Mechanism for dispatching requests in a distributed object system

Data structures, methods and devices for reducing computing overhead associated with dispatching a distributed object invocation and improving the flexibility of the dispatch framework in a distributed client/server based computing system are disclosed. In one aspect of the invention, a request received on an end point in a transport layer is dispatched from the transport layer to a subcontract in a subcontract layer where the request is partially unmarshaled and dispatched from the subcontract to a skeleton function in a skeleton layer where a servant is invoked. In another aspect of the invention a method for initializing a subcontract which includes a skeleton dispatch function arranged to dispatch a received request to an appropriate skeleton after the subcontract has unmarshaled at least a portion of the received request involves registering the subcontract with at least one cluster end point registry, creating a subcontract meta object associated with the subcontract, and storing the subcontract meta object in a subcontract registry.

BACKGROUND OF THE INVENTION
 1. Field of Invention
 The present invention relates to the fields of distributed computing
 systems, client-server computing, and object-oriented programming. More
 particularly, the invention relates to methods and devices for dispatching
 a distributed object invocation.
 2. Description of Prior Art
 A computing environment in which objects located on different computers are
 linked by a network is typically referred to as a client-server computing
 environment. Some of the computers act as providers of services or
 functionality to other computers. Others of the computers act as consumers
 of services or functionalities. The providers of service or functionality
 are known as"servers", and the consumers of the service or functionality
 are called"clients". The client-server model may also be generalized to
 the case where distinct programs running on the same computer are
 communicating with one another through some protected mechanism and are
 acting as providers and consumers of service or functionality.
 Attempts to provide such a distributed system have been made using
 object-oriented methodologies that are based upon a client-server model in
 which server objects provide interfaces to client objects that make
 requests of the server objects. Typically, in such a distributed system,
 the servers are objects consisting of data and associated methods. The
 client objects obtain access to the functionalities of the server objects
 by executing calls on them, which calls are mediated by the distributed
 system. When the server object receives a call, it executes the
 appropriate method and transmits the result back to the client object. The
 client object and server object communicate through an Object Request
 Broker (ORB) which is used to locate the various distributed objects and
 to establish communications between objects. Distributed objects may exist
 anywhere in a network, as for example in the address space of the client,
 in multiple address spaces on the client machine, and in multiple machines
 across the network.
 The software industry has responded to the need for a distributed object
 technology by forming the Object Management Group (OMG). The goal of the
 OMG is to define the Object Management Architecture (OMA), which has four
 major components: the Object Request Broker (ORB), Object Services, Common
 Facilities, and Application Objects. The Object Request Broker provides
 basic object communications and management services, thereby forming the
 basis of a distributed object system. A standard for an Object Request
 Broker is contained in the Common Object Request Broker Architecture
 (CORBA) specification.
 In typical client-server systems, performance overhead can be costly. That
 is, the speed and quality of a process within the system may be
 compromised by inefficient uses of application code and methods associated
 with gathering information from the process and with routing information
 within a process. By way of example, the performance overhead associated
 with dispatching requests on the server side of a system is often
 relatively high. Further, the internal dispatch framework within a
 client-server system is typically such that any change to the framework
 requires substantial changes to be made in application code. That is, by
 way of example, if it is desired for the relationship between a given
 transport and a skeleton to be altered, a significant amount of the
 internal dispatch framework, and, hence, the application code must be
 changed in most instances. Further, client-server systems typically do not
 efficiently include and execute application code provided by a developer.
 In other words, customizing an internal dispatch framework is often
 inefficient. As a result, there is a limited amount of flexibility in a
 typical internal dispatch framework. Consequently, the provision of
 methods and devices which would reduce the performance overhead associated
 with dispatching requests on the server side of a system and improve the
 flexibility of the internal dispatch framework is desirable.
 SUMMARY OF THE INVENTION
 To achieve the foregoing and in accordance with the purpose of the present
 invention, data structures, methods and devices for reducing computing
 overhead associated with dispatching a distributed object invocation and
 improving the flexibility of the dispatch framework in a distributed
 client/server based object oriented operating system are disclosed. In one
 aspect of the invention, a request received on an end point in a transport
 layer is dispatched from the transport layer to a subcontract in a
 subcontract layer where the request is partially unmarshaled. The
 partially unmarshaled request is then dispatched from the subcontract to a
 skeleton function in a skeleton layer where a servant is invoked. In some
 embodiments, the transport layer does not perform any unmarshaling
 functions and the dispatch of the request from the transport layer to the
 subcontract layer is accomplished by invoking a closure which identifies a
 marshal buffer which holds the request.
 In other embodiments, the skeleton function in the skeleton layer is
 arranged to unmarshal at least a part of the partially unmarshaled
 request. In alternative embodiments, additional layers may also be
 provided in order to further increase the system's modularity. By way of
 example, the subcontract layer ma y be subdivided into a two or more
 sublayers.
 In another aspect of the invention, a method for initializing a subcontract
 which includes a skeleton dispatch function arranged to dispatch a
 received request to an appropriate skeleton after the subcontract has
 unmarshaled at least a portion of the received request is disclosed. The
 subcontract initialization includes the steps of creating a subcontract
 meta object associated with the subcontract and storing the subcontract
 meta object in a subcontract registry. In some embodiments, a dedicated
 end point associated with the subcontract is initialized. In others, the
 subcontract is registered with at least one cluster end point registry and
 a unique end point with which the cluster end point registry is
 associated.
 In still another aspect of the invention, an end point dispatch registry
 data structure includes a subcontract identifier arranged to identify an
 associated subcontract and a closure arranged to identify a dispatch
 function associated with the subcontract. In some embodiments, the closure
 includes a pointer to the dispatch function and a pointer to a data
 element which contains information used by the dispatch function.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention is directed toward distributed object systems and
 will be described with reference to several preferred embodiments as
 illustrated in the accompanying drawings. The invention may be practiced
 within the context of any suitable distributed object system, including
 those defined under CORBA or any other suitable specification. However,
 for purposes of illustration, the present invention will be described
 primarily within the context of an Object Request Broker (ORB) implemented
 under the CORBA specification from the OMG, Revision 2.0, dated July 1995,
 which is incorporated herein by reference. FIG. 1a diagrammatically
 illustrates the overall architecture of a representative distributed
 object system suitable for implementing the present invention. FIG. 1b
 diagrammatically illustrates some possible flow paths that a request from
 a client to a servant object may follow within such an architecture that
 includes a three level dispatch mechanism. FIG. 1c shows one object
 reference data structure that may be used by a client to refer to a
 servant object.
 A distributed object system 10 typically includes an Object Request Broker
 (ORB) 11 as is symbolically illustrated in FIG. 1a. ORB 12 provides all of
 the location and transport mechanisms and facilities necessary to deliver
 a call from a client to a servant (target object) and to return a response
 to the client, as will be discussed below with reference to FIG. 1b. The
 client and servant may be located in the same process, in different
 processes on the same machine, or on completely different machines. For
 the purposes of this discussion, client 20 may be any code that invokes an
 operation on a distributed object and thus may or may not take the form of
 distributed object or a process. Normal object implementation 14 is a
 representation of an object type defined in a traditional object
 programming language, such as C++. A wide variety of representations are
 possible. By way of example, an object implementation 14 may be a simple
 C++ object type that has been provided by an application developer.
 Alternatively, an implementation for an object type may be developed
 within a visual application builder 15. This visual application builder
 allows a developer to visually select existing object types from a catalog
 and graphically connect the services provided by one object to the
 services needed by another (attributes, arguments, etc.) in order to
 create a new implementation for an object type.
 An object development facility 16 may be used to simplify the creation and
 the installation of distributed objects. It is used to"wrap" or
 encapsulate developer objects in distributed object code. As such, object
 development facility 16 may be used to transform a developer object into
 an ORB object implementation 14. In this example, ORB object
 implementation 14 is presented as a server as shown by its location in the
 diagram. A developer uses an interface definition language to define an
 interface for an ORB object, provides a developer object implementation
 that implements that object's behavior, and then uses the object
 development facility in order to produce an ORB object implementation 14.
 At run time, an instance of this ORB object (a servant object) is created
 that will utilize this ORB object implementation 14. It should be
 appreciated that the object development facility may also be used to
 create objects that take the role of clients at some point.
 Client 20 communicates with a servant by way of a stub 21, a method table
 dispatch 24, a subcontract layer 36, possibly a filter 40, and a transport
 layer 38. Stub 21 includes a surrogate 22, a method table 24, and a stub
 function 25. Client 20 communicates initially with surrogate 22 which
 appears to the client as the server object. Alternatively, client 20 may
 communicate directly with the server object through a dynamic invocation
 interface (DII) 26 instead of through surrogate 22, method table 24, and
 stub function 25. Dynamic invocation interface 26 is used to enable
 clients, as for example client 20, to construct dynamic requests. One
 procedure by which a client makes a call to a servant utilizing the above
 layers is described in more detail below with reference to FIG. 1b.
 Subcontract layer 36 provides the functionality required by an object in
 order to utilize subcontracts to implement various services (or features
 or object mechanisms) named by a particular subcontract, as described in
 greater detail in above-referenced U.S. patent application Ser. No.
 08/554,794, filed Nov. 7, 1995. A subcontract identifies a quality of
 service provided by the distributed object system that may be utilized by
 an individual object. For example, a subcontract may identify that the
 feature of security is to be used for a particular object. A technique by
 which a particular subcontract may be associated dynamically at run time
 with a servant object is described in above-referenced U.S. patent
 application Ser. No. 08/670,682, filed Jun. 26, 1996, now U.S. Pat. No.
 6,044,224. Filter 40, if being used, may perform a variety of tasks, such
 as compression, encryption, tracing, or debugging, which are to be applied
 to communications to and from an object.
 Transport layer 38 operates to marshal, unmarshal and physically transport
 information to and from a servant that typically does not share the same
 process as a client. A technique for marshaling/unmarshaling an object
 reference is described in above-referenced U.S. patent application Ser.
 No. 08/670,681, filed Jun. 26, 1996, now U.S. Pat. No. 6,044,409.
 Mechanisms for marshaling and unmarshaling are described in
 above-referenced U.S. patent application Ser. No. 08/673,181, filed Jun.
 26, 1996, now U.S. Pat. No. 6,032,199.
 A standard implementation suite 28 (or object adapter) represents a set of
 subcontracts that interact with ORB objects 14 in identical ways, as for
 example object key management. One such implementation suite is described
 in above-referenced U.S. patent application Ser. No. 08/669,782, filed
 Jun. 26, 1996, now U.S. Pat. No. 5,991,823. It should be duly noted that a
 subcontract may belong to multiple implementation suites. Hence, other
 implementation suites that utilize different subcontracts are possible. A
 skeleton, which may take the form of either static skeleton 32 or dynamic
 skeleton 30 is used to transform requests into a format required by a
 servant object 14. Thus, skeletons 32, 30 call an appropriate servant
 object 14. Static skeleton 32 is used to call interface-specific object
 implementations 14, while dynamic skeleton 30 is used generically when
 interface-specific objects are not available. An ORB interface 34 is the
 interface that goes directly to the ORB that is the same for all ORBs and
 does not depend upon an object's interface or object adapter. An ORB
 Daemon 46 is responsible for ensuring that object servers are active when
 invoked by clients. A technique for starting object servers is disclosed
 in U.S. patent application Ser. No. 08/408,645 which is hereby
 incorporated by reference.
 Secure Protocol 42 is a secure interoperability protocol that secures the
 internet inter-ORB protocol and helps to transmit information through
 transport layer 38 in a secure fashion. This may mean integrity
 protection, confidentiality, etc. The internet inter-ORB protocol is a
 protocol that typically communicates between processes on different
 machines. However, in some cases, the internet inter-ORB protocol may
 communicate between process on the same machine. The security server 54 is
 a security administration server that secures the services that are used
 between processes on different computers.
 Typecode/Any module 44 implements typecode and "Any" objects. Typecode
 describes an Interface Definition Language (IDL) data type, allowing type
 descriptions to be transmitted between clients and servers. An instance of
 an IDL data type may be encapsulated by an "Any" object. An Any object
 refers to typecode of the encapsulated data, and a generic encoding of the
 data.
 An implementation repository 50 is used to store information relating to
 object servers. Specifically, implementation repository 50 stores the
 information needed to start a server process. For example, implementation
 repository 50 stores information such as the location of the server
 program, any arguments to the program, and any environment variables to
 pass to the program, etc.
 Simple persistence 56 uses an Interface Definition Language (IDL)-defined
 type and the output from running that IDL type through the IDL compiler,
 together with a portion of additional code so that an IDL-defined type can
 be read from, and written to, disk. A name server 52 is used to name ORB
 objects. A client, as for example client 20, may use name server 52 to
 find a desired object by name. Name server 52 returns an object reference,
 which in turn may be used to send requests to that object. An Interface
 Repository 48 (IFR) knows about all interfaces for all objects within the
 distributed object system.
 A request made by a client using a method table ("m-table") dispatch will
 pass through a variety of the aforementioned layers of the architecture on
 its way to the servant as diagrammatically illustrated in FIG. 1b. The
 request is initiated by a client and may take any suitable form. The form
 of the request will depend to a large extent upon the nature of the
 programming language used to create the client. By way of if the client is
 written in the C++ language, the request may take the form of a C++ method
 call 62. The call is made to a designated object reference taking the form
 of a surrogate. The surrogate includes methods that comply with the
 object's interface. As will be appreciated by those skilled in the art,
 the object reference used at different locations within a distributed
 object system may vary significantly in appearance. In the embodiment
 described, the client side object reference is a dual pointer (referred to
 herein as a "fat pointer"). A fat pointer contains two distinct pointers,
 or location indicators. A first pointer points to a client representation
 ("client rep") associated with the referenced object. A second pointer
 points to a method table of the method table dispatch 24 that is
 associated with the referenced object. It should be appreciated that as
 used herein, the term "pointer" is used to identify not only locations in
 computer or network memory, but also to refer to a location indicator in
 general. A client representation is an object that has methods which
 support invocation as well as CORBA defined "pseudo" object reference
 operations. These operations include, but are not limited to, a duplicate
 method, a release method, a narrow method, a hash method, and an
 is_equivalent method.
 After the client has initiated a call, the call is processed using a method
 table dispatch mechanism 24. Such a technique is disclosed in U.S. patent
 application Ser. No. 08/307,929 and is hereby incorporated by reference.
 The method table dispatch mechanism uses a method table that contains a
 list of pointers to stub functions 25, one of which is associated with the
 method to be invoked. Stub functions 25 receive a function or procedure
 call in the "native" language of the client process, then use either a
 subcontract layer 36 or a native call to eventually call the corresponding
 servant object. The native language may be any suitable language, as for
 example a language such as C++.
 Method table dispatch 24 determines the appropriate stub function 25 to
 process the method call, and then pairs the method call with the
 appropriate stub function 25. In the event that the client making the
 method call is in the same process as the servant object, a local stub
 function is called. The local stub function sends the method call directly
 to servant object 78. A technique for routing calls within a local process
 is described in above-referenced U.S. patent application Ser. No.
 08/670,684, filed Jun. 26, 1996. Alternatively, if the servant object is
 in a different process, i.e. a remote process, a remote stub function is
 called. The remote stub function invokes the client representation, which
 delivers the invocation to servant object 78.
 Subcontracts implemented by subcontract layer 36 are logic modules which
 provide control of the basic mechanisms of object invocation and argument
 passing that are important in distributed object systems. A subcontract
 implemented by subcontract layer 36 determines a specific quality of
 service for use by an object. A subcontract is uniquely identified by a
 subcontract identifier, which is typically embedded in an object
 reference. A quality of service is a set of service properties. Among
 possible service properties which are selectable are qualities relating to
 server activation, security, transactions, filterability, and clean
 shut-down. Subcontracts are configured such that certain qualities of
 service are available. With predetermined qualities of service, the
 overhead associated with processing individual service properties is
 reduced. Realistically, only "reasonable" or commonly used combinations of
 service properties are supported with subcontracts. However, subcontracts
 may be created to meet the specific requirements of a given distributed
 object system.
 The identification of an appropriate subcontract in subcontract layer 36
 may be thought of as the identification of a desired function that is
 unique to that subcontract. For example, a marshal function or an
 unmarshal function is defined for each subcontract. A subcontract marshal
 function is used by a stub to marshal an object reference so that it may
 be transmitted to another address space, or domain. The object reference
 is typically processed by a transport mechanism in transport layer 38.
 A transport mechanism such as T1, T2, etc., which is a part of the
 transport layer 38, is used to marshal and physically transport
 information to and from servant objects. Information. i.e. the object
 reference or the request, is converted into protocols appropriate to a
 given domain. By way of example, protocols may include, but are not
 limited to, Ethernet protocols and internet interoperable protocols
 (IIOPs). In some uncommon cases, protocols may even entail the use of
 electronic mail to transmit instructions to be implemented on a server.
 After information is marshaled, the transport mechanism then transports
 information through any combination of an operating system, a device
 driver, or a network, that are all a part of hardware 70 used by the
 client side of a distributed object system. While transport mechanisms
 require a conversion of information into a protocol appropriate to a given
 domain, some transport mechanisms to do not require the encoding of
 information for different domains. One transport mechanism which does not
 require a conversion of information into a protocol appropriate to a
 domain other than the one on which information originates is termed a
 "door". Doors are essentially gateways between two different processes on
 the same host. The use of doors eliminates the need for a conversion of
 information into a canonical implementation in transport layer 38, as
 there is no need to encode information into a protocol which may be used
 by a different machine by virtue of the fact that information is remaining
 on the same host and therefore does not require a change of domain. Hence,
 information may simply be "flattened out," or marshaled into a stream
 which is not encoded for use by a different machine, and passed between
 the two processes on the host.
 Once information is transported through hardware 70 used by the client
 side, the information is then transported to hardware 70 on the server
 side of a distributed object system. Once information is routed through
 hardware 70, the server side of a distributed object system invokes a
 transport mechanism such as T1, T2 etc. to receive information on an end
 point which is a part of transport layer 38. In the event that an end
 point is not created by transport layer 38, transport layer 38 provides
 the functionally needed for the end point to be created by subcontract
 layer 36. By way of example, a door end point is typically created by
 subcontract layer 36, while other end points, including network and TCP/IP
 end points, are typically created by transport layer 38. Regardless of
 whether end points are created by subcontract layer 36 or transport layer
 38, end points "live in," i.e. are a part of, transport layer 38. End
 points are essentially ports which receive information from a different
 domain. After an end point in transport layer 38 receives information
 transported from a different domain, the end point then dispatches the
 information from transport layer 38 to subcontract layer 36. Subcontract
 layer 36, or more specifically the subcontract in subcontract layer 36
 which receives the information, then dispatches the information to the
 skeleton and the servant.
 Subcontract layer 36 provides the functionality to unmarshal at least some
 of the information it has received. That is, subcontract layer 36
 unmarshals at least part of the request. Then, the request is dispatched
 to a skeleton 31 which transforms the request into an implementation
 specific format required by servant object 78. The skeleton may either be
 a static skeleton or a dynamic skeleton as described above.
 In general, a remote request must be routed through the client side and the
 server side as described above. The method call 62 is received, method
 table dispatch layer 24 is used to identify an appropriate subcontract
 prior to the selection of a transport mechanism in transport layer 38
 which marshals the request and prepares it for transport to another
 domain. Through hardware 70, the marshaled request is transported to the
 server side where it is received on an end point which is a part of
 transport layer 38. An appropriate end point receives information
 transported across a wire, and information is dispatched from transport
 layer 38 to subcontract layer 36, which provides the functionality to at
 least partially unmarshal the information it has received. The subcontract
 then dispatches the request to skeleton 31 which transforms the request
 into a specific format required by servant object 78. This path is shown
 by arrow 77, and is the path which may be taken by both remote and local
 requests.
 However, if a client and a server are in a local process, i.e. both the
 client and the server are in the same process, the use of the path shown
 by arrow 77 as described above is unnecessarily complex. If it is known
 that the client and the server are in the same process, it is possible to
 shorten the invocation, or flow, path of a request for service. If a local
 process may be identified when an object reference is created, shortened
 flow paths, i.e. the paths represented by arrows 75 and 76, may be taken
 to send a request from what is a client to a server which are on the same
 host. The path represented by arrow 76 is more likely to be taken, as it
 uses subcontract layer 36 to identify an appropriate subcontract. However,
 in situations in which an appropriate subcontract does not need to be
 explicitly identified, the path represented by arrow 75 may be taken.
 FIG. 1c will now be used to describe an embodiment of an object reference.
 As will be familiar to those skilled in the art, object references may
 take a variety of forms depending upon the location within the process
 that they are being held at any given time. However, by way of background,
 a representative object reference for use in a system which utilizes a low
 overhead object adapter is illustrated in FIG. 1c. In the implementation
 shown therein, object reference 150 includes a host identifier 152, a port
 designation 154, and an object key 156. Object key 156 includes a
 subcontract identifier 158, a server identifier 160, an implementation
 identifier 162, and a user key 164. Host identifier 152 denotes a
 particular computer in a network, while port designation 154 identifies
 the port of the selected computer which is to be used for communication.
 Object key 156 provides further identifying information used in order to
 locate a desired servant object on its host machine.
 Server identifier 160 names a particular process or program in which the
 servant object resides, while user key 164 is a unique number or string
 used to locate the servant within the process named by server identifier
 160. Subcontract identifier 158 is used to attach the protocol of a
 particular subcontract and its associated services with a servant, and
 implementation identifier 162 names an implementation of an interface that
 is to be used with that servant object.
 Internal dispatch mechanism frameworks generally require a significant
 amount of performance overhead when they are utilized, as internal
 searches must typically be made within a server to determine the
 appropriate dispatch mechanism to use to route a particular request. The
 dispatch mechanism used to route a particular request is typically
 dependent upon a variety of factors including: 1) where the request
 originates; 2) the end point which receives the request when on the server
 side; and 3) the skeleton that is used to invoke the requested servant
 method. Also, any change which is made to any portion of the internal
 dispatch mechanism framework typically requires the alteration of a
 significant portion of code. Further, internal dispatch mechanism
 frameworks typically do not efficiently implement external application
 code provided by a developer. Therefore, an internal dispatch mechanism
 framework is not generally flexible in terms of enabling changes to the
 framework to be readily made, regardless of whether the changes to the
 framework are changes to existing code or whether the changes to the
 framework involve the addition of external code provided by an application
 developer.
 To overcome these drawbacks, the present invention utilizes a multi-layered
 dispatch mechanism framework that utilizes the concept of a subcontract
 layer which exists between the transport layer and the skeleton layer. The
 use of such a three layer dispatch mechanism, together with associated
 data structures, serves to both reduce overhead associated with
 dispatching requests and increase the flexibility of the framework. The
 dispatch layers may be thought of as levels through which a request is
 dispatched. In the described embodiment, the transport layer receives an
 incoming request and dispatches the request to an appropriate subcontract
 (out of a plurality of subcontracts in the subcontract layer) without
 unmarshaling any of the request. In alternative embodiments, the transport
 layer may do a minimal amount of unmarshaling of the request. The selected
 subcontract unmarshals at least a portion of the request, identifies the
 appropriate skeleton, and dispatches the request to an appropriate
 skeleton which in turn invokes the servant. It should be appreciated that
 this multi-layered approach provides a great deal of flexibility in
 altering the framework. Specifically, an entirely new dispatch mechanism
 can be created simply by writing and implementing a new subcontract or
 suite of subcontracts. When the new subcontract(s) is/are implemented,
 there is no need to alter either the transport layer or the skeleton
 layer. As will be appreciated by those skilled in the art, this provides a
 significant advantage when it comes to implementing new features and/or
 functionalities in the distributed object environment.
 The performance overhead associated with dispatching a request on the
 server side may be further reduced by implementing data structures. By way
 of example, an end point dispatch registry may be provided which
 identifies each (and only) dispatch mechanisms associated with a given end
 point. With this arrangement, a search for an appropriate dispatch
 mechanism to use for an end point may be narrowed to a search of the data
 structure which contains the dispatch mechanisms associated with the end
 point. Hence, the performance overhead associated with searching for a
 dispatch mechanism may be reduced. One configuration of an end point
 dispatch registry data structure will be described below with reference to
 FIG. 6.
 Other data structures which are used in the described embodiment to reduce
 the performance overhead associated with dispatching a request on the
 server side of a client-server system include a subcontract registry data
 structure and an implementation registry data structure. The subcontract
 registry data structure and the implementation registry data structure
 list subcontracts and implementation definitions, respectively, thereby
 enabling searches for subcontracts and implementation definitions to be
 concentrated on the respective registry data structures. Hence, the number
 of resources used to search for subcontracts and implementation
 definitions may be reduced, thereby improving system efficiency. Suitable
 subcontract and implementation registry data structures are illustrated in
 FIGS. 2 and 3, respectively.
 Referring next to FIG. 2 a subcontract registry data structure 200 in
 accordance with one embodiment of the present invention will be described.
 Subcontract registry data structure 200, or, simply, subcontract registry
 200, stores information that associates a particular quality of service
 with a unique subcontract identifier and with a subcontract client
 representation create function. Subcontract registry 200 registers
 information pertaining to subcontracts in a tabular form, and makes
 available subcontracts within a system available for searching. The
 tabular form is advantageous in that it allows any number of
 implementations to be associated with a particular subcontract, as is
 discussed in more detail in co-pending patent application Ser. No.
 08/670,684 filed concurrently herewith. It should be appreciated that
 although a predetermined number of permutations of features within a
 system are possible, subcontract registry 200 may identify only a subset
 of these possible subcontracts that have been implemented within the
 distributed object system. Subcontract registry 200 may be implemented as
 a hash table, linked list or any other suitable data structure.
 In the embodiment shown, subcontract registry 200 includes a subcontract
 identifier (SCID) column 202, an associated quality of service list column
 204, a subcontract client representation create function column 206, and
 pointers to other functions 208. Each row 210 of subcontract registry 200
 is termed a subcontract meta object and, by way of example, may be
 implemented as a C++ object. In the embodiment shown, a plurality of
 subcontract meta objects 212, 214 and 216 are provided in subcontract
 registry 200. The first subcontract meta object 212 has a subcontract
 identifier of "1" and will herein be identified as Subcontract 1.
 Subcontract 1 lists the following features for its quality of service:
 clean shutdown, security, persistence and server activation. The
 name-value pairs in this quality of service list indicate that a clean
 shutdown will not be implemented, an authentication protocol using NT5
 will be used for security, and persistence is turned on. A plurality of
 pointers, or location indicators, to various other functions associated
 with each subcontract meta object in other functions column 208 include
 pointers to an unmarshal function, a destringify function, and a bad
 server identifier handler.
 The second subcontract meta object 214 shown in subcontract registry 200 is
 the subcontract meta object associated with the subcontract identified as
 subcontract 2. The quality of service list for this subcontract indicates
 that server activation is present. The third subcontract meta object 216
 indicates that the subcontract identified as subcontract 3 will allow for
 transactions, clean shutdowns, and server activation.
 Subcontract registry 200 will typically have a group of associated
 functions that are used to organize and access the registry. By way of
 example, the associated functions may include an Add function, a Find
 function, a Get_First function and a Get_Next function. The Add function
 may be used to add a new quality of service to the table. In the described
 embodiment, the Add function takes as arguments a subcontract identifier
 and a subcontract meta object. A Find function takes a subcontract
 identifier as an argument and returns the subcontract meta object
 associated with that identifier. The functions Get_First and Get_Next are
 used to iterate over a subcontract registry, as for example subcontract
 registry 200, thereby searching it completely for a particular quality of
 service, and returning the appropriate meta object. When a client wishes
 to obtain an object reference, which may be a fat pointer in the C++
 language, for a particular server object, subcontract registry 200 may be
 used to look up the subcontract identifier associated with that server
 object. Once the appropriate subcontract identifier has been located, the
 appropriate subcontract client representation function is called in order
 to create a client representation suitable for the particular server
 object using the appropriate features. In the C++ embodiment, the fat
 pointer references this client representation.
 Referring next to FIG. 3, an implementation registry data structure 250 in
 accordance with one embodiment of the present invention will be described.
 Each object in a client-server system is typically associated with an
 implementation definition. Each implementation definition for an object
 includes such information as the name of the implementation, the
 subcontract to use in order to create the object, the interface identifier
 for the object, a set of call back functions associated with the object,
 and information which relates to the skeleton. The call back functions may
 be stored in an object. Implementation registry 250 includes entries which
 correspond to various implementation definitions. Through the use of
 implementation registry 250, an implementer of an object server will be
 able to provide multiple, different implementations of a single ORB object
 type in the same object server. That is, one object type may have various
 implementations that are identified by distinct implementation
 identifiers. Each implementation defines the behavior for all of the
 operations and attributes of the interface that it supports. That is, each
 interface may have many implementations. Therefore, each implementation
 identifier represents a distinct implementation for an object that uses a
 particular subcontract. In addition, each implementation may use a
 different subcontract by way of a subcontract pointer. Hence,
 implementation registry 250 aids in improving the overall efficiency of a
 client-server system in that it allows an invoking function to choose a
 particular implementation which, in turn, may use a desired subcontract of
 the subcontract registry.
 Each implementation definition represents an entry in implementation
 registry 250 that contains pointers to the stored data. Implementation
 registry 250 includes an implemenatation identifier column 252 that names
 implementations, a subcontract meta object pointer column 254, an
 interface identifier column 256, a ready flag column 257, a call back
 functions column 258, and a skeleton information column 259.
 Implementation identifiers in implementation identifier column 252 are
 names for implementations that are supplied by a developer when
 implementation definitions are created. A subcontract meta object pointer
 is a pointer from a particular implementation definition to a subcontract
 meta object contained in the subcontract registry as described above with
 respect to FIG. 2. An interface identifier is a fixed, globally unique
 name for the type of a particular interface. In some embodiments, a ready
 flag may be a boolean flag which may be set to indicate that the
 implementation associated with a particular implementation definition has
 been prepared for use as described in copending patent application Ser.
 No. 08/669,782, filed Jun. 26, 1996, now U.S. Pat. No. 5,991,823. Skeleton
 information includes functions which may be used by the skeleton
 associated with a particular implementation. Call back functions are sets
 of functions associated with each implementation. A wide variety of call
 back functions may be associated with a particular implementation. By way
 of example, the call back functions may include, but are not limited to,
 look up functions, deactivate functions, and shutdown functions.
 Implementation definition entry 262 is an example of an implementation
 definition which may be found in implementation registry 250.
 Implementation definition entry 262 is for a PRINTER implementation of the
 interface PRINTER INTERFACE and has a pointer to a subcontract in a
 subcontract registry, as for example Subcontract 1 of subcontract registry
 200 in FIG. 2. The pointer to a subcontract in a subcontract registry is a
 subcontract meta object pointer which identifies the appropriate
 subcontract for a given implementation. Implementation definition 262 also
 has unique call back functions and a unique skeleton dispatch function
 with which it is associated.
 The implementation registry 250 has a variety of associated functions
 intended to facilitate organization of and access to the registry. By way
 of example, a function Add may be used to add an implementation definition
 with a particular implementation identifier to implementation registry
 250. Similarly, a call to a function Find may take as an argument an
 implementation identifier and search through implementation registry 250
 in order to return the implementation definition which is identified by
 the implementation identifier. With implementation definitions listed in
 implementation registry 250, the performance overhead associated with
 searching for implementation definitions may be reduced. It should be
 appreciated that implementation registry 250 may be implemented as a
 hash-table, a linked list, or any other suitable data structure.
 The flexibility of an internal dispatch mechanism framework which utilizes
 dispatch layers, or levels, is improved as changes to dispatch mechanisms
 used to route a request from one level to the next may be made without
 affecting dispatch mechanisms between subsequent levels. That is,
 flexibility is enhanced because changes to dispatch method used between
 two levels of the framework may not necessarily require changes to
 dispatch methods between other subsequent levels of the framework.
 The internal dispatch mechanism for the server side of a distributed object
 system as described above with reference to FIG. 1b may be called a
 three-level dispatch mechanism. The three levels in the dispatch mechanism
 are a first level which is the transport layer, a second level which is
 the subcontract layer, and a third level which includes the skeleton layer
 and the servant object layer. The transport layer dispatches a request
 received on an end point in the transport layer to an appropriate
 subcontract in the subcontract layer which is associated with the end
 point. It should be appreciated that the association between the
 subcontract and the transport end point may be a one-to-one, a
 one-to-many, a many-to-one, or a many-to-many mapping. After the
 subcontract in the subcontract layer receives the request, the request is
 dispatched to the appropriate skeleton in the skeleton layer.
 One mechanism which may be used to dispatch a request from the transport
 layer to the subcontract layer is a closure, or more specifically, a
 subcontract dispatch closure. A closure contains a pointer to a dispatch
 function and a pointer to a data element, which is sometimes referred to
 as a "cookie." A cookie is provided by the subcontract layer when the
 closure is created. The cookie refers to data required by a specific
 subcontract to process an incoming request. Hence, the data element
 differs according to the requirements of each subcontract. In addition,
 the data element may even differ for the same subcontract if the call may
 be dispatched to different servant objects.
 Once a request is received from another domain on an end point in the
 transport layer, it is dispatched to a subcontract in the subcontract
 layer. The actual subcontract computation to be performed may be
 identified by the closure associated with the subcontract. When a
 subcontract is initialized, it is "bound to," or associated with, end
 points which may dispatch requests to the subcontract.
 Referring next to FIG. 4, a method of initializing a subcontract in
 accordance with one embodiment of the present invention will be described.
 In step 403, any "dedicated" end points that are associated with the
 subcontract are initialized. It should be appreciated that some
 subcontracts may not have any dedicated end points, while others may have
 multiple dedicated end points. A subcontract may also create or initialize
 additional dedicated end points as needed after subcontract
 initialization. As mentioned above with respect to FIG. 1b, a dedicated
 end point is an end point which is associated with only one subcontract.
 An example of a dedicated end point is a door, which is a gateway between
 processes on the same host. Doors enable information to be passed between
 two processes on a host without the having to encode the information for
 use by a different domain. The process of initializing a dedicated end
 point will be described in more detail below with reference to FIG. 5.
 After the dedicated end point is initialized, process control moves to
 step 405 where the subcontract is registered with associated "cluster" end
 points. As mentioned above with respect to FIG. 1b, a cluster end point is
 an end point which may be associated with a plurality of subcontracts.
 Registering a subcontract with associated cluster end points entails
 registering the subcontract with the end point dispatch registry
 associated with each of the cluster end points which is used by the
 subcontract. A typical end point dispatch registry will be described below
 with reference to FIG. 6. The actual registration of a subcontract may be
 accomplished by calling a register function of the cluster end point with
 the subcontract identifier and a subcontract dispatch closure as
 arguments.
 After the subcontract is registered with associated cluster end points, a
 subcontract meta object is created in step 407. In other words, step 407
 is the step of registering the subcontract with the subcontract registry
 as described above with reference to FIG. 2. A subcontract meta object may
 be thought of as a data structure within the subcontract registry which
 "connects" a particular subcontract to an associated object. In other
 words, a subcontract meta object is a collection of information which
 associates a subcontract, through the use of a subcontract identifier,
 with an object. A subcontract meta object includes, but is not limited to,
 a quality of service list, a subcontract client representation create
 function, and other functions, as for example unmarshal functions,
 associated with a given subcontract. After the subcontract meta object is
 created, process control moves to step 407 where the subcontract meta
 object is stored in the subcontract registry. Once the subcontract meta
 object is placed in the subcontract registry, the process of initializing
 a subcontract is complete.
 Referring next to FIG. 5, a method of initializing a dedicated end point in
 accordance with one embodiment of the present invention will be described.
 That is, an implementation of step 403 of FIG. 4, the step of initializing
 a dedicated end point for a subcontract, will be described. The process of
 initializing a dedicated end point begins at step 421 where a call is made
 to a "create dedicated end point" function with the subcontract dispatch
 closure as an argument. The function used to create a dedicated end point
 is dependent upon the transport with which the end point is to be
 associated. In step 423, a dedicated end point is created. After the
 dedicated end point is created, the subcontract dispatch closure is bound
 to the newly created dedicated end point in step 425. After the
 subcontract dispatch closure is bound to the created end point, the
 process of initializing a dedicated end point is complete.
 FIG. 6 is a representation of an end point registry data structure 450 in
 accordance with one embodiment of the present invention. Each end point
 which supports multiple subcontracts may have an associated end point
 registry, or end point dispatch registry. Hence, with reference to FIG. 4,
 any subcontract which is associated with a cluster end point is typically
 added to the end point registry for the cluster end point. End point
 registry 450 is comprised of a listing of keys 452 and closures 454 which
 are associated with keys 452. Key 452 is typically a subcontract
 identifier (SCID). In the described embodiment, the subcontract identifier
 is encoded as the first four bytes of the object key which is a part of an
 object reference, as described above with respect to FIG. 1c. However, in
 alternative embodiments, the form, size, location and contents of the
 subcontract identifier may be widely varied. In any case, the transport
 mechanism that implements a cluster end point that supports multiple
 subcontracts contains the appropriate logic to obtain the subcontract
 identifier of the target object being invoked. Usually, end points are
 implemented by a transport, and the transport is determined by the
 transport protocol which determines the encoding of target object
 references in the request message.
 In general, a closure is an object with a "message" on it that holds a
 state. Each closure, as for example closure 454, includes a pointer to a
 dispatch 456, or a dispatch function, and a pointer to a data element in a
 piece of memory. Specifically, the pointer to a data element is a pointer
 to opaque data which is important to the dispatch function 456 with which
 it is associated. In the described embodiment, the pointer to a data
 element is known as a cookie 458. An end point may be mapped to a specific
 subcontract with an associated closure 454, which holds pointer to a
 dispatch function 456 and cookie 458, found using end point registry 450.
 That is, given a subcontract identifier as key 452, closure 454 may be
 located. Located closure 454 may then be used to properly call the
 dispatch function 456 provided by the subcontract identified by key 452 to
 process the call. Cookie 458 points to data maintained and required by the
 subcontract to process the call, and will be passed as an argument to
 dispatch function 456.
 In the described embodiments, end point registries are only associated with
 cluster end points (i.e. those end points that may be associated with more
 than one subcontract). The use of a subcontract identifier as a key in an
 end point registry to determine an appropriate dispatch method is an
 efficient mechanism for identifying the appropriate dispatch method for a
 subcontract.
 As mentioned earlier, a three-level dispatch mechanism may be used to
 dispatch a received request on a server from the transport layer to the
 subcontract layer, then from the subcontract layer to the skeleton layer
 and the servant object. The use of subcontracts and a subcontract layer
 serves, among other purposes, the purpose of decoupling the skeleton layer
 from the transport layer. This decoupling of the skeleton layer and the
 transport layer with the subcontract layer provides for a modular
 framework which facilitates the implementation of changes into the overall
 dispatch framework. In other words, the subcontract layer may serve as an
 interface layer which enables changes to be readily made to the dispatch
 framework. As described above with reference to FIG. 6, an end point
 registry may be used to locate the proper closure, and therefore dispatch
 method, to use to dispatch a request received on an end point in the
 transport layer to the appropriate subcontract in the subcontract layer.
 The dispatch method used to dispatch a request to the appropriate
 subcontract will be referred to herein as a transport to subcontract
 dispatch method.
 The particular method used to dispatch a request from a subcontract in the
 subcontract to the skeleton layer and the servant object is dependent upon
 the implementation suite which uses the subcontract. That is, the method
 used to dispatch a request to the skeleton layer, also known as a skeleton
 dispatch method, is dependent upon both the particular client-server
 system on which the dispatch is being performed and the particular object
 being invoked. By way of example, if a persistent object is to be invoked,
 the implementation suite used may be an object adapter, as for example a
 Low-overhead Object Adapter (LOA). A subcontract to skeleton layer
 dispatch mechanism suitable for use in invoking persistent objects will be
 described below with reference to FIGS. 7a, 7b, and 7c. A more detailed
 description of the Low-overhead Object Adapter dispatch mechanism may be
 found in co-pending patent application Ser. No. 08/669,782, filed Jun. 26,
 1996, now U.S. Pat. No. 5,991,823.
 It should be appreciated that the subcontract to skeleton layer dispatch
 mechanism may be widely varied depending upon the system with which the
 dispatch mechanism is associated and the object being invoked on the
 server. By way of example, the dispatch mechanism used in the process of
 invoking a transient object may differ from the dispatch mechanism used in
 the process of invoking a persistent object. As there is usually less
 overhead associated with dispatching a transient object than with
 dispatching a persistent object, the use of separate dispatch mechanisms
 for dispatching transient objects may improve the efficiency of the
 overall process of invoking transient objects. An example of a subcontract
 to skeleton layer dispatch mechanism suitable for use in the process of
 invoking a transient object will be described below with reference to
 FIGS. 8a, 8b, and 8c.
 Referring next to FIG. 7a, a method of calling a skeleton dispatch function
 suitable for dispatching persistent objects in accordance with one
 embodiment of the present invention will be described. The process begins
 after the transport layer calls the dispatch function for a particular
 subcontract using the subcontract identifier (SCID). In some embodiments,
 end points, as for example door end points, in the transport layer may
 only have a single associated subcontract and, therefore, a single
 dispatch function. In other embodiments, as described above with respect
 to FIG. 6, end points may have end point dispatch registries which may be
 used to determine the appropriate dispatch function to use to dispatch a
 request to the appropriate subcontract identified by the subcontract
 identifier. The subcontract identifier is contained within the object
 reference as shown in FIG. 1c. Hence, the transport layer is able to
 "peek" at this subcontract identifier in order to determine which dispatch
 function should be called to access the appropriate subcontract. In some
 embodiments, a PEEK_CID method may be used to peek at the subcontract
 identifier.
 After the appropriate subcontract is accessed, the subcontract identifier
 is extracted from the object reference in the marshal buffer in step 702.
 The process of extraction means that information which is taken from the
 marshal buffer is no longer available in the marshal buffer. Next, in step
 704, this extracted subcontract identifier is compared with the current
 subcontract that is being utilized in order to verify that the appropriate
 subcontract is being used. As described above, during object development,
 an application developer associates a particular subcontract with an
 object by using a subcontract identifier.
 In step 706 the server identifier (ID) is extracted from the object
 reference in the marshal buffer. The server object that is the subject of
 the invocation is present within a particular server process on a host
 computer; thus, it is important to verify that this server identifier
 matches the identifier of the current server. In step 708 this extracted
 server identifier is compared with the identifier of the current server in
 order to determine if the object reference is referencing the appropriate
 server process. Step 710 determines if the extracted server identifier is
 valid and does indeed match the identifier of the current server. If the
 server identifiers do not match, the indication is that the server
 identifier is invalid and appropriate action should be taken. This action
 may be performed by a bad server identifier handler function. Step 712 is
 the determination of whether an appropriate bad server identifier handler
 is registered in the subcontract meta object associated with the current
 subcontract. Because the subcontract identifier has already been extracted
 from the object reference, this step may be performed by using the
 subcontract registry of FIG. 2. The subcontract identifier acts as a key
 to a particular row of the subcontract registry that allows a search of
 the "other functions" column in order to determine if the bad server
 identifier handler is present for the current subcontract. If the handler
 is not present, then in step 716, the system throws an exception relating
 to this situation, and the dispatch function ends. If, however, the
 handler is registered in the subcontract meta object, then the registered
 bad server identifier handler is called with the subcontract identifier
 and the marshal buffer as arguments in step 714. After the bad server
 handler is called, the dispatch function ends.
 Returning now to step 710 which is the determination of whether the
 extracted server identifier is valid, if it is determined that the
 extracted server identifier does match with the current server, then
 process control moves to step 718 where the implementation identifier is
 extracted from the object reference in the marshal buffer. In step 720
 this extracted implementation identifier is used as a key to the
 implementation registry of FIG. 3 in order to find the implementation
 definition which corresponds to the implementation identifier. The use of
 the implementation registry is explaned above with reference to FIG. 3.
 The step of finding the appropriate implementation definition may be
 performed conceptually by searching through the column of possible
 implementation identifiers of the implementation registry in order to
 determine if the implementation identifier extracted in step 718 is
 present in the registry.
 Referring next to FIG. 7b, the remaining steps in the skeleton dispatch
 function suitable for dispatching persistent objects, as described with
 reference to FIG. 7a, in accordance with one embodiment of the present
 invention will be described. In step 722, a determination is made
 regarding whether an appropriate implementation definition was found in
 the implementation registry. If it is determined that an appropriate
 implementation definition was not found in step 722, an exception is
 thrown in step 724 relating to this condition, and the dispatch function
 ends. If, however, an appropriate implementation definition is found, then
 the process flow may proceed with use of the definition.
 However, in some cases, even if an implementation definition is found, it
 may not necessarily be ready for use. An implementation definition is
 ready, or prepared, if the step of preparing an implementation definition
 has been executed. A method of preparing an implementation definition is
 described in co-pending patent application Ser. No. 08/669,782, filed Jun.
 26, 1996, now U.S. Pat. No. 5,991,823. Once the step of preparing an
 implementation definition has been executed, a "ready flag" corresponding
 to a particular implementation definition will be set to indicate that the
 implementation definition is ready for use. The determination of whether
 the ready flag corresponding to the implementation definition found in
 step 720 of FIG. 7a has been set is made in step 726. If it is determined
 that the ready flag has not been set, the dispatch function enters a wait
 state in which it waits for the implementation definition to become ready
 in step 727. That is, the dispatch function waits for the associated ready
 flag to be set to a state which is indicative of the readiness of an
 implementation definition for use. Once the implementation definition is
 ready, control moves from the determination of whether the appropriate
 ready flag is set in step 726 to step 728, where the lookup function is
 extracted from the implementation definition. This lookup function is one
 of the call back functions of located in the implementation registry shown
 in FIG. 3, and will be used to produce a localized pointer to the called
 servant. In step 730, the user key is extracted from the object reference
 in the marshal buffer. Next, in step 732, the lookup function is called
 with the user key as an argument. Once the lookup function is executed, it
 will return a pointer to the servant. The pointer to a servant may be
 implemented in the local language of the server. By way of example, this
 pointer may be a C++ object pointer that references the servant C++
 object.
 Once a local pointer has been obtained to point to the called servant
 object, the dispatch function is ready to execute the appropriate method
 upon the servant object that was originally requested by the client. In
 step 733, a method descriptor is extracted from the marshal buffer. A
 method descriptor is a data structure which holds the method name, as
 understood by the client, which is defined upon the servant that the
 client wishes to invoke. In step 734, the skeleton dispatch function is
 extracted from the implementation definition. This skeleton dispatch
 function may be found in the implementation registry of FIG. 3 in the
 column which holds skeleton information. The extracted skeleton dispatch
 function is called, in step 736, with arguments which include the servant
 pointer, the marshal buffer, and the method descriptor. The skeleton
 dispatch function will be described below with reference to FIG. 7c. The
 skeleton dispatch function achieves the result of executing the method
 upon the servant that the client originally requested. Once this
 operation, i.e. the execution of the requested method, has taken place,
 the invocation of the server object by the client is typically completed.
 However, in some embodiments, an additional function may be executed. The
 post invoke function is a developer defined function for each
 implementation that achieves a functionality that the developer desires.
 In step 738, the post invoke function is extracted from the implementation
 definition. The post invoke function is then called with the user key as
 an argument in step 740. The post invoke function may be used by the
 developer to perform a particular action after the invocation of an
 object.
 Referring next to FIG. 7c, a method of executing the skeleton dispatch
 function, i.e. step 736 of FIG. 7b, in accordance with one embodiment of
 the present invention will be described. The call to the skeleton dispatch
 function begins at step 742 where an unmarshaling mechanism, which
 corresponds to the method descriptor with which the call to the skeleton
 dispatch function is made, is selected. In some embodiments, the
 unmarshaling mechanism may be a sequence of code that is selected by a
 switch statement. As other information has previously been extracted from
 the marshal buffer, the only information remaining in the marshal buffer
 at this point are the arguments used by the method to be called. In step
 744, the selected unmarshaling mechanism is used to unmarshal the
 remainder of the marshal buffer into invocation arguments. After
 invocation arguments are obtained, in step 746, the method descriptor is
 used to invoke the method defined upon the servant using the invocation
 arguments. Invoking the method in the servant using invocation arguments
 has the effect of executing the method that was originally requested by
 the client. It is possible that the invoked method may return no value and
 may instead perform other functions, or it may be that the method returns
 a value to the client. After the method in the servant is invoked, a
 determination is made in step 748 regarding whether the invoked method
 produces a reply. If no reply is produced then the execution of the
 skeleton dispatch function is done. If a reply is produced, control moves
 to step 750 where an appropriate marshaling mechanism which corresponds to
 the method descriptor is selected. After the marshaling mechanism is
 selected, the selected marshaling mechanism is used to marshal the reply
 into a marshal buffer in step 752. The marshaling mechanism will typically
 marshal a reply header before marshaling reply arguments. In the event
 that the reply contains more bytes than the marshal buffer which contained
 the object reference is capable of encapsulating, a new marshal buffer may
 be created to encapsulate the reply. Otherwise, the reply is marshaled
 into the marshal buffer which contained the request. After the reply is
 marshaled into the marshal buffer, the reply is ready to be returned
 through the transport layer to the client.
 Although one dispatch mechanism has been described with reference to FIGS.
 7a, 7b, and 7c, it should be appreciated that a wide variety of other
 mechanisms may be provided in accordance with the multi-level dispatch
 mechanism of the present invention. By way of another example, a dispatch
 mechanism which is easier to customize than the dispatch mechanism
 described above may be implemented. The differences in the three-level
 dispatch mechanisms are due, for the most part, to the fact that
 implementation as described with respect to FIGS. 7a, 7b, and 7c does not
 readily support the incorporation of custom objects. By way of example, as
 described above with reference to FIGS. 7a, 7b, and 7c, the dispatch
 process generally involves the identification of an appropriate
 implementation, whereas the dispatch process used in the invocation which
 supports custom objects will not involve the identification of an
 appropriate implementation, as there is typically only one associated
 implementation, and perhaps even only one associated servant object.
 Referring next to FIG. 8a, a method of calling a skeleton dispatch function
 suitable for dispatching persistent objects in accordance with a second
 embodiment of the present invention will be described. The process begins
 after the transport layer calls the dispatch function for a particular
 subcontract using the subcontract identifier (SCID). In some embodiments,
 end points, as for example door end points, in the transport layer may
 only have a single associated subcontract and, therefore, a single
 dispatch function. In such embodiments, the end point will dispatch
 directly to the associated subcontract after a request is received. In
 other embodiments, as described above with respect to FIG. 6, end points
 may have end point dispatch registries which may be used to determine the
 appropriate dispatch function to use to dispatch a request to the
 subcontract identified by the subcontract identifier. The subcontract
 identifier is contained within the object reference as shown in FIG. 1c.
 Hence, in such embodiments, the transport layer is able to peek at this
 subcontract identifier in order to determine which dispatch function
 should be called in order to access the appropriate subcontract. In some
 embodiments, a PEEK_SCID method may be used to peek at the subcontract
 identifier.
 After the appropriate subcontract is accessed, that is, after the request
 has been dispatched to the appropriate subcontract, the subcontract
 identifier is extracted from the object reference in the marshal buffer in
 step 802. The process of extraction means that information which is taken
 from the marshal buffer is no longer present in the marshal buffer. Next,
 in step 804, this extracted subcontract identifier is compared with the
 current subcontract that is being utilized in order to verify that the
 appropriate subcontract is being used. As described above, during object
 development, an application developer associates a particular subcontract
 with an object by using a subcontract identifier.
 In step 806 the server identifier (ID) is extracted from the object
 reference in the marshal buffer. The server object that is the subject of
 the invocation is present within a particular server process on a host
 computer; thus, it is important to verify that this server identifier
 matches the identifier of the current server. In step 808 this extracted
 server identifier is compared with the identifier of the current server in
 order to determine if the object reference is referencing the appropriate
 server process. Step 810 determines if the extracted server identifier is
 valid and does indeed match the identifier of the current server. If the
 server identifiers do not match, the indication is that the server
 identifier is invalid and appropriate action should be taken. This action
 may be performed by a bad server identifier handler function. Step 812 is
 the determination of whether an appropriate bad server identifier handler
 is registered in the subcontract meta object associated with the current
 subcontract. Because the subcontract identifier has already been extracted
 from the object reference, this step may be performed by using the
 subcontract registry of FIG. 2. The subcontract identifier acts as a key
 to a particular row of the subcontract registry that allows a search of
 the "other functions" column in order to determine if the bad server
 identifier handler is present for the current subcontract. If the handler
 is not present, then in step 816, the system throws an exception relating
 to this situation, and the dispatch function ends. If, however, the
 handler is registered in the subcontract meta object, then the registered
 bad server identifier handler is called with the subcontract identifier
 and the marshal buffer as arguments in step 814. After the bad server
 handler is called, the dispatch function ends.
 Returning now to step 810 which is the determination of whether the
 extracted server identifier is valid, if it is determined that the
 extracted server identifier does match with the current server, then
 process control moves to step 830 of FIG. 8b, where a user key is
 extracted from the object reference encapsulated in the marshal buffer.
 Referring next to FIG. 8b, the remaining steps in the method of calling a
 skeleton dispatch function suitable for dispatching persistent objects, as
 described with reference to FIG. 8a, in accordance with a second
 embodiment of the present invention will be described. In step 830, the
 user key is extracted from the object reference in the marshal buffer.
 Next, in step 833, a method descriptor is extracted from the marshal
 buffer. A method descriptor, as mentioned above, is a data structure which
 holds the method name, as understood by the client, which is defined upon
 the servant that the client wishes to invoke. In step 834, the skeleton
 dispatch function is located in the marshal buffer using the user key
 extracted in step 830. The extracted skeleton dispatch function is called,
 in step 836, with arguments which include the user key, the marshal
 buffer, and the method descriptor. The skeleton dispatch function will be
 described below with reference to FIG. 8c. The skeleton dispatch function
 achieves the result of executing the method upon the servant that the
 client originally requested. Once this operation, i.e. the execution of
 the requested method, has taken place, the invocation of the server object
 by the client is typically completed.
 Referring next to FIG. 8c, a method of executing the skeleton dispatch
 function, i.e. step 836 of FIG. 8b, in accordance with a second embodiment
 of the present invention will be described. It should be appreciated that
 the method of executing the skeleton dispatch function described with
 reference to FIG. 8c differs from the method of executing the skeleton
 dispatch function described with reference to FIG. 7c in that the method
 described with reference to FIG. 8c enables externally provided modules of
 code to be executed.
 The call to the skeleton dispatch function begins at step 840 where custom
 objects are constructed. In other words, any custom objects, or modules of
 application code provided by a developer, are constructed. It should be
 appreciated that if custom objects are not present, then no custom objects
 are constructed. In step 841, a call is made to a custom pre-dispatch
 (CUSTOM.PRE_DISPATCH) method. The custom pre-dispatch method is a method
 provided by an application developer, and typically uses the user key,
 which is passed as an argument in a call to the skeleton dispatch
 function, to determine the appropriate servant to use to invoke the method
 identified in the method descriptor. The provision of allowing for the
 inclusion of a custom pre-dispatch method is possible through the use of a
 "custom hook." A custom hook enables external code to be inserted into and
 executed as part of the call to the skeleton invoke method. If the custom
 hook is not used, i.e. if there is no external code, then process flow
 does not execute a custom pre-dispatch method. If the custom hook is used,
 then the custom pre-dispatch method provided by an application developer
 is executed. It should be appreciated that this custom hook, like others,
 is provided to enable external code to be efficiently executed as a part
 of the call to a skeleton dispatch method.
 After the call to the custom pre-dispatch method, process flow moves to
 step 842 where an unmarshaling mechanism, which corresponds to the method
 descriptor with which the call to the skeleton dispatch function is made,
 is selected. In some embodiments, the unmarshaling mechanism may be a
 sequence of code that is selected by a switch statement. As other
 information has previously been extracted from the marshal buffer, the
 only information remaining in the marshal buffer at this point are the
 arguments used by the method to be called.
 In step 844, the selected unmarshaling mechanism is used to unmarshal the
 remainder of the marshal buffer into invocation arguments. After
 invocation arguments are obtained, in step 845, a call is made to a custom
 pre-operation (CUSTOM.PRE_&lt;&lt;OPERATION&gt;&gt;) method. The name of the
 "operation" is dependent upon the name of the IDL method called. In some
 embodiments, the operation may be the acquisition of either a read lock or
 a write lock. A read lock may be used to prevent write operations from
 occurring on a global variable at any given time, whereas a write lock may
 be used to prevent any read operations and more than one write operation
 from occurring on a global variable at any given time. The call to the
 custom pre-operation is associated with a custom hook. That is, if
 external custom pre-operation code is provided by a user, the custom hook
 is considered to be active, and the external custom pre-operation code is
 executed. If no external code is provided, the presence of the inactive
 custom hook does not in any way compromise the efficiency of the call to
 the skeleton dispatch method.
 After the call to the custom pre-operation method, in step 846, the method
 descriptor is used to invoke the method defined upon the servant using the
 invocation arguments. Invoking the method in the servant using invocation
 arguments has the effect of executing the method that was originally
 requested by the client. It is possible that the invoked method may return
 no value and may instead perform other functions, or it may be that the
 method returns a value to the client. In some embodiments when a costume
 pre-operation method is used, the invoked method may not perform any
 functions, as the invocation of the method may be a part of the custom
 pre-operation method. After the method in the servant is invoked, a call
 is made in step 847 to a custom post-operation (CUSTOM.POST_&lt;&lt;OPERATION&gt;&gt;)
 method. The name of the "operation" is dependent upon the name of the IDL
 method called. The custom post-operation method is paired with the custom
 pre-operation method. In other words, if a custom pre-operation method is
 in existence, the custom post-operation method typically must also be in
 existence. In embodiments where the custom pre-operation method is the
 acquisition of either a read lock or a write lock, the corresponding
 custom post-operation method is typically the release of either the read
 lock or the write lock, respectively. As was the case for the custom
 pre-operation method, the inclusion of a custom post-operation method is
 also made possible by the use of a custom hook. After the custom
 post-operation method is called, a determination is made in step 848
 regarding whether the invoked method produces a reply. If no reply is
 produced then process control proceeds to step 853 where a call is made to
 a custom post-dispatch (CUSTOM.POST_DISPATCH) method. If a reply is
 produced, control moves to step 850 where an appropriate marshaling
 mechanism which corresponds to the method descriptor is selected.
 Once the marshaling mechanism is selected, the selected marshaling
 mechanism is used to marshal the reply into a marshal buffer in step 852.
 In the event that the reply contains more bytes than the marshal buffer
 which contained the object reference is capable of encapsulating, a new
 marshal buffer may be created to encapsulate the reply. Otherwise, the
 reply is marshaled into the marshal buffer which contained the request.
 After the reply is marshaled into the marshal buffer, a call is made to a
 custom post-dispatch (CUSTOM.POST_DISPATCH) method in step 853. The custom
 post-dispatch method, which is paired with a custom pre-dispatch method,
 is typically used for reference counting. That is, the custom
 post-dispatch method is typically used to keep track of threads, which are
 used to execute methods, in the sense of counting the number of threads
 which are executing methods at the time the custom post-dispatch method is
 called. Again, a custom hook is used to make it possible to include
 application code pertaining to the custom post-dispatch method. Once the
 call is made to the custom post-dispatch method, the reply is ready to be
 returned through the transport layer to the client.
 It should be appreciated that the subcontract dispatch method may be
 further divided into a plurality of sub-layers to improve modularity. By
 way of example, two sub-layers, a lower sub-layer and a higher sub-layer
 may be used. This division of the subcontract dispatch method usually
 occurs when a subcontract supports multiple transports and, hence, is
 registered with different types of end points provided by the many
 transports utilized by the subcontract. This subcontract may register a
 different dispatch closure with each end point, due to the fact that the
 method used to unmarshal the request header which includes the target
 object reference, i.e. the method identifier, differs depending on the
 transport and, hence, the end point, used to deliver the request message.
 In other words, the method in the lower sub-layer used to unmarshal the
 request provides the "logic glue" that is used to decode the transport
 specific request header information required by the subcontract. The
 dispatch function referenced by a transport-to-subcontract dispatch
 closure may implement this logic glue. This logic glue eventually calls
 the function in the upper sub-layer that implements transport-independent,
 subcontract-dependent processing. This processing may include a lookup of
 servant object implementation definitions, transaction processing,
 security checks, calling skeleton dispatch functions, etc.
 The subdivision of the subcontract dispatch method into sub-layers,
 providing transport-dependent and transport-independent processing,
 provides additional flexibility in evolving the subcontract layer. By way
 of example, one subcontract which supports a transport may be incorporated
 into a more powerful subcontract in order to support another transport,
 with minimal changes to the overall system.
 Referring next to FIG. 9, a subcontract dispatch mechanism which is divided
 into sub-layers in accordance with a third embodiment of the present
 invention will be described. FIG. 9 is a diagrammatic representation of
 transport layer 38, subcontract layer 36, and skeleton layer 31 on the
 server side of a distributed object systems. Transport layer 38 includes
 three transport mechanisms, namely "transport 1" 902, "transport 2" 904
 and "transport 3" 906. "Transport 1" 902 has an end point, "end point 1"
 908, and a closure, "closure 1" 910, associated therewith. As shown, "end
 point 1" 908 is a dedicated end point. "Closure 1" 910 may be used to
 dispatch information from transport layer 38 to subcontract layer 36.
 "Transport 2" 904 has an associated end point dispatch registry 912 and
 associated closures, as for example "closure 2A" 914 and "closure 2B" 916.
 End point dispatch registry 912 is used as end points associated with
 "transport 2" 904 are cluster end points. "Transport 3" 906 has an
 associated dedicated end point, "end point 3" 918, which is associated
 with "closure 3" 920.
 As mentioned earlier, the subcontract dispatch method may be divided into a
 plurality of sub-layers to improve modularity. "Subcontract 1" 922 and
 "subcontract 3" 924 are each divided into a lower sub-layer, 922a and
 924a, respectively, and an upper sub-layer, 922b and 924b, respectively.
 The division of the subcontract dispatch method usually occurs when a
 subcontract, as for example "subcontract 1" 922, supports multiple
 transports. The methods in lower sub-layer 922a, for example, are a
 transport specific, i.e. transport-dependent, processing method 926 which
 is associated with "transport 1" 902, and a transport specific processing
 method 928 which is associated with "transport 2" 904. Methods 926,928 are
 used to decode the transport specific request header information required
 by "subcontract 1" 922, and are eventually used to call the
 transport-independent subcontract processing function 930 in upper
 sub-layer 922b that implements transport-independent,
 subcontract-dependent processing. This processing may include a lookup of
 servant object implementation definitions in implementation repository 932
 which may be used to access a skeleton dispatch function 934 in skeleton
 layer 31.
 The subdivision of the subcontract dispatch method into sub-layers further
 provides additional flexibility in evolving subcontract layer 36. As
 mentioned earlier, one subcontract which supports a transport may be
 incorporated into a more powerful subcontract in order to support another
 transport, with minimal changes to the overall system. "Subcontract 1" 922
 is incorporated as a part of "subcontract 3" 924, which is more "powerful"
 than "subcontract 1" 922 in that "subcontract 3" 924 supports "transport
 3" 906. In other words, "subcontract 1" 922 is incorporated as a part of
 "subcontract 3" 924, thereby enabling "subcontract 3" 924 to support
 "transport 1" 902, "transport 2" 904, and "transport 3" 906. This
 incorporation involves a transport specific processing method 936 in lower
 sub-layer 924a of "subcontract 3" 924 which utilizes transport-independent
 subcontract processing method 930 associated with "subcontract 1" 922. The
 use of sub-layers in the subcontract dispatch method, and, hence, the use
 of the same transport-independent subcontract processing method for more
 than one subcontract increases the flexibility of subcontract layer 36, as
 subcontracts may be incorporated as components of other subcontracts, with
 minimal modifications being made to subcontract layer 36.
 The present invention also relates to an apparatus for performing the
 operations as described above. This apparatus may be specially constructed
 for the required purposes, or it may be a general purpose computer
 selectively activated or reconfigured by a computer program stored in the
 computer. The processes presented herein are not inherently related to any
 particular computer or other apparatus. In particular, various general
 purpose machines may be used with programs written in accordance with the
 teachings herein, or it may be more convenient to construct a more
 specialized apparatus to perform the required method steps. The required
 structure for a variety of these machines will appear from the description
 given above.
 The present invention as described above employs various process steps
 involving data stored in computer systems. These steps are those requiring
 physical manipulation of physical quantities. Usually, though not
 necessarily, these quantities take the form of electrical or magnetic
 signals capable of being stored, transferred, combined, compared, and
 otherwise manipulated. It is sometimes convenient, principally for reasons
 of common usage, to refer to these signals as bits, values, elements,
 variables, characters, data structures, or the like. It should be
 remembered, however, that all of these and similar terms are to be
 associated with the appropriate physical quantities and are merely
 convenient labels applied to these quantities.
 Further, the manipulations performed are often referred to in terms such as
 identifying, running, or comparing. In any of the operations described
 herein that form part of the present invention these operations are
 machine operations. Useful machines for performing the operations of the
 present invention include general purpose digital computers or other
 similar devices. In all cases, there should be borne in mind the
 distinction between the method of operations in operating a computer and
 the method of computation itself. The present invention relates to method
 steps for operating a computer in processing electrical or other physical
 signals to generate other desired physical signals.
 In addition, the present invention further relates to computer readable
 media that include program instructions for performing various
 computer-implemented operations. The media and program instructions may be
 those specially designed and constructed for the purposes of the present
 invention, or they may be of the kind well known and available to those
 having skill in the computer software arts. Examples of computer readable
 media include, but are not limited to, magnetic media such as hard disks,
 floppy disks, and magnetic tape; optical media such as CD-ROM disks;
 magneto-optical media such as optical disks; and hardware devices that are
 specially configured to store and perform program instructions, such as
 read-only memory devices (ROM) and random access memory (RAM). Examples of
 program instructions include both machine code, such as produced by a
 compiler, and files containing higher level code that may be executed by
 the computer using an interpreter.
 FIG. 10 illustrates a typical computer system in accordance with the
 present invention. The computer system 100 includes any number of
 processors 102 (also referred to as central processing units, or CPUs)
 that is coupled to memory devices including primary storage devices 104
 (typically a read only memory, or ROM) and primary storage devices 106
 (typically a random access memory, or RAM). As is well known in the art,
 ROM 104 acts to transfer data and instructions uni-directionally to the
 CPU and RAM 106 is used typically to transfer data and instructions in a
 bi-directional manner. Both primary storage devices 104, 106 may include
 any suitable computer-readable media as described above. A mass memory
 device 108 is also coupled bi-directionally to CPU 102 and provides
 additional data storage capacity. The mass memory device 108 may be used
 to store programs, data and the like and is typically a secondary storage
 medium such as a hard disk that is slower than primary storage devices
 104, 106. Mass memory storage device 108 may take the form of a magnetic
 or paper tape reader or some other well-known device. It will be
 appreciated that the information retained within the mass memory device
 108, may, in appropriate cases, be incorporated in standard fashion as
 part of RAM 106 as virtual memory. A specific mass storage device such as
 a CD-ROM 114 may also pass data uni-directionally to the CPU.
 CPU 102 is also coupled to one or more input/output devices 110 that may
 include, but are not limited to, devices such as video monitors, track
 balls, mice, keyboards, microphones, touch-sensitive displays, transducer
 card readers, magnetic or paper tape readers, tablets, styluses, voice or
 handwriting recognizers, or other well-known input devices such as, of
 course, other computers. Finally, CPU 102 optionally may be coupled to a
 computer or telecommunications network using a network connection as shown
 generally at 112. With such a network connection, it is contemplated that
 the CPU might receive information from the network, or might output
 information to the network in the course of performing the above-described
 method steps. The above-described devices and materials will be familiar
 to those of skill in the computer hardware and software arts.
 Although only a few embodiments of the present invention have been
 described, it should be understood that the present invention may be
 embodied in many other specific forms without departing from the spirit or
 the scope of the present invention. In the described embodiments, although
 only two subcontract layer to skeleton layer dispatch functions, i.e.
 skeleton dispatch functions, have been described, it should be appreciated
 that the skeleton dispatch functions may be widely varied within the scope
 of the present invention, as the skeleton dispatch function used as a part
 of a three-level dispatch mechanism is dependent upon the particular
 system on which the dispatch mechanism is implemented. By way of example,
 object adapters other than Low-overhead Object Adapters may be utilized.
 Further, in the described embodiments, the transport layer to subcontract
 layer dispatch mechanism is a closure. It should be appreciated that the
 transport to subcontract layer dispatch mechanism may also be widely
 varied, depending upon the particular system on which the three-level
 dispatch mechanism is implemented. By way of example, end point to
 subcontract mappings may vary depending upon the configuration of the
 transport layer in which the end point is located and the configuration of
 the subcontract layer.
 Additionally, in the described embodiments, specific end point,
 subcontract, and implementation registries have been described. It will be
 apparent that these registries can be widely varied within the scope of
 the present invention. Further, steps involved with methods of calling
 dispatch functions, as for example a skeleton dispatch function, may be
 reordered. Steps may also be removed or added without departing from the
 spirit or the scope of the present invention. Therefore the described
 embodiments should be taken as illustrative and not restrictive, and the
 invention should be defined by the following claims and their full scope
 of equivalents.