20 , such as during start-up, is stored in ROM  24 . 
   The computer  20  further includes a hard disk drive  27 , a magnetic disk drive  28 , e.g., to read from or write to a removable disk  29 , and an optical disk drive  30 , e.g., for reading a CD-ROM disk  31  or to read from or write to other optical media. The hard disk drive  27 , magnetic disk drive  28 , and optical disk drive  30  are connected to the system bus  23  by a hard disk drive interface  32 , a magnetic disk drive interface  33 , and an optical drive interface  34 , respectively. The drives and their associated computer-readable media provide nonvolatile storage of data, data structures, computer-executable instructions, etc. for the computer  20 . Although the description of computer-readable media above refers to a hard disk, a removable magnetic disk and a CD, it should be appreciated by those skilled in the art that other types of media which are readable by a computer, such as magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, and the like, can also be used in the exemplary operating environment. 
   A number of program modules can be stored in the drives and RAM  25 , including an operating system  35 , one or more application programs  36 , other program modules  37 , and program data  38 . 
   A user can enter commands and information into the computer  20  through a keyboard  40  and pointing device, such as a mouse  42 . Other input devices (not shown) can include a microphone, joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to the processing unit  21  through a serial port interface  46  that is coupled to the system bus, but can be connected by other interfaces, such as a parallel port, game port or a universal serial bus (USB). A monitor  47  or other type of display device is also connected to the system bus  23  via an interface, such as a video adapter  48 . In addition to the monitor, computers typically include other peripheral output devices (not shown), such as speakers and printers. 
   The computer  20  can operate in a networked environment using logical connections to one or more other computer systems  5 . The other computer systems  5  can be servers, routers, peer devices or other common network nodes, and typically include many or all of the elements described relative to the computer  20 , although only a memory storage device  49  has been illustrated in  FIG. 2 . The logical connections depicted in  FIG. 2  include a local area network (LAN)  51  and a wide area network (WAN)  52 . Such networking environments are common in offices, enterprise-wide computer networks, intranets and the Internet. 
   When used in a LAN networking environment, the computer  20  is connected to the local network  51  through a network interface or adapter  53 . When used in a WAN networking environment, the computer  20  typically includes a modem  54  or other means for establishing communications (e.g., via the LAN  51  and a gateway or proxy server  55 ) over the wide area network  52 , such as the Internet. The modem  54 , which can be internal or external, is connected to the system bus  23  via the serial port interface  46 . In a networked environment, program modules depicted relative to the computer  20 , or portions thereof, can be stored in the remote memory storage device. It will be appreciated that the network connections shown are exemplary and other means of establishing a communications link between the computer systems  5  (including an Ethernet card, ISDN terminal adapter, ADSL modem, 10BaseT adapter, 100BaseT adapter, ATM adapter, or the like) can be used. 
   In accordance with the practices of persons skilled in the art of computer programming, the illustrated ADPS is described below with reference to acts and symbolic representations of operations that are performed by the computer  20 , unless indicated otherwise. Such acts and operations are sometimes referred to as being computer-executed. It will be appreciated that the acts and symbolically represented operations include the manipulation by the processing unit  21  of electrical signals representing data bits which causes a resulting transformation or reduction of the electrical signal representation, and the maintenance of data bits at memory locations in the memory system (including the system memory  22 , hard drive  27 , floppy disks  29 , and CD-ROM  31 ) to thereby reconfigure or otherwise alter the computer system&#39;s operation, as well as other processing of signals. The memory locations where data bits are maintained are physical locations that have particular electrical, magnetic, or optical properties corresponding to the data bits. 
   Component Object Overview 
   With reference now to  FIG. 3 , in the COIGN system, the computer  20  ( FIG. 2 ) executes “COIGN,” a component-based application that is developed as a package of component objects. COIGN&#39;s component objects conform to the Microsoft Component Object Model (“COM”) specification (i.e., each is implemented as a “COM Object”  60 , alternatively termed a “COM component”). COIGN executes using the COM family of services (COM, Distributed COM (“DCOM”), COM+) of the Microsoft Windows NT Server operating system, but alternatively can be implemented according to other object standards (including the CORBA (Common Object Request Broker Architecture) specification of the Object Management Group) and executed under object services of another operating system. 
   COIGN automatically partitions and distributes other component-based applications. Like COIGN, the component-based applications automatically partitioned and distributed by COIGN are implemented in conformity with COM and executed using COM services, but alternatively can be implemented according to another object standard and executed using object services of another operating system. 
   COM: Binary Compatibility 
   The COM specification defines binary standards for objects and their interfaces which facilitate the integration of software components into applications. COM specifies a platform-standard binary mapping for interfaces, but does not specify implementations for interfaces. In other words, an interface is defined, but the implementation of the interface is left up to the developer. The binary format for a COM interface is similar to the common format of a C++ virtual function table. Referring to  FIG. 3 , in accordance with COM, the COM object  60  is represented in the computer system  20  ( FIG. 2 ) by an instance data structure  62 , a virtual function table  64 , and member methods (also called member functions)  66 – 68 . The instance data structure  62  contains a pointer  70  to the virtual function table  64  and data  72  (also referred to as data members, or properties of the object). A pointer is a data value that holds the address of an item in memory. The virtual function table  64  contains entries  76 – 78  for the member methods  66 – 68 . Each of the entries  76 – 78  contains a reference to the code  66 – 68  that implements the corresponding member methods. A reference to an interface is stored as a pointer to the pointer  70 . 
   While extremely simple, the binary mapping provides complete binary compatibility between COM components written in any language with any development tool. Any language that can call a function through a pointer can use COM components. Any language that can export a function pointer can create COM components. Language-neutral binary compatibility is an important feature of COM. 
   COM: Strongly Typed Interfaces and Interface Descriptor Language 
   The pointer  70 , the virtual function table  64 , and the member methods  66 – 68  implement an interface of the COM object  60 . By convention, the interfaces of a COM object are illustrated graphically as a plug-in jack as shown in objects  110  and  130  in  FIG. 4 . Also, interfaces conventionally are given names beginning with a capital “I.” In accordance with COM, the COM object  60  can include multiple interfaces, which are implemented with one or more virtual function tables. The member function of an interface is denoted as “IInterfaceName::MethodName.” 
   All first-class communication in COM takes place through well-defined, binary-standard interfaces, which are strongly typed references to a collection of semantically related functions. 
   Programmatically, interfaces are described either with an Interface Definition Language (IDL) or with a package of compiled metadata structures called a type library. Whether expressed in IDL or a type library, the interface definition enumerates in detail the number and type of all arguments passed through interface functions. Each interface function can have any number of parameters. To clarify semantic features of the interface, IDL attributes can be attached to each interface, member function, or parameter. In IDL syntax, attributes are enclosed in square brackets ([ ]). Attributes specify features such as the data-flow direction of function arguments, the size of dynamic arrays, and the scope of pointers. Syntactically, IDL is very similar to C++. Moreover, the interface definition has a purpose similar to that of a function prototype in C++; it provides a description for invocation, but not an implementation. An IDL compiler maps the interface definitions into a standard format for languages such as C++, Java, or Visual Basic. For example, the Microsoft IDL compiler, MIDL, can map interfaces into C++ or export compiled IDL metadata to a type library. (For a detailed discussion of COM and OLE, see Kraig Brockschmidt, Inside OLE, Second Edition, Microsoft Press, Redmond, Wash. (1995)). 
   COM: Globally Unique Identifiers 
   In COM, classes of COM objects are uniquely associated with class identifiers (“CLSIDs”), and registered by their CLSID in the registry. The registry entry for a COM object class associates the CLSID of the class with information identifying an executable file that provides the class (e.g., a DLL file having a class factory to produce an instance of the class). Class identifiers are 128-bit globally unique identifiers (“GUIDs”) that the programmer creates with a COM service named “CoCreateGUID” (or any of several other APIs and utilities that are used to create universally unique identifiers) and assigns to the respective classes. The interfaces of a component are also immutably associated with interface identifiers (“IIDs”), which are also 128-bit GUIDs. If an interface changes, it receives a new IID. 
   COM: Implementation 
   The virtual function table  64  and member methods  66 – 68  of the COM object  60  are provided by an object server program  80  (hereafter “object server DLL”) which is stored in the computer  20  ( FIG. 2 ) as a dynamic link library file (denoted with a “.dll” file name extension). In accordance with COM, the object server DLL  80  includes code for the virtual function table  64  and member methods  66 – 68  of the classes that it supports, and also includes a class factory  82  that generates the instance data structure  62  for an object of the class. 
   Other objects and programs (referred to as a “client” of the COM object  60 ) access the functionality of the COM object by invoking the member methods through the COM object&#39;s interfaces. First, however, the COM object must be instantiated (i.e., by causing the class factory to create the instance data structure  62  of the object); and the client must obtain an interface pointer to the COM object. 
   Before the COM object  60  can be instantiated, the object is first installed on the computer  20 . Typically, installation involves installing a group of related objects called a package. The COM object  60  is installed by storing the object server DLL file(s)  80  that provides the object in data storage accessible by the computer  20  (typically the hard drive  27 , shown in  FIG. 2 ), and registering COM attributes (e.g., class identifier, path and name of the object server DLL file  80 , etc.) of the COM object in the system registry. The system registry is a per-machine component configuration database. 
   COM: Component Instantiation 
   A client requests instantiation of the COM object locally or on a remote computer using system-provided services and a set of standard, system-defined component interfaces based on class and interface identifiers assigned to the COM Object&#39;s class and interfaces. More specifically, the services are available to client programs as application programming interface (API) functions provided in the COM library, which is a component of the Microsoft Windows NT operating system in a file named “OLE32.DLL.” The DCOM library, also a component of the Microsoft Windows NT operating system in “OLE32.DLL,” provides services to instantiate COM objects remotely and to transparently support communication among COM objects on different computers. 
   In particular, the COM library provides “activation mechanism” API functions, such as “CoCreateInstance( ),” that the client program can call to request local or remote creation of a component using its assigned CLSID and an IID of a desired interface. In response to a request, the “CoCreateInstance( )” API looks up the registry entry of the requested CLSID in the registry to identify the executable file for the class. The “CoCreateInstance( )” API function then loads the class&#39; executable file either in the client program&#39;s process, or into a server process which can be either local or remote (i.e., on the same computer or on a remote computer in a distributed computer network) depending on the attributes registered for the COM object  60  in the system registry. The “CoCreateInstance( )” API uses the class factory in the executable file to create an instance of the COM object  60 . Finally, the “CoCreateInstance( )” API function returns a pointer of the requested interface to the client program. 
   Referring to  FIG. 4 , a system including a local client  100  and a remote component  140  is described. A local client  100  instantiates and accesses the services of a remote component  140  using services provided by DCOM. DCOM provides the low-level services supporting instantiation of component  140  in another process or on another machine. After instantiation, DCOM supports cross-process or cross-machine communication. 
   More specifically, after the “CoCreateInstance” API  102  of the OLE32 DLL  104  is called by a client  100 , the “CoCreateInstance” API  102  determines from the system registry, from an explicit parameter, or from a moniker, the class of the component  140  and in which machine or process the component  140  should be instantiated. In  FIG. 4 , the component  140  is to be activated  106  on a remote machine. A local Service Control Manager  108  connects to a remote Service Control Manager  144 , which requests creation of the component  140  through the “CoCreateInstance” API  102 . An executable file  80  for the class is then loaded into a remote server process, and the class factory  82  in the executable file  80  is used to create an instance of the COM object  140 . Finally, the “CoCreateInstance( )” API  102  function returns to the client  100  an interface pointer to an interface proxy  110  for the requested component  140 . Whether a component is instantiated locally or remotely, the pointer returned to the client program refers to a location in local address space. So to a client, all component instantiations appear to be in-process. 
   COM: In-Process, Cross-Process, and Cross-Machine Communication 
   Binary compatibility gives COM components true location transparency. A client can communicate with a COM component in the same process, in a different process, or on an entirely different machine. Stated more succinctly, COM supports in-process, cross-process, or cross-machine communication. The location of the COM component is completely transparent to the client because in each case the client still invokes the component by calling indirectly through an interface&#39;s virtual function table. Location transparency is supported by two facilities: MIDL generation of interface proxies and stubs, and the system registry. 
   Referring again to  FIG. 4 , cross-machine communication occurs transparently through an interface proxy  110  and stub  130 , which are generated by software such as the MIDL compiler. The proxy  110  and stub  130  include information necessary to parse and type function C) arguments passed between the client  100  and the component  140 . For example, this information can be generated from an Interface Description Language (IDL) description of the interface of the component  140  that is accessed by the client  100 . The proxy  110  and stub  130  can provide security for communication between the client  100  and the component  140 . A client  100  communicates with the proxy  110  as if the proxy  110  were the instantiated component  140 . The component  140  communicates with the stub  130  as if the stub  130  were the requesting client  100 . The proxy  110  marshals function arguments passed from the client into one or more packets that can be transported between address spaces or between machines. Data for the function arguments is stored in a data representation understood by both the proxy  110  and the stub  130 . In DCOM, the proxy  110  and stub  130  copy pointer-rich data structures using deep-copy semantics. The proxy  110  and stub  130  typically include a protocol stack and protocol information for remote communication, for example, the DCOM network protocol, which is a superset of the Open Group&#39;s Distributed Computing Environment Remote Procedure Call (DCE RPC) protocol. The one or more serialized packets are sent over the network  120  to the destination machine. The stub unmarshals the one or more packets into function arguments, and passes the arguments to the component  140 . In theory, proxies and stubs come in pairs-the first for marshaling and the second for unmarshaling. In practice, COM combines code for the proxy and stub for a specific interface into a single reusable binary. 
   The client  100  invokes the component  140  through an indirect call on an interface virtual function table  64 . In this case, however, following the interface pointer provided to the client  100 , the virtual function table  64  belongs to the proxy  110 . The proxy  110  marshals function argument into one or more serialized packets and sends the packets to the destination machine using DCOM Network Protocol. The stub  130  unmarshals the arguments and calls the component  140  through the interface virtual function table  64  in the target address space. As a call is returned, the process is reversed. In this way, in-process communication between client  100  and component  140  is emulated in a distributed computing environment, invisibly to both the client  100  and the component  140 . 
   Invocation of cross-process components is very similar to invocation of cross-machine components. Moreover, cross-process communication uses the same interface proxies and stubs as cross-machine communication. The important difference is that once the function arguments have been marshaled into a buffer, DCOM transfers execution to the address space of the component. As with cross-machine invocation and communication, cross-process invocation and communication are completely transparent to both client and component. 
   COM insures location transparency because all communication takes place through calls on interface virtual function tables. The client does not know whether the code pointed to by the virtual function table belongs to the component or to an interface proxy that will forward the message to the remote component. 
   COM: Standard Interfaces 
   Once the client of the COM object  60  has obtained the first interface pointer of the COM object, the client can obtain pointers of other desired interfaces of the component using the interface identifier associated with the desired interface. 
   The “Iunknown” interface includes a member function named “QueryInterface( ).” The “QueryInterface( )” function can be called with an interface identifier as an argument, and returns a pointer to the interface associated with that interface identifier. The “IUnknown” interface of each COM object also includes member functions, “AddRef( )” and “Release( ).” Whenever a client of a component creates a new reference (e.g., an interface pointer) to the component, it calls “AddRef( ).” When it is finished using the reference, it calls “Release( ).” Through the “AddRef( )” and “Release( )(” functions, a component knows exactly how many clients have references to it. When its reference count goes to zero, the component is responsible for freeing itself from memory. By convention, the “IUnknown” interface&#39;s member functions are included as part of each interface on a COM object. Thus, any interface pointer that the client obtains to an interface of a COM object can be used to call the “QueryInterface( )” function. 
   Com: Interface Design Considerations 
   By design, the COM binary standard restricts the implementation of an interface and components to the degree necessary to insure interoperability. To summarize, COM places four specific restrictions on interface design to insure component interoperability. First, a client accesses a component through its interface pointers. Second, the first item pointed to by an interface pointer must be a pointer to a virtual function table. Third, the first three entries of the virtual function table must point to the “QueryInterface( )”, “AddRef( )” and “Release( )” functions for the interface. Finally, if a client intends to use an interface, it must insure that the interface&#39;s reference count has been incremented. As long as a component programmer obeys the four rules of the COM binary standard, he or she is completely free to make any other implementation choices. 
   During implementation, the component programmer chooses a memory layout for component and per-instance interface data. Memory layout is influenced by the number of supported interfaces, the existence of unique instances of the same interface for different clients, the expected lifetimes of interface instances, the amount of per-instance and per-component data, and internal, component-specific design factors. 
   Most components support at most roughly a dozen interfaces with each interface having only a single instance. Referring to  FIG. 5 , the relationship between a client  100  and a component  140  exposing multiple interfaces to the client is explored in some detail. The client includes an interface pointer  160  to the IUnknown interface, and other interface pointers  162 – 166  for other interfaces exposed by the client. The interface pointers  160 – 166  point to an instance data structure  62  for the component  140 . COM defines several standard interfaces generally supported by COM objects including the “IUnknown” interface. A pointer  170  to the virtual table  180  is listed first in the instance data structure  62  of the component  140 . The instance data structure  62  contains one VTBL pointer  170 – 173  per interface, a per-component reference count  176 , and internal component data  178 . Each VTBL pointer  170 – 173  points to a virtual table  180 – 183 , which in turn contain pointers to member functions  190 – 195  of the interfaces. Every interface includes the “QueryInterface( )”  190 , “AddRef( )”  191 , and “Release( )”  192  functions. In addition, interfaces can include other member functions. For example, Interfaces includes the additional functions  193 – 195 . Within the component&#39;s member functions, a constant value is added to the “this” pointer to find the start of the memory block and to access component data  178 . All of the component interfaces use a common pair of “AddRef( )” and “Release( )” functions to increment and decrement the component reference count  176 . 
   Sometimes, a component supports multiple copies of a single interface. Multiple-instance interfaces are often used for iteration. A new instance of the interface is allocated for each client. Multiple-instance interfaces are typically implemented using a tear-off interface. A tear-off interface is allocated as a separate memory block. The tear-off interface contains the interface&#39;s VTBL pointer, a per-interface reference count, a pointer to the component&#39;s primary memory block, and any instance-specific data. In addition to multiple-instance interfaces, tear-off interfaces are often used to implement rarely accessed interfaces when component memory size is desirably minimized, (i.e., when the cost of the extra four bytes for a VTBL pointer per component instance is too expensive). 
   Components commonly use a technique called delegation to export interfaces from another component to a client. Delegation is often used when one component aggregates services from several other components into a single entity. The aggregating component exports its own interfaces, which delegate their implementation to the aggregated components. In the simple case, the delegating interface simply calls the aggregated interface. The simple case is interface specific, code intensive, and requires an extra procedure call during invocation. The simple solution is code intensive because delegating code is written for each interface type. The extra procedure call becomes particularly important if the member function has a large number of arguments or multiple delegators are nested through layers of aggregation. 
   A generalization of delegation is the use of a universal delegator. The universal delegator is essentially a type-independent, re-usable delegator. The data structure for a universal delegator consists of a VTBL pointer, a reference count, a pointer to the aggregated interface, and a pointer to the aggregating component. Upon invocation, a member function in the universal delegator replaces the “this” pointer on the argument stack with the pointer to the delegated interface and jumps directly to the entry point of the appropriate member function in the aggregated interface. The universal delegator is “universal” because its member functions need know nothing about the type of interface to which they are delegating; they reuse the invoking call frame. Implemented in a manner similar to tear-off interfaces, universal delegators are instantiated on demand, one per delegated interface with a common VTBL shared among all instances. 
   Alternative Object Standards 
   Although COIGN is described with reference to applications designed according to COM, aspects of COIGN are equally applicable to applications designed according to other object standards. For example, the following aspects, later described in detail, are equally applicable to COM and non-COM applications: automatic distributed partitioning of an application binary; recording summarized pair-wise component communication; deriving a network-independent representation of application communication; re-instrumenting an application for distribution using pre-processed metadata; reversible static linking of a library to an application; in-line redirection of object creation requests in an ADPS; dynamic classification; quickly estimating network latency and bandwidth; and automatically detecting location constraints. 
   Alternative Distributed Communications Services 
   The COIGN system is described with reference to communication support provided by the COM family of services. Other distributed communication services provide cross-process and cross-machine transparency, but not in-process location transparency. This prevents a server process from running in the same address space as a client process, and thus prevents a distributed application from using inexpensive in-process communication between components also capable of distributed communication. In contrast, the COM family of services provides true location transparency, so non-distributed components pay no performance penalty for exposing potentially distributable interfaces. 
   Even so, a true location-transparent component system similar to COM could be built with some effort upon other distribution services, as in fact COM builds on the Distributed Computing Environment Remote Procedure Call (“DCE RPC”) standard. The COIGN system could then be ported to the new system. 
   Overview of the Illustrated ADPS 
   It is both possible and beneficial to partition and distribute applications automatically. Quantitatively, the benefit of automatic distributed partitioning is determined by the performance of the chosen distribution. It is possible to determine a distribution for a given application that minimizes communication costs for the application in a given distributed computing environment. Ultimately, however, the performance of a selected application distribution also depends on the granularity and quality of the application&#39;s units (e.g., COM objects in the COIGN system ADPS), and, where applicable, on the appropriateness of the profiling scenarios (described below) used to measure internal application communication. While the present invention cannot improve a completed application&#39;s design, it can achieve the best possible distribution of that design subject to the profiling scenarios. 
   Automatic distributed partitioning reduces the programmer&#39;s burden. Rather than code for a specific distribution, the programmer is encouraged to create easily distributed application units. Emphasis is placed on code reusability, application unit autonomy, and choice of appropriate algorithm and data abstractions—all elements of good software engineering. In essence, automatic distributed partitioning makes the most of good software engineering by raising the level of abstraction for the distributed application programmer. In contrast, manual distributed partitioning forces the programmer to be keenly aware of how an application will be distributed. 
   Distributed partitioning is complicated by interactions between code modules, between data structures, and between both code and data. For instance, one data structure can contain a pointer to another data structure. If either data structure is naively relocated to another machine without modification, an attempt to de-reference the pointer will fail, most likely producing a virtual memory fault. Automatic distributed partitioning requires that either the programmer or the computer system explicitly manage code and data interactions crossing machine boundaries. For example, in the COIGN system, the COM family of services manages code and data interactions across machine and process boundaries. 
   In general, an ADPS takes an application as its input. For output, the ADPS modifies the application to produce a distributed version of the application that minimizes network communication costs. 
   Referring to  FIG. 6 , an application  200  is automatically partitioned for distribution according to the illustrated embodiment of the present invention. In the illustrated ADPS, the application  200  is of design known in the art. In the COIGN system, for example, the application  200  is an application binary, including executable files, dynamic link libraries, and other object code representations of software. In the COIGN system, the application binary is desirably designed according to an object model with suitable granularity, location transparency, and interface description, for example, Microsoft&#39;s COM, but alternatively can be designed according to other standards. 
   An application description set  220  describing the behavior of the application is prepared at step  210  for the application  200 . The application description set  220  can be supplied by an external source that analyzes the application  200  in advance, or can be generated by the illustrated ADPS itself. The application description set  220  can include static and/or dynamic metadata describing the application. For example, in the COIGN system, the application description set  220  can include static metadata derived from metadata provided by a Microsoft IDL compiler (MIDL). Alternatively, the application description set  220  can include static metadata generated by the illustrated ADPS through static analysis techniques. Dynamic analysis techniques can be used by the illustrated ADPS to include dynamic metadata (such as dynamic descriptions of units, descriptions of actual inter-unit communication between the units of the application  200 , and descriptions of how much time was spent in each unit in computation) in the application description set  220 . 
   An environment description set  230  describes the distributed computing environment in which the application  200  is to be distributed. The environment description set  230  can be a description of an idealized computer network with identical computers and no communication costs. Alternatively, the environment description set  230  includes a high level description of a particular physical network on which the application  200  is to be distributed. The environment description set  230  can include a high level behavioral classification scheme used to determine which units should run on particular machines in a distributed computing environment. The environment description set  230  can also include descriptions of network characteristics such as latency and bandwidth, or descriptions of location constraints for particular units. In an alternative embodiment, the application description set  220  implicitly contains description of the behavior of a distributed computing environment along with description of the behavior of an application, for example real-time measurements of communications between distributed units of an application. 
   The environment description set  230  and application description set  220  are analyzed at step  240  to determine where units of the application  200  should be located in the distributed computing environment, for example according to the following pseudocode:
         If (unit behavior=x) locate unit on machine Y   Else locate unit on machine Z.       

   In the COIGN system, a more complicated algorithm, for example, a commodity flow algorithm, is applied to a representation of units and communication between the units. 
   A distribution scheme  50  is the result of applying the environment description set  230  to the application description set  220 . The distribution scheme  250  includes a mapping of application units to locations in a distributed computing environment. The units can be classified using static metadata of the units. Alternatively, where run-time profiling was used to dynamically describe the units, the units can be classified according to dynamic behavior. At run-time, units of the application  200  are mapped using the distribution scheme  250  for location on an appropriate computer in the distributed computing environment. 
   The various aspects of the present invention can be organized according to the three sub-areas they involve: discovering how the application can be partitioned, deciding how the application should be distributed, and achieving a chosen distribution. 
   Discovery: Discovering how the Application can be Partitioned. 
   An application description set  220  describes the behavior of the application. In the illustrated ADPS, these descriptors can be supplied by an external source and include static and/or dynamic metadata about the application. In the COIGN system, COIGN generates the application description set using an instrumentation package attached to the application, identifying individual units of the application, and identifying and quantifying relationships between the units. The mechanism by which the instrumentation package is attached to the application is described in detail below. 
   The illustrated ADPS requires knowledge of the structure and behavior of the target application. Data is gathered or supplied on how the application can be divided into units and how those units interact. ADPS functionality and effectiveness are limited by the granularity of distribution units, availability of structural metadata to identify units, choice of application analysis technique, representation of communication information, and mechanisms for determining location constraints on application units. 
   Granularity of Distributable Units 
   The granularity at which an application is divisible severely impacts the potential for improving performance of its distribution. Distribution granularity dictates the smallest independently distributable unit of the application. The number of potential distributions is inversely related to the distribution granularity. If the number of distributions is insufficient, none may offer good performance. However, if the granularity is too small, the tasks of choosing and realizing a distribution may become prohibitively expensive. 
   Perhaps even more importantly, the choice of partitioning unit shapes the relationships between partitioned granules. For instance, many distributed share memory (DSM) systems partition programs into VM pages. A single VM page often contains objects whose only commonality is their locality in creation time. The relationship between adjacent VM pages may be even more tenuous. Ideally, data within a distribution granule will exhibit good temporal and contextual locality. 
   The illustrated ADPS cannot choose granularity directly. The choice of distribution granularity is determined by the choice of operating environment. For instance, the distribution granularity in COIGN is a direct result of implementing the system on COM. An ideal environment for automatic distributed partitioning should provide a granularity of distribution with sufficient options to make automated partitioning worthwhile. The ideal granularity should match available metadata and provide a good “fit” to the application&#39;s structure. 
   Structural Metadata to Identify Units and Manage Communication 
   Distributed partitioning divides an application into units. Measurement of communication between units and division of units require access to appropriate metadata describing program structure. Program metadata can be derived from any of several sources including a compiler intermediate representation (IR), application debugging information, an interface definition language (IDL), and memory access data from the virtual memory (VM) system. Structural metadata provides the illustrated ADPS with sufficient information to separate application units and to manage code and data interactions among remote units of the application. 
   For example, in the COIGN system, IDL metadata and type libraries are provided by the Microsoft IDL compiler. IDL metadata is used to identify the number and type of arguments passed to and from interface functions. IDL metadata facilitates the identification and separation of components. Further, during distributed execution, IDL metadata is used to create proxies and stubs for cross-process and cross-machine communication. 
   Alternatively, other types of structural or program metadata can be used to identify application units. 
   Dynamic Application Analysis 
   The illustrated ADPS generates the application description set  220 . To do so, the illustrated ADPS can analyze (step  210 ) the structure of the application  200  and the communication between identified units of the application  200 . 
   The choice of application analysis technique determines the type of application behavior visible to an ADPS. To work satisfactorily on applications in which application units are dynamically created and destroyed, a fully functional ADPS requires whole program analysis with complete information about the application&#39;s units, their dynamic instantiation relationships, and their communication patterns. 
   Dynamic analysis provides insight into an application&#39;s run-time behavior. The word “dynamic,” as it is used here, refers to the use of run-time analysis as opposed to static analysis to gather data about the application. Major drawbacks of dynamic analysis are the difficulty of instrumenting an existing application and the potential perturbation of application execution by the instrumentation. Techniques such as sampling or profiling reduce the cost of instrumentation. In sampling, from a limited set of application executions, a generalized model of application behavior is extrapolated. Sampling is only statistically accurate. In profiling, an application is executed in a series of expected situations. Profiling requires that profile scenarios accurately represent the day-to-day usage of the application. A scenario is a set of conditions and inputs under which an application is run. In the COIGN system, scenario-based profiling can be used to estimate an application&#39;s run-time behavior. 
   Referring to  FIG. 7 , scenario-based profiling of an application  200  to generate an application description set  220  is described. At step  202 , structural metadata describing the application  200  is obtained. This structural metadata can be provided by an external source, or generated by the illustrated ADPS, as described in the preceding section. During later dynamic analysis, structural metadata can be used to determine how much data is between units of an application. For example, in the COIGN system, IDL metadata can be used to exactly identify function parameters, then measure the size of those parameters. With accurate interception and access to structural information, communication measurement is a straightforward process. 
   At step  204 , the application  200  is executed in a scenario meant to model the expected use of the application  200 . During execution, the application behaves normally while the numbers, sizes, and endpoints of all inter-unit messages are measured. At step  206 , the user decides if profiling is finished. The application can be run through an arbitrary number of profiling scenarios. After profiling of the application is completed, the results from the scenario-based profiling are written (step  208 ) to the application description set  220 . The application description set  220  can include structural description of the application as well as description of communication between units of the application. 
   Through scenario-based profiling, an ADPS can create a profile for each application unit instantiated during profiling runs of the application. The profile identifies and quantifies communication between the application unit and other units. The collection of profiles for all units in the application, together with the records of communications between units, can be included within the application description set  220  and used to decide where units should be placed in the network. 
   Network-Independent Representation 
   An ADPS partitions an application to minimize its distributed communication costs. A correct distributed partitioning decision requires both realistic information about the network on which the application will be distributed, and accurate information about communications between units of an application. 
   In the illustrated ADPS, an appropriate inter-unit cost representation for an application is network-independent, but also incorporates realistic analysis of distribution tradeoffs prior to distribution. For example, referring to  FIG. 6 , an application description set  220  comprising a network-independent abstraction of inter-unit communication costs of an application can be combined with an environment description set  230  comprising basic statistics about a physical network to calculate concrete, network-dependent communication costs. While the environment description set  230  can be generated at the same time as the application description set, it can also be generated before or after. The environment description set  230  can be generated immediately before the application is to be distributed in a distributed computing environment, in this way describing the most recent state of the environment. 
   Network-independent representations of communication costs provide an application with a great degree of flexibility to adapt to future changes in network topology including changes in the relative costs of bandwidth, latency, and machine resources. In this way, a single application can be optimally bound to different networks, and a single application can be optimally bound and re-bound to a changing network. The ADPS preserves application flexibility by insulating the programmer from the final distributed partitioning decision. The programmer is responsible for exposing as many partitioning choices as possible by dividing the application into distributable units, but the ADPS is responsible for correctly distributing the application units for a given execution of the application based on the network environment. In essence, the ADPS allows late binding of an application to a particular network and its topology. 
   Late binding of an application across a specific network is facilitated by two mechanisms, described in detail below. First, compression of information about application communication reduces ADPS run-time overhead during profiling, and thereby enables more accurate and efficient summarization of network-independent communication costs. Second, quick estimation of the latency and bandwidth of a network allows the ADPS to delay partitioning until current estimates are needed. Combined, these techniques make it possible to delay binding of a distribution to a network until the latest possible moment, thus facilitating automatic adaptation to new networks. 
   In an alternative embodiment, estimates of latency and bandwidth are periodically taken during execution of a distributed application. If the new estimates deviate beyond a preset threshold from previous estimates, the application is re-partitioned and distributed using the new estimates. In another embodiment, inter-unit communication is measured during distributed execution. If the communication characteristics of the distributed application deviate beyond a preset threshold from the communication characteristics used to determine the current distribution scheme, the distributed application is re-partitioned and re-distributed. 
   Alternatively, at a time when the characteristics of the distributed application deviate beyond a preset threshold, a notification can be given to the user. In response to the notification, the user can re-bind the application or ignore the notification. 
   Communication Representation 
   In the illustrated ADPS, during scenario-based profiling, communication between the application units is measured. Later, the illustrated ADPS partitions the application by comparing the inter-unit communication costs and network costs of alternative distributions. Because precise distributed partitioning analysis requires an accurate picture of the cost to distribute each unit of an application, the illustrated ADPS requires an accurate picture of the communication between units of an application. 
   During scenario-based profiling, the illustrated ADPS can measure the number and size of communications sent between any two application units. Pertinent features describing an inter-unit message are the source unit, the destination unit, and the amount of data sent from source to destination. For practical reasons, it is important to minimize perturbation of the application by the illustrated ADPS during scenario-based profiling. While the illustrated ADPS might ideally log all data about every message, doing so would most likely have a severe impact on application execution during profiling. Moreover, data about application communication needs to be preserved until the application is actually partitioned. If the size of the communication data is extremely large, preserving it can be prohibitively expensive. An inclusive log of all messages can be extremely large. It is conceivable that an application scenario could involve millions of messages. 
   Rather than store this information in a lengthy trace file, in the COIGN system, the number and size of inter-unit messages is selectively summarized. Various techniques can be used to compress application communication information. 
   The communication log can be compressed somewhat by storing messages with the same source and destination in a single collection. The source and destination need only be written once with subsequent records containing the size of the message only. However, the communication log might still be prohibitively large. 
   The communication log can be compressed even farther by noting that the important feature of the message in the partitioning decision is not the size of the message, but rather the communication cost of the message. The communication log for a source-to-destination pair could be compressed into a single number by summing the cost of all messages. However, to preserve generality it is desirable to separate the network dependent portion of the communication costs from the network independent portion. 
   The cost of sending a message consists of a latency factor, which is fixed for all messages, and a bandwidth factor, which is a function of the message size. The correlation of message size to bandwidth is nearly linear. Assuming that the bandwidth-cost function is in fact linear, instead of storing each message size, an alternative ADPS according to the invention stores the number of messages and the sum of the message sizes, as shown in the following equation 1: 
                   ∑     m   =   1     n     ⁢           ⁢     Cost   ⁡     (   m   )         =       n   ·   Latency     +     s   Bandwidth         ,       where   ⁢           ⁢   s     =       ∑     m   =   1     n     ⁢           ⁢       Size   ⁡     (   m   )       .                 (   1   )             
 
   Unfortunately, the bandwidth-cost function is not strictly linear for most networks. Instead, the bandwidth-cost function is made up of discontinuous, near-linear ranges. The discontinuities occur when a message of size n+1 requires one more network packet than a message of size n. Not coincidentally, the discontinuities are a function of the network maximum transmission unit (MTU) and the network protocols. Compressing message sizes under the assumption that the bandwidth-cost function is strictly linear introduces an average error of 15% for a 10BaseT Ethernet. Similar errors are introduced for other networks. 
   An alternative approach to compress the log of messages is to compress each near-linear sub-range separately. For example, all messages from 0 to 1350 bytes could be linearly compressed into the number of messages and sum of message lengths. All messages from 1351 to 2744 bytes could also be linearly compressed. All messages above some large threshold value could be linearly compressed as MTU-induced discontinuities become less pronounced. MTU-induced non-linearities in the bandwidth-cost function are much more important for small messages than for large messages. As messages become larger, the amortized cost of each additional network packet becomes minimal. Unfortunately, compression based on the near-linear sub-ranges of a specific network is network dependent, which is something to be avoided. 
   Rather than linearly compress sub-ranges based on the MTU of a specific network, the ADPS of the present invention can linearly compress a number of exponentially larger sub-ranges starting with a very small range. For each sub-range, the decompression algorithm (i.e., the algorithm to calculate the cost of the compressed messages) is given by the following equation 2: 
                 ∑     m   =   1     n     ⁢           ⁢     Cost   ⁡     (   m   )         =     n   ·       (       Latency   small     +       (       s   n     -     Size   small       )     ⁢         Latency   large     -     Latency   small           Size   large     -     Size   small             )     .               (   2   )             
 
where 
         s   =       ∑     m   =   1     n     ⁢           ⁢     Size   ⁡     (   m   )           ,       
     Latency small =Latency of the smallest message size in the sub-range,   Latency large =Latency of the largest message size in the sub-range,   Size small =Size of the smallest message in the sub-range, and   Size large =Size of the largest message in the sub-range.   

   In the COIGN system, the following sub-ranges for network-independent linear compression are used: 0–31 bytes, 32–63 bytes, 64–127 bytes, 128–255 bytes, 256–511 bytes, 512–1023 bytes, 1024–2047 bytes, 2048–4095 bytes, and 4096 bytes and larger. Compressing with these sub-ranges and then calculating values results in an average error of just over 1% for a 10BaseT Ethernet. 
   Determining Location Constraints 
   An ADPS can consider location constraints when partitioning application units for distribution. All prior work in ADPS systems has relied on programmer intervention to determine location constraints for application units. In the illustrated ADPS, location constraints can be desirably automatically detected and recorded, freeing the programmer from the task of identifying, tracking, and indicating location constraints. 
   Per-unit location constraints indicate which application units run better on a particular machine of the network or will not run at all if removed from a particular machine. The most common form of per-unit constraint is application unit communication through second-class communication mechanisms. A typical example of a second-class communication mechanism is a Unix file descriptor. The file descriptor represents a communication channel between the operating system and application. The file descriptor is a second-class mechanism because it cannot be directly distributed with first-class mechanisms, such as shared memory in a DSM system or interfaces in COM. The file descriptor implicitly constrains program location. In the COIGN system, system service libraries called by application units are analyzed to automatically detect second-class communication mechanisms and other per-unit location constraints. Alternatively, per-unit location constraints can be automatically detected by analyzing other application unit interactions with system resources. 
   Pair-wise location constraints indicate which combinations of application units must be located together. Pair-wise distribution constraints cannot be violated without breaking the application. For example, in COM, pair-wise constraints occur when two components must be co-located because they communicate either through an undocumented interface or through an interface that is not remotable because it uses opaque data types. In the COIGN system, pair-wise constraints are automatically detected during analysis of interaction between application units. If communication (e.g., function call parameters, data types) between two application units is not understood well enough to quantify the communication during profiling, a pair-wise location constraint is placed upon the two application units. Alternatively, if communication between the two application units is not understood well enough to remote the interaction (e.g., by marshalling and unmarshalling parameters over processes or machines) during distributed execution, a pair-wise location constraint is placed upon the two application units. 
   Decision: Deciding how the Application Should be Distributed. 
   While an application can be partitioned in many ways, not all of them will yield equivalent performance. Application distributions that reduce the number and size of distributed messages are most likely to exhibit good performance. Because distributed communication is much more expensive than local communication, a distribution should minimize the amount of inter-machine communication. In addition to communication overhead, the illustrated ADPS can take into consideration relative computation costs and resource availability. A simple classification algorithm can be used to generate a distribution scheme  250  from an application description set  220  and an environment description set  230 . Abstractly, the distribution decision consists of a communication model and cost metric that encode the decision problem for a particular application on a particular network, and an algorithm for optimizing the model. 
   An ADPS can model the tradeoffs between candidate distributions. Distribution costs can be modeled either directly or indirectly. Direct models specifically include communications costs between application units and resource availability. Indirect models consider contributing factors such as data or temporal locality. The choice of model determines which kinds of input data are required and which factors the optimizing algorithm maximizes. One very useful model of the distribution problem represents the application as a connected graph. Nodes represent units of the application and edges represent interactions between units. Edges are weighted with the relative cost of the interaction if remote. 
   Distribution Optimization Algorithms 
   The distribution optimization algorithm accepts a model of the decision problem and maps it onto a computer network. After all data has been gathered, it is the optimization algorithm that decides where application units will be placed in the network. In the COIGN system, the problem of deciding where to place application units is mapped to the common problem of cutting a commodity flow network. As described below with reference to  FIG. 8 , the application units and inter-unit communication form a commodity flow network. After this mapping, known graph-cutting algorithms can be used for automatic distributed partitioning. 
   A commodity flow is a directed graph  250  G=(N,E) with two special nodes (s  251  and t  252 ) designated respectively the source and sink. A steady supply of a commodity is produced by the source s  251 , flows through the graph  250 , and is consumed by the sink t  252 . The graph  250  contains an arbitrary number of nodes  253  through which the commodity flows. Each node  253  may be connected to another node  253  by an edge  254 . A node  253  may be connected to an arbitrary number of other nodes. Each edge  254  of the graph  250  has a capacity  255  that determines how much of the commodity may flow through it at a given time. The total flow through the graph is limited by the aggregate edge capacity  256 . An important concept related to commodity flows is the cut  258 . A cut (S, T) of a flow network G=(N,E) is a partition of the nodes N into two sets, S and T, such that the source sεS and the sink tεT and for all nεN, nεS or nεT. The capacity of a cut  258  is the capacity of all of the edges connecting S to T; in other words, the capacity of the edges that cross the cut  258 . A minimum cut is a cut of the commodity-flow graph with the smallest capacity. 
   In the case of a simple client-server network, the optimization algorithm can be a MIN-CUT MAX-FLOW algorithm, a type of optimization algorithm known in the art. The MIN-CUT MAX-FLOW theorem states that the capacity of the minimum cut is equal to the maximum flow through the flow graph. The capacity of the MIN-CUT is determined by the same edges that constrain the MAX-FLOW. The most efficient known algorithms to solve the MIN-CUT MAX-FLOW problem belong to the preflow-push family. The basic idea of the preflow-push algorithms is to use an iterative technique in which the commodity (limited by edge capacities) is pushed breadth-first through each edge from the source  251  to the sink  252 . Excess commodity (when more commodity flows into a node than flows out) is iteratively pushed back to the sink again using a breadth-first algorithm. The simplest preflow-push algorithm runs in O(N 2 E) time. Another algorithm used to partition client-server application across two machines, the lift-to-front algorithm, is a known preflow-push algorithm that runs in time O(N 3 ), which is asymptotically at least as good as O(N 2 E). The best known pre-flow push algorithm to date runs in time O(NE log (N 2 /E)). Alternatively, other known optimization algorithms can be applied to a model of the decision problem. 
   While the problem of partitioning a graph into two sets (one containing the source and one containing the sink) can be solved in polynomial time, partitioning a graph into three or more sets (creating a multi-way cut) according to known algorithms in the general case is NP-hard. For this reason, practical multi-way graph cutting relies on approximation algorithms known in the art. 
   In the COIGN system, the algorithm to map a client-server distributed partitioning problem onto the MIN-CUT problem is as follows: Create one node for each unit in the application. Create one edge between every pair of communication units. The weight on the edge should be the difference between communication cost (communication time) for the remote case (when the two application units are placed on separate machines) and the local case (when the two application units are placed on the same machine). Create two additional nodes: the source and the sink. The source represents the client. For each application unit that must reside on the client—for instance, because it directly accesses GUI functions—create an edge with infinite weight from the source to the application unit. For each application unit that must reside on the server—because it directly accesses storage—create an edge with infinite weight between the sink and the application unit. Find the minimum cut of the graph. Since the minimum cut contains edges with the smallest weights (capacities), those edges represent the line of minimum communication between the client and server. 
   Each edge in the commodity-flow graph effectively represents the cost in time of distributing that edge. Because the common currency of graph edges is time, other time-based factors that affect distribution choice can be mapped readily onto the same MIN-CUT problem with communication costs. A good example is the problem of deciding where to place application units when client and server have different speed processors. For this case, two additional edges are attached to each application unit. An edge from the application unit to the source s has a weight equal to the execution time of the application unit on the server. A second edge from the application unit to the sink has a weight equal to the execution time of the application unit on the client. 
   Each “computation” edge represents the cost in execution time if application unit is moved to the other computer. The MIN-CUT algorithm will cut through the edge that is least expensive (when considered with the other edges in the graph), thus leaving the application unit attached to the computer on which its aggregate communication and computation time is the lowest. 
   Each of the edges in the commodity flow graph is weighted with the same linear “currency”. Because communication costs are most readily converted into time, the graph can be augmented with other time-based costs. In an ideal environment, one would also like to map discontinuous features into the graph problem. A common influencing factor in the choice of distribution is memory overhead. It is often desirable to keep memory footprint per client to a minimum on the server in order to maximize scalability of the server across multiple clients. Similarly, a client may not have enough memory to accommodate all application units that would ideally be placed upon it if considering time-based costs alone. The only known method to map memory overhead onto the graph-cutting problem uses a multi-commodity flow graph. Unfortunately, multi-commodity flow graphs are provable NP-complete in the general case. 
   Choosing a Distribution Online 
   In the illustrated ADPS, accurate values of latency and bandwidth for a particular network can be quickly estimated using a small number of samples, enabling adaptation to changes in network topology including changes in the relative costs of bandwidth, latency, and machine resources. 
   A correct distributed partitioning decision requires realistic information about the network on which the application will be distributed. If all distributed partitioning decisions are made offline, data for a particular network can be gathered from a large number of samples. For example, average latency and bandwidth values for a network can be derived from a large number of test packets sent on the network. In a dynamic environment where bandwidth and network availability can change from one execution to another, or within a given execution, it is desirable to make distributed partitioning decisions online at application startup. Data for online decision-making is gathered while the user waits. This creates a serious constraint on the number of samples used to determine available latency and bandwidth and model of network communication costs. 
   An ADPS minimizes communication costs between distributed application units by comparing alternative distributions. When comparing two application distributions, the communication costs in the first distribution are compared with the communication costs in the second distribution. The communication cost for any message is composed of two sub-costs: a fixed sub-cost due to network latency and a variable sub-cost due to network bandwidth. For some message m, the cost can be represented according to the following equation 3: 
               Cost   ⁡     (   m   )       =     Latency   +         Size   ⁡     (   m   )       Bandwidth     .               (   3   )             
 
   The cost of an application distribution is the sum of the costs of all n messages sent between the partitioned application units given by the following equation 4: 
               Distribution   ⁢           ⁢   Cost     =         ∑     m   =   1     n     ⁢           ⁢     Cost   ⁡     (   m   )         =       n   ·   Latency     +           ∑     m   =   1     n     ⁢           ⁢     Size   ⁡     (   m   )         Bandwidth     .                 (   4   )             
 
   Measuring the real communication costs for a given network is extremely simple in theory, but somewhat error-prone in practice. For instance, to measure the average latency of a network, one sends a number of messages from one machine to another and back. One can compute the average round-trip time from either individual round trips using the following equation 5: 
                 T   ave     =         ∑     i   =   1     n     ⁢           ⁢     T   i       n       ,           (   5   )             
         or from the cumulative time for all of the round trips using the following equation 6: 
               T   ave     =         T   total     n     .             (   6   )             
       

   In practice, the round-trip time for a packet is unpredictable, making it hard to estimate average network behavior. This is particularly true for IP-based networks. Consider the round trip for a typical network message. The application initiates a message by creating a packet and invoking the operating system. The message passes through various layers in a protocol stack before the operating system eventually invokes the network interface. While travelling through the protocol stack, the message may be delayed by cache faults in the memory hierarchy. The network interface places the message onto the network medium. In many cases, such as shared medium token-ring or Ethernet, the network adapter may have to wait before actually transmitting the message. The message may travel over multiple physical networks; passing through routers to cross networks. At any router, the message may be dropped due to insufficient queue capacity on the router, forcing a re-transmission. When the message finally arrives at the receiver, it is placed in an incoming buffer. Again, the message may be dropped if the receiver has insufficient buffer capacity. In fact, the vast majority of message losses in typical networks are due to insufficient buffer capacity on the receiving machine. The network interface alerts the operating system, which picks up the message, passes it through the protocol stack, and finally delivers it to the receiving process. The receiving process takes appropriate action, then returns a reply to the sending process. The reply may wind its way back to the original process only to find that the original process was rescheduled after losing its scheduling quantum. 
   A message may be delayed at any point in the journey from the sender to the receiver and back. By measuring average round-trip time, an ADPS in fact measures the cumulative average effect of each source of delay. The more sources of spurious delay, the more measurements must be taken in order to calculate accurately the average round-trip time. Unfortunately, it takes time to make each network measurement. If network performance is unstable over time, then individual measurements will be unstable and the ADPS will therefore need more measurements to obtain an accurate view of current network performance. In contrast to average latency, minimum latency remains quite stable throughout all of the sources of delay typically introduced in networks. Stability in calculating the minimum network latency hints at the stochastic nature of packet-switched networks. No matter how heavy traffic is on a network, there are almost always a few packets that travel through the network at peak speeds. In fact, short-term performance of packet-switched networks is extremely unpredictable. If this were not the case, almost all packets would take a long time to travel through a heavily used network. In other words in a non-stochastic network, average latency and minimum latency would converge. Moreover, minimum latency fairly accurately tracks average latency for most networks. 
   In the illustrated ADPS, minimum latency and maximum bandwidth can be quickly measured with a short-term sample of measurements because even in congested networks, a few measurement packets pass through undelayed. Moreover, because minimum latency and maximum bandwidth reasonably track average values, minimum latency and maximum bandwidth values can be used in the illustrated ADPS. 
   Alternatively, an ADPS can utilize a combination of long-term values and short-term values. First, the ADPS can compute the average latency and bandwidth over an entire usage cycle—either a full day or a full week—and partition the application once accordingly. At the same time, the ADPS can create a library of stored average latency and bandwidth numbers—say one set of averages for each hour in the day—and depending on the time of day, partition the application according to the pre-computed network statistics. Second, after quickly estimating minimum latency and maximum bandwidth, these values can be matched to the closest stored average latency and bandwidth values, and the application then partitioned accordingly. 
   Distribution: Achieving a Chosen Distribution. 
   Ultimately, an ADPS modifies the execution of the application to achieve a desired distribution. In the COIGN system, described in detail below, COIGN modifies the application by inserting an instrumentation package specially designed for distributing the application according to the desired distribution. This instrumentation package can be included with the instrumentation package used to identify units and measure communication, or can be a separate, lighter overhead package. Once the application is instrumented, achieving a distribution consists of two important steps: identifying application units and distributing them to the correct machine. 
   In general, through scenario-based profiling or static analysis, the illustrated ADPS creates a profile for each application unit instantiated. The profile characterizes the application unit&#39;s communication with other units and any constraints on its location. Information from the profiling scenarios or static analysis is generalized to predict application behavior for later executions. A mapping of generalized application unit profiles to specific machines in the network is generated. Application units instantiated during application execution are then matched to similar application unit profiles, and located on the appropriate machine in the network. The actual distribution is an approximate solution to the distributed partitioning problem: the optimal solution for a particular application execution can only be determined after execution has completed. The underlying assumption of automatic distributed partitioning is that past profiles are statistically accurate in describing future application executions. If, in fact, past profiles accurately predict future application executions, then future executions can be partitioned using the distribution derived from the profiles. 
   Difficulties in classification by profile arise when application units are dynamic objects, such as COM components, for example. Component lifetimes are dynamic. A component may be instantiated or deleted at almost any point in program execution. Multiple instances of the same static type of component may exist concurrently. Moreover, separate instances of the same static type of component may have vastly different behavior and communication patterns due to their different usage contexts. For example, a single component in the document processing application, Octarine, is instantiated multiple times in a typical execution. Some instances hold references to operations invoked by menu commands. Some instances hold references to parts of a document including footers, headers, and body. Still other instances hold references to components in dialog boxes or spreadsheet cells. Two components with the same static type and similar communication patterns may need to be placed on separate machines if their sets of communicating partners are significantly different. In applications that are input-driven, user input typically drives the dynamic instantiation of application components. For this reason, component behavior varies tremendously between executions. 
   Component instances need to be classified not by their static type, but rather by their behavior and “where” they fit into the application. In essence, an instance needs to be classified by its usage context. The context in which a component is used determines its pattern of communication with other components. Usage context also determines the quantity of data communicated to other components. 
   Identification by Dynamic Classification 
   The illustrated ADPS can identify application units for distribution according to a dynamic classification scheme. The word “dynamic,” as it is used here, refers to classification incorporating information on how the application unit was used during run-time. 
   Scenario-based profiling provides adequate information about the behavior and usage context of components to create component profiles used in dynamic component classification, assuming that the programmer or other user of the ADPS is sufficiently prudent to select profiling scenarios that accurately reflect the application&#39;s day-to-day usage. In practice, this is a reasonable assumption because the illustrated ADPS places no restriction on application execution that would make it impractical to use real-life scenarios for profiling. Dynamic component classification can be used to decide which component profile matches a component instance during distributed execution, or across multiple profiling scenarios. Moreover, component classification can be used within a single profiling scenario to classify component instances with identical or nearly identical behavior. 
   In a distribution scheme, a specific component profile can represent different combinations of component instances, depending on application behavior and on the chosen set of profiling scenarios. For example, a component profile can represent a single instance of a component in a single profiling scenario, or a single instance across multiple profiling scenarios. A component profile can represent a group of instances in a single profiling scenario, or groups of similar instances across multiple profiling scenarios. 
   A component is instantiated if a client uses it. For this reason, a component is dynamically classified at the time of instantiation using contextual information available at instantiation. The client must exist, in some form, if the component is instantiated. In the COIGN system, a component instance can be dynamically classified by examining the application state to determine context at the time of instantiation. An application&#39;s entire state (or at least an approximation thereof) is available at the time of component instantiation to aid in classification. However, to be tractable, component classification must use only a limited subset of the application state. Contextual information readily available at the time of component instantiation includes the execution call stack and arguments to the instantiation function. 
   According to the illustrated ADPS, various classification mechanisms can be used to dynamically classify components. Although some of these mechanisms, including procedure-call-chains, have been used in the field of dynamic memory allocation, none of these mechanisms has been used to dynamically classify components in automatic partitioning and distribution. 
   Referring to  FIG. 9 , various types of component instance classifiers are described for a component of type “type” instantiated by code fragment  260 . 
   An incremental classifier  261  tracks the number of times the function “CoCreateInstance( )” has been called. To the extent the ordering of component instantiation varies between executions of an application, the incremental classifier has limited value. 
   A component static type classifier  262  describes the type of component. A static-type CCC classifier  263  (T3C) creates a classification descriptor by concatenating the static type of the component to be instantiated with the static types of the components in the CCC. 
   In the illustrated ADPS, a procedure-call-chain (PCC) classifier  264  can be used for dynamic classification. In the field of dynamic memory allocation, PCCs have been used to identify allocation sites for storing objects in memory. The PCC classifier  264  creates a classification descriptor by concatenating the static type of the component with the PCC of the instantiation request. A PCC consists of the return address from each of the invocation frames in the call stack. A depth-n PCC is a PCC containing the return addresses from the topmost n invocation frames. The depth of the PCC can be tuned to evaluate implementation tradeoffs. Accuracy in predicting allocation lifetimes increases as the depth of a PCC increases. While a PCC can be adequate for dynamic classification in procedure-based application, component-based applications have more call context because they are inherently object-oriented. The possible PCCs form a sparse, one-dimensional space: the range of valid return addresses. Object-oriented programming adds a second dimension: the identity of the component executing the code. 
   In the COIGN system, a component call chain (CCC) is used for dynamic classification. Entries in a CCC belong to a sparse, two-dimensional space: the product of the caller&#39;s instance identity and return address. A complete CCC identifies a component instantiation. Components with matching CCCs are assumed to have matching profiles. CCCs are stored in a persistent dictionary across profiling scenarios. As new instances are created, their CCCs are added to the profiling dictionary. To partition the application, each instance class, as identified by its unique CCC, is assigned to a specific network machine. 
   There are two major variants on the CCC. The first variant contains only the entry points into each component. The entry-point component call-chain (EP3C) classifier  265  concatenates the component&#39;s static type with an entry-point component call-chain (the EP3C). The EP3C contains one tuple for each component in the dynamic call-chain. The tuple contains the return address pointer and the component instance identifier of the calling component. The EP3C does not contain entries for component-internal functions. Like the PCC classifier, the depth of the call chain in the EP3C classifier can be tuned to evaluate implementation tradeoffs. 
   The internal component call chain ( 13 C) classifier  266  creates a classification descriptor by concatenating the static type of the component with the full CCC of the instantiation request (the I3C). The I3C contains contains one tuple for each entry point component in the dynamic call-chain, as well as additional tuples for any procedures internal to the calling component. Put another way, the I3C is the procedure-oriented dynamic call-chain augmented with component instance identifiers. The EP3C is the I3C with all entries but one removed for each component in the chain. Again, the depth of the CCC used for classification can be tuned to evaluate implementation tradeoffs. 
   Tradeoffs in call-chain depth and classifier implementations include processing overhead to create a call chain, memory overhead of the profile dictionary, accuracy of the classifier, and limitations on distribution granularity imposed by the classifier. While component granularity sets an ultimate upper bound on the divisibility of the application, the classifier can further reduce the upper bound. A component instance classifier desirably identifies as many unique component classifications as possible in profiling scenarios in order to preserve distribution granularity. The partitioning system distributes the application by component classification. All of the instances of the same classification are placed on the same machine because they are indistinguishable to the distribution runtime. Therefore, a component instance classifier is desirably reliable and stable; it correctly determines when two component instances are the “same,” whether they are instantiated in the same application execution or in another application execution. Each classifier uses a specific descriptor to identify classes of similar component instances. Call-chain-based classifiers form a descriptor from the execution call stack. 
   Distributing Components to the Correct Machine 
   During distributed execution, application units are created in appropriate processes on appropriate machines in a distributed computing environment. This distribution is achieved by manipulating an application&#39;s execution. 
   Generally, there are three classes of solutions to accomplish this task according to the present invention: modify the application&#39;s source code, modify the application&#39;s binaries prior to execution, or manipulate the application&#39;s execution through run-time intervention. Static modification of application source code or binaries is extremely difficult because it requires problematic whole-program static analysis. Manipulating the application&#39;s execution through run-time intervention is relatively straightforward but has some limitations. In general, an application&#39;s execution can be manipulated to produce a chosen distribution efficiently by intercepting unit creation calls and executing them on the appropriate remote host. 
   Referring to  FIG. 10 , techniques for intercepting unit creation calls according to the illustrated embodiment are described. 
   Referring to code fragment  280 , using call replacement in application source code, calls to the COM instantiation functions can be replaced with calls to the instrumentation by modifying application source code. The major drawback of this technique is that it requires access to the source code. Using call replacement in application binary code ( 281 ), calls to the COM instantiation functions can be replaced with calls to the instrumentation by modifying application binaries. While this technique does not require source code, replacement in the application binary does require the ability to identify all applicable call sites. To facilitate identification of all call sites, the application is linked with substantial symbolic information. 
   Another technique is DLL redirection  282 . In this technique, the import entries for COM APIs in the application can be modified to point to another library. Redirection to another DLL can be achieved either by replacing the name of the COM DLL in the import table before load time or by replacing the function addresses in the indirect jump table after load. Unfortunately, redirecting to another DLL through either of the import tables fails to intercept dynamic calls using LoadLibrary and GetProcAddress. 
   The only way to guarantee interception of a specific DLL function is to insert the interception mechanism into the function code, a technique called DLL replacement. One method is to replace the COM DLL with a new version containing instrumentation ( 283 ). DLL replacement requires source access to the COM DLL library. It also unnecessarily penalizes all applications using the COM DLL, whether they use the additional functionality or not. 
   Borrowing from debugger techniques, breakpoint trapping of the COM DLL ( 284 ), instead of replacing the DLL, inserts an interception mechanism into the image of the COM DLL after it has been loaded into the application address space. At run time, the instrumentation system inserts a breakpoint trap at the start of each instantiation function. When execution reaches the function entry point, a debugging exception is thrown by the trap and caught by the instrumentation system. The major drawback to breakpoint trapping is that debugging exceptions suspend all application threads. In addition, the debug exception is caught in a second operating-system process. Interception via break-point trapping has a high performance cost. 
   The most favorable method for intercepting DLL functions is to inline the redirection call ( 286 ). In the COIGN system, inline indirection is used to intercept component instantiation calls. As described in detail below, component instantiation calls are intercepted by the COIGN Runtime, which is part of the COIGN system. The requested component is identified and classified according to the distribution scheme. If appropriate, the component instantiation call is re-directed to a remote computer. Otherwise, the component instantiation call is executed locally. 
   Usage and Architecture of the COIGN System 
   The COIGN system automatically partitions and distributes COM applications. Following a brief overview of the COIGN system, a detailed example is described in which COIGN is applied to an existing COM application, and the architecture of COIGN is described in detail. 
   Brief Overview of the COIGN System 
   Given an application built with COM components (in binary form), COIGN inserts an instrumentation package to enable scenario-based profiling of the application. COIGN uses scenario-based profiling on a single computer to quantify inter-component communication within the application. A network profile describing the behavior of a network is generated. Location constraints on the placement of components are automatically detected. Inter-component communication is modeled as a graph in which nodes representing components and edges represent inter-component communication and location constraints. Using graph-cutting algorithms, COIGN selects an optimal distribution scheme for the application for a distributed environment. COIGN then inserts an instrumentation package that incorporates the optimal distribution scheme into the application. At run time, COIGN manipulates program execution to produce the desired distribution. 
   COIGN analyzes an application, chooses a distribution, and produces the desired distribution without access to application source files. By leveraging the COM binary standard, COIGN automatically distributes an application without any knowledge of the application source code. As a corollary, COIGN is completely language neutral; it neither knows nor cares about the source language of the components in the application. Finally, by analyzing binaries only, COIGN automatically produces distributed applications without violating the primary goal of the COM component system: building applications from reusable, binary components. 
   Application of COIGN to an Existing COM Application 
   The application used in this example is a version of an existing COM application, Microsoft Corporation&#39;s Microsoft Picture It!®. Picture It!® is a consumer application for manipulating digitized photographs. Taking input from high-resolution, color-rich sources such as scanners and digital cameras, Picture It!® produces output such as greeting cards, collages, or publications. Picture It!® provides tools to select a subset of an image, apply a set of transforms to the subset, and insert the transformed subset into another image. The original Picture It!® application is entirely designed to run on a single computer. It provides no explicit support for distribution. Picture It!® is composed of approximately 112 COM component classes in 1.8 million lines of C++ source code. 
   Referring to Table 1, starting with the original binary files “pi.exe” for Picture It!®, the “setCOIGN” utility is used to insert COIGN&#39;s profiling instrumentation package, which includes a profiling logger, a NDR interface informer, and an EP3C classifier in this example. 
   Table 1 also shows file details for the application binary being instrumented. SetCOIGN makes two modifications to the pi.exe binary file. First, it inserts an entry to load the COIGN Runtime Executive (RTE) DLL (COIGNrte.dll) into the first slot in the application&#39;s DLL import table. Second, setCOIGN adds a data segment containing configuration information to the end of pi.exe. The configuration information tells the COIGN RTE how the application should be profiled and which of several algorithms should be used to classify components during execution. 
   
     
       
         
             
           
             
               TABLE 1 
             
             
                 
             
             
               Instrumenting the Application with Profiling Instrumentation 
             
             
               Using SetCOIGN 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
          
             
               D:\apps\pictureit\bin&gt; setcoign /p pi.exe 
             
             
               Config: 
             
          
         
         
             
             
             
          
             
                 
               Logger: 
               Coign Profile Logger 
             
             
                 
               Informer: 
               Coign NDR Interface Informer 
             
             
                 
               Classifier: 
               Coign EP3C Classifier 
             
          
         
         
             
          
             
               PE Executable: 
             
          
         
         
             
             
             
             
          
             
                 
               Initialized Data: 
               487424 
               (00077000) 
             
             
                 
               Image size: 
               1609728 
               ( 189000) 
             
             
                 
               Section Alignment: 
               4096 
               (  1000) 
             
             
                 
               File Alignment: 
               512 
             
             
                 
               File Size: 
               1579520 
             
             
                 
               Optional header: 
               224 
             
          
         
         
             
             
             
             
             
          
             
               Directories: 
                    VAddr 
                    VSize 
                    VAEnd 
                 
             
          
         
         
             
             
             
             
             
             
          
             
                 
               Exports: 
               15ac60 
               5563 
               1601c3 
                 
             
             
                 
               Imports: 
               157148 
               12c 
               157274 
             
             
                 
               Resources: 
               173000 
               15868 
               188868 
             
             
                 
               Debug: 
               111a60 
               54 
               111ab4 
             
             
                 
               IAT: 
               110000 
               1a58 
               111a58 
             
          
         
         
             
             
             
             
             
             
             
             
             
             
          
             
               Sections: 4 
                    VAddr 
                    VSize 
                    VAEnd 
                    FAddr 
                    FSize 
               R 
               L 
               R 
               L 
             
          
         
         
             
             
             
             
             
             
             
             
             
             
             
          
             
                 
               .text 
               1000 
               10e343 
               10f343 
               400 
               10e400 
               0 
               0 
               0 
               0 
             
             
                 
               .rdata 
               110000 
               501c3 
               1601c3 
               10e800 
               50200 
               0 
               0 
               0 
               0 
             
             
                 
               .data 
               161000 
               11224 
               172224 
               15ea00 
               d400 
               0 
               0 
               0 
               0 
             
             
                 
               .rsrc 
               173000 
               15868 
               188868 
               16be00 
               15a00 
               0 
               0 
               0 
               0 
             
             
                 
               .coign 
               189000 
               6cd0 
               18fcd0 
               181800 
               6e00 
               0 
               0 
               0 
               0 
             
          
         
         
             
          
             
               Debug Directories: 
             
          
         
         
             
             
             
             
          
             
                 
               0. 
               00000000 
               00181800 . . . 00181910 −&gt; 00188600 . . . 00188710 
             
             
                 
               1. 
               00000000 
               00181910 . . . 001819c0 −&gt; 00188710 . . . 001887c0 
             
             
                 
               2. 
               00000000 
               001819c0 . . . 001819ea −&gt; 001887c0 . . . 001887ea 
             
          
         
         
             
             
          
             
               Extra Data: 
               512 ( 181a00 - 181800) 
             
          
         
         
             
          
             
               Coign Extra Data: 
             
          
         
         
             
             
          
             
                 
               {9CEEB02F-E415-11D0-98D1-006097B010E3} : 4 bytes. 
             
             
                 
                 
             
             
                 
               @ 
             
          
         
       
     
   
   Because it occupies the first slot in the application&#39;s DLL import table, the COIGN RTE will always load and execute before the application or any of its other DLLs. It therefore has a chance to modify the application&#39;s address space before the application runs. The COIGN RTE takes advantage of this opportunity to insert binary instrumentation into the image of system libraries in the application&#39;s address space. The instrumentation modifies for redirection all of the component instantiation functions in the COM library. Before returning control to the application, the COIGN RTE loads any additional COIGN components as stipulated by the configuration information stored in the application. 
   Referring to Table 2, with the COIGN runtime configured for profiling, the application is ready to be run through a set of profiling scenarios in which the source, destination, and size of all communications are measured. Because the binary has been modified transparently to the user (and to the application itself), profiling runs behave from the user&#39;s point of view as if there were no instrumentation in place. The instrumentation gathers profiling information in the background while the user controls the application. The only visible effect of profiling is a slight degradation in application performance. In a simple profiling scenario, start Picture It!® is started, a file is loaded for preview, and the application is exited. For more advanced profiling, scenarios can be driven by an automated testing tool, for example, Visual Test. 
   During profiling, the COIGN instrumentation maintains running summaries of the inter-component communication within the application. COIGN quantifies every inter-component function call through a COM interface. The instrumentation measures the number of bytes that would have to be transferred from one machine to another if the two communicating components were distributed. The number of bytes is calculated by invoking portions of the DCOM code that use IDL structural metadata for the application, including the interface proxy and stub, within the application&#39;s address space. COIGN measurement follows precisely the deep-copy semantics of DCOM. Referring to Table 2, after calculating communication costs, COIGN compresses and summarizes the data online so that the overhead to store communication information does not grow linearly with execution time. If desired, the application can be run through profiling scenarios for days or even weeks to more accurately track user usage patterns. 
   
     
       
         
             
           
             
               TABLE 2 
             
             
                 
             
             
               Running the Application through a Profiling Scenario 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
          
             
               D:\apps\pictureit\bin&gt; pi.exe 
             
             
               [Coign Runtime Environment: 00000080 636f6900 00000000] 
             
             
               [Coign EP3C Classifier/9999] 
             
             
               [Coign NDR Interface Informer] 
             
             
               [Coign Profiling Logger (16 cycles)] 
             
             
               [CoignRTE: DLL — PROCESS — ATTACH] 
             
             
               [CoignRTE: DLL — THREAD — ATTACH] 
             
             
               [CoignRTE: DLL — THREAD — ATTACH] 
             
             
               [CoignRTE: DLL — THREAD — ATTACH] 
             
             
               [CoignRTE: DLL — THREAD — ATTACH] 
             
             
               [CreateFileMoniker ( D:\apps\pictureit\docs\MSR.mix )] 
             
             
               (StgOpenStorage ( D:\apps\pictureit\docs\MSR.mix )] 
             
             
               [CoignRTE: DLL — THREAD — DETACH] 
             
             
               [CoignRTE: DLL — THREAD — DETACH] 
             
             
               [Elapsed time: 26400 ms] 
             
             
               [CoignRTE: DLL — PROCESS — DETACH] 
             
          
         
         
             
             
          
             
               [Inter-component communication: 
               ] 
             
          
         
         
             
             
             
             
             
             
             
             
             
             
          
             
               [ 
               Messages  : 
                    16 
                    64 
                   256 
                   1024 
                   4096 
                   16384 
                   Totals 
               ] 
             
             
               [ 
               In Counts      : 
               105240 
               1629 
               473 
               1599 
               66 
               45 
               109052 
               ] 
             
             
               [ 
               Out Counts: 
               102980 
               4303 
               843 
               783 
               131 
               12 
               109052 
               ] 
             
             
               [ 
               In Bytes    : 
               782022 
               57912 
               49616 
               815034 
               157619 
               237963 
               2100166 
               ] 
             
             
               [ 
               Out Bytes : 
               455207 
               130140 
               95473 
               304592 
               239239 
               70019 
               1294670 
               ] 
             
             
                 
             
          
         
       
     
   
   At the end of the profiling, COIGN writes the summary log of inter-component communication to a file for later analysis. In addition to information about the number and sizes of messages and components in the application, the profile log also contains information used to classify components and to determine pair-wise component location constraints. Log files from multiple profiling executions can be combined and summarized during later analysis. Alternatively, at the end of each profiling execution, information from the log file can be inserted into the configuration record in the application executable (the pi.exe file in this example). The latter approach uses less storage because summary information in the configuration record accumulates communication from similar interface calls into a single entry. 
   Invoking “adpCOIGN” initiates post-profiling analysis, as shown in Table 3. AdpCOIGN examines the system service libraries to determine any per-component location constraints on application components. For example, for client-server distributions, adpCOIGN recognizes components that must be placed on the client in order to access the Windows GUI libraries or that must be placed on the server in order to access persistent storage directly. 
   
     
       
         
             
           
             
               TABLE 3 
             
             
                 
             
             
               Initiating Post-Profiling Analysis 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
          
             
                 
               D:\apps\pictureit\bin&gt; adpcoign pi.log 
             
             
                 
               Binaries: 
             
             
                 
                pi.exe 
             
             
                 
                mso97d.dll 
             
             
                 
                mfc42d.dll 
             
             
                 
                mfco42d.dll 
             
             
                 
                oleaut32.dll 
             
             
                 
               Dependencies: 
             
             
                 
                 01 D:\apps\pictureit\bin\pi.exe 
             
             
                 
                  D:\apps\pictureit\bin\piserv.dll 
             
             
                 
                   piperf.dll 
             
             
                 
                   oleaut32.dll 
             
             
                 
                 00 D:\apps\pictureit\bin\piserv.dll 
             
             
                 
                  D:\apps\pictureit\bin\mfco424.dll 
             
             
                 
                   mfc42d.dll 
             
             
                 
                 00 D:\apps\pictureit\bin\mfco42d.dll 
             
             
                 
                  C:\winnt\system32\ole32.dll 
             
             
                 
                 00 C:\winnt\system32\ole32.dll 
             
          
         
         
             
             
             
          
             
                 
               Objects: 
               112 
             
             
                 
               Interfaces: 
               792 
             
             
                 
               Calls: 
               38286 
             
             
                 
               Bytes: 
               743534 
             
             
                 
               Proc. Speed: 
               200MHz 
             
             
                 
                 
             
          
         
       
     
   
   Combining location constraints and information about inter-component communication, adpCOIGN creates an abstract graph model of the application. In one implementation, adpCOIGN combines the abstract graph model with data about the network configuration to create a concrete model of the cost of distribution on a real network. AdpCOIGN then uses a graph-cutting algorithm to choose a distribution with minimum communication costs. Alternatively, the construction of the concrete model and the graph-cutting algorithm are performed at application execution time, thus potentially producing a new distribution tailored to current network characteristics. 
   After analysis, the application&#39;s inter-component communication model is written into the configuration record in the application binary using the setCOIGN utility, as shown in Table 4. Any residual profiling logs are removed from the configuration record at this time. The configuration record is also modified to disable the profiling instrumentation. In its place, a lightweight version of the instrumentation is loaded to realize (enforce) the distribution chosen by the graph-cutting algorithm. 
   
     
       
         
             
           
             
               TABLE 4 
             
             
                 
             
             
               Instrumenting the Application with Distribution Instrumentation 
             
             
               Using SetCOIGN 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
          
             
               D:\apps\pictureit\bin&gt; setcoign /f:pi.set pi.exe 
             
          
         
         
             
             
          
             
               Config: 
               pi.set 
             
          
         
         
             
             
             
          
             
                 
               Informer: 
               Coign Light Interface Informer 
             
             
                 
               Classifier: 
               Coign EP3C Classifier 
             
          
         
         
             
             
             
             
          
             
                 
               Initialized Data: 
               487424 
               (00077000) 
             
             
                 
               Image size: 
               1646592 
               ( 192000) 
             
             
                 
               Section Alignment: 
               4096 
               (  1000) 
             
             
                 
               File Alignment: 
               512 
             
             
                 
               File Size: 
               1612800 
             
             
                 
               Optional header: 
               224 
             
          
         
         
             
             
             
             
             
          
             
               Directories: 
                    VAddr 
                    VSize 
                    VAEnd 
                 
             
          
         
         
             
             
             
             
             
             
          
             
                 
               Exports: 
               15ac60 
               5563 
               1601c3 
                 
             
             
                 
               Imports: 
               190f18 
               140 
               191058 
             
             
                 
               Resources: 
               173000 
               15868 
               188868 
             
             
                 
               Debug: 
               111a60 
               54 
               111ab4 
             
             
                 
               IAT: 
               110000 
               1a58 
               111a58 
             
          
         
         
             
             
             
             
             
             
             
             
             
             
          
             
               Sections: 5 
                    VAddr 
                    VSize 
                    VAEnd 
                    FAddr 
                    FSize 
               R 
               L 
               R 
               L 
             
          
         
         
             
             
             
             
             
             
             
             
             
             
             
          
             
                 
               .text 
               1000 
               10e343 
               10f343 
               400 
               10e400 
               0 
               0 
               0 
               0 
             
             
                 
               .rdata 
               110000 
               501c3 
               1601c3 
               10e800 
               50200 
               0 
               0 
               0 
               0 
             
             
                 
               .data 
               161000 
               11224 
               172224 
               15ea00 
               d400 
               0 
               0 
               0 
               0 
             
             
                 
               .rsrc 
               173000 
               15868 
               188868 
               16be00 
               15a00 
               0 
               0 
               0 
               0 
             
             
                 
               .coign 
               189000 
               83f8 
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               8400 
               0 
               0 
               0 
               0 
             
          
         
         
             
          
             
               Debug Directories: 
             
          
         
         
             
             
             
             
          
             
                 
               0. 
               00000000 
               00189a00 . . . 00189b10 −&gt; 00189c00 . . . 00189d10 
             
             
                 
               1. 
               00000000 
               00189b10 . . . 00189bc0 −&gt; 00189d10 . . . 00189dc0 
             
             
                 
               2. 
               00000000 
               00189bc0 . . . 00189bea −&gt; 00189dc0 . . . 00189dea 
             
          
         
         
             
          
             
               Coign Extra Data: 
             
          
         
         
             
             
          
             
                 
               {9CEEB022-E415-11D0-98D1-006097B010E3} : 4980 bytes. 
             
             
                 
               {9CEEB030-E415-11D0-98D1-006097B010E3} : 904 bytes. 
             
             
                 
               {9CEEB02F-E415-11D0-98D1-006097B010E3} : 4 bytes. 
             
             
                 
                 
             
          
         
       
     
   
   Aside from the inter-component communication model, perhaps the most important information written into the application configuration is data for the component classifier. The component classifier matches components created during distributed executions to components created during the profiling scenarios. The abstract model of inter-component communication contains nodes for all known components and edges representing the communication between components. To determine where a component should be located in a distributed execution, the classifier tries to match it to the most similar component in the profiling scenario. The premise of scenario-based profiling is that profiled executions closely match post-analysis executions. Therefore, if the circumstances of a component&#39;s creation are similar to those of a component in a profiling execution, then the components will most likely have similar communication patterns. Based on the chosen distribution for similar profiled components, the classifier decides where new components created during the distributed execution should be instantiated. 
     FIG. 11  shows a graphical representation  300  of the distribution chosen for a profiled scenario in which the user loads and previews an image in Picture It!® from a server. Each of the large dots  302  in  FIG. 11  represents a dynamic component in the profiled scenario. Lines  304  between the large dots  302  represent COM interfaces through which the connected components communicate. The lines  304  can be colored according to the amount of communication flowing across the interface. Heavy black lines  306  represent interfaces that are not remotable (i.e., pairs of components that must reside on the same machine). An interface can be non-remotable for any of the following reasons: the interface has no IDL or type library description; one or more of the interface parameters is opaque, such as a “void *”; the client directly accesses the component&#39;s internal data; or the component must reside on the client or the server because it directly accesses system services. The “pie” slice  308  in the top half of  FIG. 11  contains those components that should be located on the server to minimize network traffic and thus execution time. In the described example, the operating storage services, the document file component, and three “property set” components are all located on the server. Note that approximately one dozen other “property set” components (of the “PI.PropSet” class) are located on the client. In order to achieve optimal performance, a component-based ADPS is able to place components of the same class on different machines. 
   After the abstract distribution model is written into the binary, the application is prepared for distribution. When the application user instructs Picture It!® to load an image from the server, the lightweight version of the COIGN runtime will intercept the related instantiation request and relocate it to the server. The four components within the pie slice  308  in  FIG. 11  are automatically distributed to the server. COIGN distributes components to the server by starting a surrogate process on the server. The surrogate acts as a distributed extension of the application; distributed components reside in its address space. A distributed version of the COIGN runtime maintains communication links between the original application process on the client and the surrogate process on the server. 
   COIGN has automatically created a distributed version of Picture It!® without access to the application source code or the programmer&#39;s knowledge of the application. The automatic distributed application is customized for the given network to minimize communication cost and maximize application throughput. 
   In the one embodiment, COIGN is used with other profiling tools as part of the application development process. COIGN shows the developer how to distribute the application optimally and provides the developer with feedback about which interfaces are communication “hot spots.” The programmer can fine-tune the distribution by inserting custom marshaling and caching on communication-intensive interfaces. The programmer also enables or disables specific distributions by inserting or removing location constraints on specific components and interfaces. Alternatively, the programmer creates a distributed application with minimal effort simply by running the application through profiling scenarios and writing the corresponding distribution model into the application binary without modifying application sources. 
   In an alternative embodiment, COIGN is used on-site by the application user or system administrator to customize the application for a network. The user enables application profiling through a simple GUI to the setCOIGN utility. After “training” the application to the user&#39;s usage patterns—by running the application through representative scenarios—the GUI triggers post-profiling analysis and writes the distribution model into the application. In essence, the user has created a customized version of the distributed application without any knowledge of the underlying details. 
   Alternatively, COIGN can automatically decide when usage differs significantly from profiled scenarios, and silently enables profiling for a period to re-optimize the distribution. The COIGN runtime already contains sufficient infrastructure to allow “fully automatic” distribution optimization. The lightweight version of the runtime, which relocates component instantiation requests to produce the chosen distribution, can count messages between components with only slight additional overhead. Run time message counts could be compared with relative message counts from the profiling scenarios to recognize changes in application usage. 
   Architecture of the COIGN System 
   Referring to  FIGS. 12 and 13 , the COIGN runtime is composed of a small collection of replaceable COM components. The most important components are the COIGN Runtime Executive (RTE)  400 , the interface informer  410 , the information logger  420 , the component classifier  430 , and the component factory  440 . 
   In general, the RTE  400  provides low-level services to the other components in the COIGN runtime. The interface informer  410  identifies interfaces by their static type and provides support for walking the parameters of interface function calls. The information logger  420  receives detailed information about all component-related events in the application from the RTE and the other COIGN runtime components. The information logger  420  is responsible for recording relevant events for post-profiling analysis. The component classifier  430  identifies components with similar communication patterns across multiple program executions. The component factory  440  decides where component instantiation requests should be fulfilled and relocates instantiation requests as needed to produce a chosen distribution. In an alternative embodiment, the component factory  440  is implemented in a separate object from a component relocator  450 . Similarly, the functions of the other illustrated components could be divided or united in other configurations of components to perform the functions of the present invention. 
   Runtime Executive 
   The COIGN RTE  400  is the first DLL loaded into the application address space. As such, the RTE  400  runs before the application or any of its components. The RTE  400  patches the COM library and other system services to intercept component instantiation requests and re-direct them. The RTE  400  reads the configuration information written into the application binary by the setCOIGN utility. Based on information in the configuration record, the RTE loads other components of the COIGN runtime. For example, the sets of DLLs for profiling and “regular” program execution, i.e., the heavyweight and lightweight instrumentation packages, differ in the choice of components  410 ,  420 ,  430 ,  440 , and  450  to run on top of the RTE  400 . The heavyweight instrumentation package includes a different interface informer  410  and information logger  420  from the lightweight instrumentation package. The heavyweight interface informer includes more detailed structural metadata and more elaborate information logger than the lightweight interface informer. According to the model of the COIGN system, arbitrary combinations of modules, and arbitrary combinations of different versions of modules, enable tailoring of instrumentation packages for a wide range of analysis and adaptation tasks. 
   The RTE  400  provides a number of low-level services to the other components in the COIGN runtime. Services provided by the RTE  400  include interface wrapping, component identification and tagging, interception and redirection of component instantiation requests, interface wrapping, and address space and stack management. 
   As described in detail below, the RTE “wraps” all COM interfaces by replacing the component interface pointer with a pointer to a COIGN instrumentation interface. The RTE manages interface wrappers  402 . Once an interface is wrapped, the COIGN runtime can intercept all function calls between components that cross the interface. An interface is wrapped using information from the interface informer  410 . The RTE also invokes the interface informer  410  to process the parameters to interface function calls in profiling. The results of the processing can be stored in the information logger  420 . 
   As described in detail below, to identify components communicating within an application, the RTE frames components  404  in conjunction with the interface wrappers  402 . In this way, components can be dynamically identified by the component classifier  430  and information about components, rather than just interfaces, can be stored in the information logger  420 . 
   The RTE  400  provides a set of functions to access information in the configuration record created by setCOIGN. The RTE  400 , in cooperation with the information logger  420 , provides other components with persistent storage through the configuration record. 
   As described in detail below, the RTE redirects all component instantiation requests made by the application through the function of the COM runtime  406 . It invokes the component classifier  430  to identify the about-to-be-instantiated component. The RTE  400  then invokes the component factory  440 , which fulfills the instantiation request at the appropriate location based on its component classification. 
   The RTE tracks all binaries (.DLL and .EXE files) loaded in the application&#39;s address space. The RTE also provides distributed, thread-local stack used by the other components to store cross-call context information. 
   Interface Informer 
   The interface informer  410  locates and manages interface metadata. With assistance from the interface informer  410 , other components of the COIGN system can determine the static type of a COM interface, and walk both the input and output parameters of an interface function call. COIGN includes multiple versions of interface informers. 
   A first version of interface informer is included in the heavyweight instrumentation package and operates during scenario-based profiling. This “profiling” interface informer uses format strings generated by the MIDL compiler and interface marshaling code to analyze all function call parameters and precisely measure inter-component communication. The profiling interface informer adds a significant amount of overhead to execution run-time. 
   A second version of interface informer is included in the lightweight instrumentation package, and is used after profiling to produce the distributed application. This “distributed” informer examines function call parameters only enough to locate interface pointers. Before the execution of the distributed application, the interface metadata of the heavyweight, profiling interface informer is aggressively edited to remove metadata unnecessary for the identification of interface pointers. As a result of aggressive pre-execution optimization of interface metadata, the distributed informer imposes minimal execution overhead on most applications. 
   In an alternative embodiment, a third version of interface informer includes less interface metadata than the profiling interface informer, but more interface metadata than the distributed interface informer. This “intermediate” interface informer can be used for lightweight profiling of an application during distributed execution, for example, to determine if an application execution conforms to expected use parameters set forth after scenario-based profiling. 
   While described in the context of the COIGN system, the processing of interface metadata to yield a lightweight instrumentation package from a heavyweight instrumentation package has more general applicability to the field of instrumentation. 
   Information Logger 
   The information logger  420  summarizes and records data for automatic distributed partitioning analysis. Under direction of the RTE  400 , COIGN runtime components pass information about a number of events to the information logger  420 . The logger  420  is free to process the events as it wishes. Depending on the logger&#39;s version, it might ignore the event, write the event to a log file on disk, or accumulate information about the event into in-memory data structures. COIGN includes multiple versions of information loggers. 
   The profiling logger, included in the heavyweight instrumentation package, summarizes data describing inter-component communication into in-memory data structures. At the end of execution, these data structures are written to disk for post-profiling analysis. 
   The event logger, which can be included in the lightweight instrumentation package, creates detailed traces of all component-related events during application execution. Traces generated by the event logger can drive detailed simulations of the execution of component-based applications. 
   The null logger, which alternatively can be included in the lightweight instrumentation package, ignores all events. Use of the null logger insures that no extra files are generated during execution of the automatically distributed application. 
   Alternatively, an information logger  420  can process information in some arbitrary way tailored for a specific instrumentation package. 
   Component Classifier 
   The component classifier  430  identifies components with similar patterns across multiple executions of an application. COIGN includes eight component classifiers that were created for evaluation purposes, including classifiers that use static classification methods and classifiers that use PCCs and various types of CCCs. Alternatively, other component classifiers can identify similar components using different classification methods. 
   Information used to generate COIGN&#39;s dynamic classifiers is gathered during scenario-based profiling by the component classifier  430 . COIGN&#39;s scenario-based approach to automatic distribution depends on the premise that the communication behavior of a component during a distributed application can be predicted based on the component&#39;s similarity to another component in a profiling scenario. Because in the general case it is impossible to determine a priori the communication behavior of a component, the component classifier  430  groups components with similar instantiation histories. The classifier  430  operates on the theory that two components created under similar circumstances will display similar behavior. The output of the post-profiling graph-cutting algorithm is a mapping of component classifications to computers in the network. 
   During distributed execution, the component classifier  430  matches a component created during distributed executions to the most similar component listed in the distribution scheme. When dynamic classification is used, the component classifier  430  in effect matches a component created during distributed execution to the most similar component created during the profiling scenarios. Based on the chosen distribution for similar profiled components, the classifier decides where new components created during the distributed execution should be instantiated. 
   Component Factory 
   The component factory  440  produces the distributed application. Using output from the component classifier  430  and the graph-cutting algorithm, the component factory  440  moves each component instantiation request to the appropriate computer within the network. During distributed execution, a copy of the component factory  440  is replicated onto each machine. The component factories act as peers. Each redirects component instantiation requests on its own machine, forwards them to another machine as appropriate, and fulfills instantiation requests destined for its machine by invoking COM to create the new component instances. The job of the component factory is straightforward since most of the difficult problems in creating a distributed application are handled either by the underlying DCOM system or by the component classifier  430 . 
   COIGN can contain a symbiotic pair of component factories. Used simultaneously, the first factory handles communication with peer factories on remote machines while the second factory interacts with the component classifier and the interface informer. 
   Implementation of the COIGN Automatic Distributed Partitioning System 
   The COIGN system includes numerous features specific to an ADPS for applications built from COM components. These features are described in detail below for a version of the COIGN system on the Microsoft Windows NT platform. 
   COIGN is an ADPS for component-based applications. It instruments, measures, partitions, and distributes applications at the level of binary-standard COM components. While the instrumentation aspects of COIGN are described below in the context of automatic distributed partitioning, a number of the aspects, including interface wrapping, static re-linking, and handling undocumented interfaces, are applicable to any instrumentation system for COM components. 
   To understand component behavior, COIGN gathers intimate knowledge of how an application and its components interact with the COM run-time services. COIGN is a binary-level system. The COIGN runtime penetrates the boundary between the application and the COM runtime transparently to the application. COIGN inserts itself between the application and the COM runtime services. 
   COM components are dynamic objects. Instantiated during an application&#39;s execution, components communicate with the application and each other through dynamically bound interfaces. A component frees itself from memory after all references to it have been released by the application and other components. COIGN is particularly aware of component instantiations. Applications instantiate COM components by calling API functions exported from a user-mode COM DLL. Applications bind to the COM DLL either statically or dynamically. 
   Static binding to a DLL is very similar to the use of shared libraries in most UNIX systems. Static binding is performed in two stages. At link time, the linker embeds in the application binary the name of the DLL, a list of all imported functions, and an indirect jump table with one entry per imported function. At load time, the loader maps all imported DLLs into the application&#39;s address space and patches the indirect jump table entries to point to the correct entry points in the DLL image. 
   Dynamic binding occurs entirely at run time. A DLL is loaded into the application&#39;s address space by calling the LoadLibrary Win32 function. After loading, the application looks for procedures within the DLL using the GetProcAddress function. In contrast to static binding, in which all calls use an indirect jump table, GetProcAddress returns a direct pointer to the entry point of the named function. 
   The COM DLL exports approximately 50 functions capable of instantiating new components. With few exceptions, applications instantiate components exclusively through the CoCreateInstance function or its successor, CoCreateInstanceEx. From the instrumentation perspective there is little difference among the COM API functions. For brevity, CoCreateInstance is a placeholder for any function that instantiates new COM components. 
   Intercepting Component Instantiation Requests and In-line Redirection 
   To correctly intercept and label all component instantiations, the COIGN instrumentation is called at the entry and exit of each of the component instantiation functions. 
   Referring to  FIG. 14 , at load time, the first few instructions  502  of the target function  500  are replaced with a jump instruction  504  to the instrumentation detour function  506  in the instrumentation. The first few instructions  502  are normally part of the function prolog generated by a compiler and not the targets of any branches. The replaced instructions  502  are used to create a trampoline function  508 . When the modified target function  501  is invoked, the jump instruction  504  transfers execution to the detour function  506  in the instrumentation. The detour function  506  passes control to the remainder of the target function by invoking the trampoline function  508 . After the moved instructions  502  are executed in the trampoline  508 , a jump instruction  510  transfers execution back to a spot in the target function  501 . The trampoline function  508  allows the detour function  506  to invoke the target function without interception. 
   Although inline indirection is complicated somewhat by the variable-length instruction set of certain processors upon which the COIGN system runs, for example, the Intel x86 architecture, its low run-time cost and versatility more than offset the development penalty. Inline redirection of the CoCreateInstance function, for example, creates overhead that is more than an order of magnitude smaller than the penalty for breakpoint trapping. Moreover, unlike DLL redirection, inline redirection correctly intercepts both statically and dynamically bound invocations. Finally, inline redirection is much more flexible than DLL redirection or application code modification. Inline redirection of any API function can be selectively enabled for each process individually at load time based on the needs of the instrumentation. 
   To apply inline redirection, the COIGN runtime, a collection of DLLs, is loaded into the application&#39;s address space before the application executes. One of these DLLs, the COIGN run-time executive (RTE), inserts the inline redirection code. 
   In addition to exporting function entry points to applications, DLLs in Windows NT also export a special entry point to the operating system, the DllMain function. The DllMain function is invoked by the operating system on initialization or termination of an application or any of its threads. DllMain gives the DLL first-chance execution on program initialization and last-chance execution on termination. One use for DllMain is to invoke static C++ constructors and destructors. When loaded into an application&#39;s address space, the DllMain function of the COIGN RTE DLL applies inline redirection to the COM API functions. 
   Linking the COIGN Runtime to the Application 
   Using one of several mechanisms, the COIGN runtime is loaded into the application&#39;s address space before the application executes. The COIGN runtime is packaged as a collection of dynamic link libraries. The COIGN run-time executive (RTE) is the most important DLL; it loads all other COIGN DLLs, so is loaded first into the application&#39;s address space. The COIGN RTE can be loaded by static or dynamic binding with the application. 
   According to one method of static binding of the COIGN RTE into an application, the application binary is modified to add the RTE DLL to the list of imported DLLs. Static binding insures that the RTE executes with the application. Referring to  FIG. 15 , an application binary  600  in a common object file format (“COFF”) includes a header section  610 , a text section  616 , a data section  620 , a list of imports  630 , and a list of exports  640 . The header section  610  includes pointers  611 – 614  to other sections of the application binary  600 . The text section  616  describes the application. The data section  620  includes binary data for the application. Within the binary data, function calls to functions provided by other DLLs are represented as address offsets from the pointer  613  in the COFF header  610  to the imports section  630 . The list of imports includes two parallel tables. The first table, the master table  632 , contains string descriptions of other libraries and functions that must be loaded for the application to work, for example, necessary DLLs. The second table, the bound table  634 , is identical to the master table before binding. After binding, the bound table contains corresponding addresses for bound functions in the application image in address space. Function calls in the data section  620  are directly represented as offsets in the bound table. For this reason, the ordering of the bound table should not be changed during linking. The exports list  640  includes functions that the application binary  600  exports for use by other programs. 
   To statically bind the COIGN RTE into an application, COIGN uses binary rewriting to include the COIGN RTE in the list of imports  630 . To load the rest of the COIGN runtime DLLs before any of the other DLLs are loaded, and to modify COM instantiation APIs at the beginning of application execution, the COIGN RTE DLL is inserted at the beginning of the master table  632  in the list of imports  630 . Because the application is in binary form, merely inserting the COM RTE DLL into the master table of the list of imports is not possible without replacing the first entry on the master table  632  (assuming the first entry reference had the same length), or corrupting the binary file. For this reason, a new imports section  650  is created. Into the master table  654  of the new imports section  650 , the binary rewriter inserts an entry to load the COIGN RTE DLL, and appends the old master table  632 . A dummy entry for the COIGN RTE DLL is added to the bound table  652  of the new imports section  650  to make it the same size as the master table, but the dummy entry is never called. The bound table is otherwise not modified, so the references within the COFF binary data to spots within the bound table are not corrupted. The header section  610  of the application points  618  to the new imports section  650  instead of the old imports section  630 . At load time, the libraries listed in the new master table  650  are loaded. Addresses are loaded into the new bound table  654 . Function calls from the data  620  of the COFF continue to point successfully to offsets in a bound table. In this way, the COIGN RTE DLL is flexibly included in the list of imports without corrupting the application binary. The application is thereby instrumented with COIGN RTE, and the package of other COIGN modules loaded by the COIGN RTE according to its configuration record. 
   To dynamically bind the COIGN RTE DLL into an application without modifying the application binary, a technique known as DLL injection can be used. Using an application loader, the RTE DLL is forcefully injected into the application&#39;s address space. Inserting a code fragment into an application&#39;s address space is relatively easy. With sufficient operating-system permissions, the Windows NT virtual memory system supports calls to allocate and modify memory in another process. After the application loader inserts a code fragment into the application&#39;s address space, it causes the application to execute the fragment using one of several methods. The code fragment uses the LoadLibrary function to dynamically load the RTE DLL. 
   One method of invoking an external code fragment in an application is through the Windows NT debugging API. To execute the injected code fragment, the application loader suspends the application, changes the program counter on the application&#39;s startup thread to point to the injected code fragment, and resumes execution of the thread. After loading the COIGN RTE DLL, the injected code fragment triggers a debugging breakpoint. The application loader then restores the original program counter and resumes application execution. The primary disadvantage of invoking a code fragment through the debugging API is its penalty on application execution. Once a loader has attached to an application using the debugging API, it cannot detach itself from the application. As long as it is attached, the loader will be invoked synchronously for all debugging related events. Debugging related events include process creation and termination, thread creation and termination, virtual memory exceptions, and application exceptions. Each of these events necessitates two full context switches: one to the loader and one back to the application. A secondary disadvantage to invoking a code fragment through the debugging API is that only one program can attach to an application through the debugging API at a time. The application cannot be debugged if the COIGN application loader uses the debugging API. 
   An alternative method of invoking a code fragment within the application is to inject a new thread of execution into the application. The Win32 API supported by Windows NT includes a function called CreateRemoteThread. CreateRemoteThread starts a new thread within another operating-system process at an address specified by the caller. Using this method, COIGN loads the application in a suspended state using a special flag to the CreateProcess call. COIGN injects the RTE-loading code fragment into the application and starts a new thread to invoke the RTE-loading code. After the code fragment executes, it terminates its thread. COIGN then resumes application execution. Invoking a code fragment with CreateRemoteThread has little side effect on application execution. After the remote thread has executed, the application loader can terminate, leaving the instrumentation runtime firmly embedded in the application&#39;s address space. 
   Using the debugging API to invoke dynamically injected code is prohibitively expensive. Injecting the COIGN RTE DLL using the CreateRemoteThread call is only marginally more expensive than including the DLL through static binding, but is much more complex to implement. The primary advantage of static binding is simplicity. The statically bound application is invoked without a special loader or special command line parameters. 
   Static Re-Linking of Libraries to an Application 
   In  FIG. 15 , COIGN uses binary rewriting to insert the instruction to load the COIGN RTE in a new import section  650 . The header section  610  of the application binary  600  is modified to point to the new import section  650 . In the COIGN system, the linking of a library to an application is made reversible, and static re-linking to the same application binary to a second library is flexibly enabled. Although static re-linking is described in the context of the COIGN system, it is applicable to linking of applications in general. 
   As shown in  FIG. 16 , an application binary  600  in common object file format (“COFF”) includes a header  610 , text  616 , data  620 , an imports list  630 , and an exports list  640 . The imports section  630  includes master  632  and bound  634  tables. To reversibly link a library to the application binary  600 , a header  660  is appended to the application binary  600 . In COIGN, the appended header  660  is called a COIGN header. The original COFF header  610  is copied to the appended header for storage. 
   A new imports section  670  is created following the appended header, and the first entry in the master table  672  of the new imports section  670  is a reference  673  to the first library to be loaded. For example, in COIGN, the first entry  673  can be for the COIGN RTE DLL. Following the first entry  673 , the original master table  632  is appended. 
   The binary rewriter can also append arbitrary data  680  to the extended COFF file. For example, a COIGN configuration record can be appended to the end of the application. Alternatively, other types of data can be appended. For example, each unit of data in the COIGN system can include a GUID describing the type of data, an offset to the next unit of data, as well as the data itself. The COIGN configuration record can contain information used by the distributed runtime to produce a chosen distribution. 
   Finally, the original COFF header  610  is modified by the binary rewriter to point  619  to the new imports section  670 . 
   At load time, the libraries listed in the master table  672  of the new import section  670  are loaded, and addresses are loaded into the bound table  674 . During execution, an application instrumented according to the added library  673  in the imports section can access and store data  680  appended to the extended COFF file. For example, in COIGN, the COIGN instrumentation can access and store data in the COIGN configuration record. 
   To re-link the application binary, the original COFF header  610  is restored from the appended header  660 . The appended header  660 , new imports section  670 , and any appended data  680  are discarded. Because the original COFF header  610  contained a pointer  613  to the original imports section  630 , the application binary  600  is restored. At this point, the process can be repeated using the original application binary, or using a second library instead of the first library. Alternatively, the first entry  673  in the master table  672  of the new imports section  670  can be overwritten with a binary rewriter to include the second library instead of the first, and the application re-binded. 
   In this way, multiple instrumentation packages can be added to an application binary  600  without recompiling the application binary. Moreover, because a new imports section  670  is used, changes to the imports section  670  can be of arbitrary length and still not corrupt the application binary  600 . 
   Instrumenting Interfaces of COM Components to Measure Communication, Assist Distribution, and Identify Components by Interface 
   All first-class communication between COM components takes place through interfaces. In many respects, the COIGN runtime is an interface instrumentation system. Much of its functionality is dedicated to identifying interfaces, understanding their relationships to each other, and quantifying the communication through them. 
   To measure communication between components, the COIGN runtime intercepts all inter-component communication through interfaces. By standard, an interface is a pointer to a virtual function table (VTBL, pronounced “V-Table”). A component client always accesses an interface through an interface pointer (a pointer to the pointer to a virtual function table). The component is responsible for allocating and releasing the memory occupied by an interface. Quite often, components place per-instance interface data immediately following the virtual-function-table pointer.  FIG. 5  shows the memory layout of a typical component. With the exception of the virtual function table and the pointer to the virtual function table, the component memory area is opaque to the client. 
   Invoking an interface member function is similar to invoking a C++ member function. Clients invoke interface member functions through the interface pointer. The first argument to any interface member function is the “this” pointer, the pointer to the interface. For example, typical syntax to invoke an interface member function is: 
   
     
       
         
             
             
             
           
             
                 
                 
             
           
          
             
                 
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               pIStream−&gt;Seek(nPos); 
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               pIStream−&gt;pVtbl−&gt;pfSeek(pIStream, nPos) 
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   The initial interface pointer to a component is returned by the instantiating API function. By intercepting all component instantiation requests, COIGN has an opportunity to instrument the interface before returning the interface pointer to the client. 
   Rather than return the component&#39;s interface pointer, the interception system returns a pointer to an interface of its own making, a specialized universal delegator called an interface wrapper. The process of creating the wrapper and replacing the interface pointer with a pointer to an interface wrapper is referred to as wrapping the interface. Interfaces are referred to as being wrapped or unwrapped. A wrapped interface is one to which clients receive a pointer to the interface wrapper. An unwrapped interface is one either without a wrapper or with the interface wrapper removed to yield the component interface. 
     FIG. 17  shows an interface wrapper  700  used in the COIGN system. The client  100  holds a pointer  702  to the interface wrapper  700 . The interface wrapper  700  holds a pointer  704  to a virtual table  710  for the COIGN instrumentation system and an interface type description  706  for the wrapped interface. The interface type description  706  includes information that can be used to access the component interface through the instance data structure  62  and pointer  70  to the virtual table for the interface, as described above with reference to  FIGS. 3 and 5 . The interface type description  706  includes description of the parameters of the wrapped interface, and can include a GUID. Further, the interface wrapper can hold arbitrary data  708  associated with the wrapped interface. The virtual table  710  for the COIGN instrumentation system includes pointers  711 – 713  to the IUnknown functions  722 – 726 , and a pointer  714  to an instrumentation function  728 . When the client  100  attempts to invoke an interface member function, the pointer  702  to the interface wrapper  700  is followed and COIGN has intercepted the interface member-function invocation. An instrumentation function  728  is invoked that processes member-function parameters and then calls the component interface of the member function, using the information supplied in the interface type description  706 . Upon return from the member-function call, the instrumentation function  728  processes the outgoing parameters, and returns execution to the client  100 . Any information useful to the COIGN instrumentation system can be recorded in the data section  708  of the interface wrapper  700 . In this way, access to information about the interface wrapper  700  is easily organized and accessible. Even for components that reuse the same implementation of “QueryInterface( )”, “AddRef( )”, and “Release( )” in multiple interfaces of dissimilar types, interface-specific information  708  is organized and accessible. 
   In one embodiment of COIGN, each interface has a corresponding interface wrapper. In an alternative embodiment, an interface wrapper is provided for each extended class of interface, with the interface type description used to differentiate function calls for the various interfaces within a class. 
   In addition to providing a mechanism for COIGN to intercept member function calls and measure the parameters, interface wrappers can be used by COIGN to identify communications as coming from or directed to a particular component. COM does not provide components with strongly-typed identities. Instead, COM components are loosely-coupled collections of interfaces. Despite this lack of a COM-supplied component identity, the interfaces of a component can be identified as common to the component using interface wrappers. In an interface wrapper, the identity of the owner of the interface can be stored. 
     FIG. 18  shows data structures used to track interface wrappers for all of the interfaces of components in an application. A number of clients  100  holds pointers  702  to interface wrappers  700 . A table  800  of interface wrappers  700  includes an interface wrapper  700  for each interface created. Each of these interface wrappers  700  includes the same pointer  704  to the same instrumentation function table  710 . Each interface wrapper also includes an interface type description  706  and can include other data  708  associated with the interface. The interface type description  706  and associated interface data  708  can be different for each of the interfaces. 
   A client can receive an interface pointer in one of four ways: from one of the COM component instantiation functions; by calling “QueryInterface( )” on an interface to which it already holds a pointer; as an output parameter from one of the member functions of an interface to which it already holds a pointer; or as an input parameter on one of its own member functions. For each new interface created by an instantiation function such as “CoCreateInstance( ),” the interface is wrapped with an interface wrapper  700  identifying the created component. Whenever an unwrapped interface is returned to a client as a parameter, it is wrapped with an interface wrapper  700  identifying the originating component. Each new interface returned by a “QueryInterface( )” call is wrapped with an interface wrapper identifying the called component. By induction, if an interface is not wrapped, it belongs to the current component. 
   COIGN uses a hash table that maps interfaces to interface wrappers to help manage interface wrappers. When COIGN detects an interface pointer to be returned to a client, it consults the hash table. If the interface is wrapped, a pointer  702  to the interface wrapper for the interface is returned to a client. If the interface is not wrapped, an interface wrapper is added to the table  800  and a pointer  702  to the added interface wrapper is returned to the client. Because an interface wrapper points to the instrumentation virtual table  710 , interface wrappers can be distinguished from normal interfaces, and multiple wrappings prevented. 
   At any time the COIGN runtime knows exactly which component is executing. The identity of the current component is noted as a thread-local variable and used to identify interfaces. For example, when a member-function of a component interface is called through an interface wrapper, the called component can be identified as the current component by pushing the component identity on a local stack. When the component is done executing, the component identity is then popped from the local stack. 
   At any time, COIGN can examine the top values of the stack to determine the identity of the current component and any calling components. In this way, interface wrappers can be used to measure inter-component communication. 
   COIGN can also examine the identities of components currently pushed on the stack to determine the sequence of component calls preceding a component instantiation request. In this way, interface wrappers enable dynamic classification of components by tracing component identities on the local stack. 
   While clients should only have access to interfaces through interface wrappers, a component should never see an interface wrapper to one of its own interfaces because the component uses its interfaces to access instance-specific data. A component could receive an interface wrapper to one of its own interfaces if a client passes an interface pointer back to the owning component as an input parameter on another call. The solution is simply to unwrap an interface pointer parameter whenever the pointer is passed as a parameter to its owning component. 
   Structural Metadata, Static Analysis Techniques, and Pre-Processing of Metadata 
   Interface wrapping requires static metadata about interfaces. In addition to needing the information for the interface type description, an interface wrapper uses static metadata in the lightweight instrumentation package to identify all interface pointers passed as parameters to an interface member function. 
   There are a number of sources for COIGN to acquire static interface metadata. Possible sources include the IDL description of an interface, COM type libraries, and interface proxies and stubs. 
   Static interface metadata is used to generate interface proxies and stubs. The Microsoft IDL (MIDL) compiler generates proxies and stubs from IDL source code. COIGN can acquire marshaling byte code directly from interface proxies and stubs. The MIDL compiler supports a number of optimization levels to reduce the size of interface proxies and stubs. One of the optimization levels uses a byte-code interpreter to marshal interface parameters. Static interface metadata can be acquired easily by interpreting the marshaling byte codes. Although the marshaling byte codes are not publicly documented, the meanings of all byte codes emitted by the MIDL compiler can be determined by experimentation. Using MIDL generated byte-codes means that COIGN must be updated with each new release of the MIDL runtime. This is not a serious problem because changes in the MIDL byte-codes are always backward compatible and new versions of the runtime are generally released only with major operating-system upgrades. 
   Acquiring static interface metadata from the IDL description of an interface is another entirely acceptable method. It does however require static analysis tools to parse and extract the appropriate metadata from the IDL source code. In essence, it needs an IDL compiler. When components are distributed with IDL source code, but without interface proxies and stubs, a programmer can easily create interface proxies and stubs from the IDL sources with the MIDL compiler. 
   Another alternative is to acquire static interface metadata from the COM type libraries COM type libraries allow access to COM components from interpreters for scripting languages, such JavaScript or Visual Basic. While compact and readily accessible, type libraries are incomplete. The metadata in type libraries does not identify whether function parameters are input or output parameters. In addition, the metadata in type libraries does not contain sufficient information to determine the size of dynamic array parameters. 
   The COIGN toolkit contains an interpreter and a precompiler to process the marshaling byte codes. The interpreter is used during application profiling. The interpreter parses interface parameters and provides the COIGN runtime with complete information about all interface pointers passed as parameters. More importantly, the profiling interpreter calculates the size of all parameters. This size information is used to accurately predict inter-component communication costs. 
   The byte-code precompiler uses dead-code elimination and constant folding to produce an optimized metadata representation. The simplified metadata representation is used by the lightweight instrumentation package of the COIGN runtime during distributed executions of the application. The simplified metadata describes all interface pointers passed as interface parameters, but does not contain information to calculate parameter sizes. Processed by a secondary interpreter, the simplified metadata allows the non-profiling runtime instrumentation package to wrap interfaces in a fraction of the time required when using the COM marshaling byte codes. 
   Automatic Detection of Pair-Wise Component Location Constraints and Handling Undocumented Interfaces 
   A final difficulty in interface wrapping is coping with undocumented interfaces, those without static metadata. While all component interfaces should have static metadata, occasionally components from the same vendor will use an undocumented interface to communicate with each other. Function calls on an undocumented interface are not marshallable, so two components communicating through an undocumented interface cannot be separated during distribution. The profiling instrumentation package runtime records this fact for use during distributed partitioning analysis. 
   Of immediate importance to the COIGN runtime, however, is the impossibility of determining a priori the number of parameters passed in a call to an undocumented interface. 
   When a function call on a documented interface is intercepted, the incoming function parameters are processed, a new stack frame is created, and the component interface is called. Upon return from the component&#39;s interface, the outgoing function parameters are processed, and execution is returned to the client. Information about the number of parameters passed to the member function is used to create the new stack frame for calling the component interface. For documented interfaces, the size of the new stack frame can easily be determined from the marshaling byte codes. 
   When intercepting an undocumented interface, the interface wrapper has no static information describing the size of stack frame used to call the member function. A stack frame cannot be created to call the component, so the existing stack frame is reused. In addition, the execution return from the component is intercepted in order to preserve the interface wrapping invariants used to identify components and to determine interface ownership. 
   For function calls on undocumented interfaces, the interface wrapper replaces the return address in the stack frame with the address of a trampoline function. The original return address and a copy of the stack pointer are stored in thread-local temporary variables. The interface wrapper transfers execution to the component directly using a jump rather than a call instruction. 
   When the component finishes execution, it issues a return instruction. Rather than return control to the caller—as would have happened if the interface wrapper had not replaced the return address in the stack frame—execution passes directly to the trampoline function. As a fortuitous benefit of COM&#39;s callee-popped calling convention, the trampoline can calculate the function&#39;s stack frame size by comparing the current stack pointer with the copy stored before invoking the component code. The trampoline saves the frame size for future calls, then returns control to the client directly through a jump instruction to the temporarily stored return address. By using the return trampoline, the COIGN runtime continues to function correctly even when confronted with undocumented interfaces. 
   The return trampoline is used only for the first invocation of a specific member function. Subsequent calls to the same interface member function are forwarded directly through the interface wrapper. 
   Interface metadata is crucial to the COIGN system. During partitioning, the interception system measures the DCOM message size for every interface invocation. COIGN&#39;s marshaling-byte-code interpreter follows the exact same control logic as the COM marshaling interpreter to measure the size of DCOM message packets. The COIGN runtime summarizes the DCOM message size data. At the end of execution, communication summarization information is written to a profiling file for later analysis. 
   With accurate interception and access to information from the interface proxies and stubs, communication measurement is a straightforward process. The COIGN runtime measures the numbers, sizes, and endpoints of all inter-component messages. The COIGN analysis tools combine physical network measurements with logical data from the COIGN runtime to determine the exact communication costs for a given network. 
   Automatic Detection of Per-Component Location Constraints 
   COIGN uses location-constraint analysis to determine which component instances should be constrained to a particular host regardless of communication cost. COIGN&#39;s algorithm for discovering per-component location constraints is based on the following hypothesis: if a component accesses a location dependent resource, that access will occur through system API functions listed in the component&#39;s binary as links to system libraries. 
   On platforms with shared or dynamically linked libraries, applications usually access system resources through system API functions. On Windows NT, system API functions are exported from system DLLs. By simple analysis of binaries, it is determined which system DLLs an application or a component use. It is also determined which functions are used from each system DLL. 
   During scenario-based profiling, the COIGN runtime creates a mapping of components to binary files. Whenever a component is instantiated, the COIGN runtime traces entries in the component&#39;s interface VTBL back to their original binary file. COIGN records the binary file of each component. 
   During a post-profiling analysis phase, COIGN examines the binary files for each component to determine which system DLLs and system API functions are accessed by the component. A list of location-specific system API functions which “constrain” a component&#39;s distribution is created by the programmer or included with COIGN. For client-server applications, constraining functions are divided into those that should be executed on the client and those that should be executed on the server. Client constraining functions include those that access the video system, such as CreateWindow, and those that access the multimedia system, such as PlaySound. Server constraining functions are restricted mostly to file access functions such as CreateFile. A component is constrained to execute on either the client or the server if it uses any of the client or server constraining functions. 
   Determining application constraints based on the usage of system API functions is not infallible. Occasionally, a component is flagged as being constrained to both the client and the server because it uses functions assigned to both. For these cases, the application programmer manually assigns the component to a machine. 
   In a more frequently occurring case, COIGN decides that a component should be located on a particular machine when, in fact, the constraint is not needed. This overly conservative constraint occurs when constraining API functions execute only once, such as during installation. When a COM component is first installed on a computer, it registers itself with the system registry. The code used to register the component during installation resides in the component binary although it is never executed after installation. COIGN&#39;s constraint detection system has no way to know that a constraining function used during installation is not used during application execution. Therefore, installation code is desirably isolated from application-execution code. 
   From the models of application communication, network behavior, and location constraints, COIGN uses an optimization algorithm to select an optimal distribution scheme of the application components. To effect a desired distribution, COIGN intercepts component instantiation requests to the appropriate machine. COIGN intercepts all COM component instantiation requests and invokes the appropriate static or dynamic component classification system to determine which component is about to be instantiated. COIGN then determines the appropriate host for the component instantiation using the component placement map created during post-profiling analysis. A remote instantiation request is forwarded to the appropriate host for execution. After the remote instantiation requests completes, the interface pointer to the newly instantiated component is marshaled, and returned to the calling machine. Each interface pointer is wrapped before being returned to the application. 
   Remote instantiation requests execute in a surrogate process on the remote machine. Surrogate processes are created by the COIGN runtime on each machine used by the application. Surrogate processes communicate with each other and with the application through a redirection interface. The redirection interface provides remote access to all of the COM instantiation functions. In addition to the COM instantiation functions, the redirection interface also provides access to COIGN-specific utility functions. For example, one of these functions retrieves a remote stack walk for component classification across multiple machines. 
   Having described and illustrated the principles of our invention with reference to an illustrated embodiment, it will be recognized that the illustrated embodiment can be modified in arrangement and detail without departing from such principles. Moreover, it will be recognized that the COIGN system is one possible refinement of the illustrated embodiment. It should be understood that the programs, processes, or methods described herein are not related or limited to any particular type of computer apparatus, unless indicated otherwise. Various types of general purpose or specialized computer apparatus may be used with or perform operations in accordance with the teachings described herein. Elements of the illustrated embodiment shown in software may be implemented in hardware and vice versa. 
   In view of the many possible embodiments to which the principles of our invention may be applied, it should be recognized that the detailed embodiments are illustrative only and should not be taken as limiting the scope of our invention. Rather, I claim as my invention all such embodiments as may come within the scope and spirit of the following claims and equivalents thereto. 
   Appendix A includes “COIGN.h,” a source code compendium of system accessible COIGN definitions. 
   Appendix B includes “COIGN.idl,” an interface description language file for the COIGN system.