Patent Publication Number: US-6983285-B2

Title: Apparatus and method for dynamically verifying information in a distributed system

Description:
REFERENCE TO RELATED APPLICATIONS 
   This is a divisional of application Ser. No. 09/044,932, filed Mar. 20, 1998, now U.S. Pat. No. 6,466,947, which is incorporated herein by reference in its entirety. 
   The following identified U.S. patent applications are relied upon and are incorporated by reference in this application as if fully set forth. 
   Provisional U.S. patent application No. 60/076,048, entitled “Distributed Computing System,” filed on Feb. 26, 1998. 
   U.S. patent application Ser. No. 09/044,923, entitled “Method and System for Leasing Storage,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,838, entitled “Method, Apparatus, and Product for Leasing of Delegation Certificates in a Distributed System,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,834, entitled “Method, Apparatus, and Product for Leasing of Group Membership in a Distributed System,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,916, entitled “Leasing for Failure Detection,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,933, entitled “Method for Transporting Behavior in Event Based System,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,919, entitled “Deferred Reconstruction of Objects and Remote Loading for Event Notification in a Distributed System,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,938, entitled “Methods and Apparatus for Remote Method Invocation,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/045,652, entitled “Methods and System for Deterministic Hashes to Identify Remote Methods,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,790, entitled “Method and Apparatus for Determining Status of Remote Objects in a Distributed System,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,930, entitled “Downloadable Smart Proxies for Performing Processing Associated with a Remote Procedure Call in a Distributed System,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,917, entitled “Suspension and Continuation of Remote Methods,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,835, entitled “Method and System for Multi-Entry and Multi-Template Matching in a Database,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,839, entitled “Method and System for In-Place Modifications in a Database,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,945, entitled “Method and System for Typesafe Attribute Matching in a Database,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,931, entitled “Dynamic Lookup Service in a Distributed System,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/944,939, entitled “Apparatus and Method for Providing Downloadable Code for Use in Communicating with a Device in a Distributed System,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,826, entitled “Method and System for Facilitating Access to a Lookup Service,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/030,840, entitled “Method and Apparatus for Dynamic Distributed Computing Over a Network,” and filed on Feb. 26, 1998. 
   U.S. patent application Ser. No. 09/044,936, entitled “An Interactive Design Tool for Persistent Shared Memory Spaces,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,934, entitled “Polymorphic Token-Based Control,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,915, entitled “Stack-Based Access Control,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,944, entitled “Stack-Based Security Requirements,” and filed on the same date herewith. 
   U.S. patent application Ser. No. 09/044,837, entitled “Per-Method Designation of Security Requirements,” and filed on the same date herewith. 

   FIELD OF THE INVENTION 
   The present invention relates to a system and method for transmitting objects between machines in a distributed system and more particularly to dynamically verifying information in a distributed system. 
   BACKGROUND OF THE INVENTION 
   Distributed programs which concentrate on point-to-point data transmission can often be adequately and efficiently handled using special-purpose protocols for remote terminal access and file transfer. Such protocols are tailored specifically to the one program and do not provide a foundation on which to build a variety of distributed programs (e.g., distributed operating systems, electronic mail systems, computer conferencing systems, etc.). 
   While conventional transport services can be used as the basis for building distributed programs, these services exhibit many organizational problems, such as the use of different data types in different machines, lack of facilities for synchronization, and no provision for a simple programming paradigm. 
   Distributed systems usually contain a number of different types of machines interconnected by communications networks. Each machine has its own internal data types, its own address alignment rules, and its own operating system. This heterogeneity causes problems when building distributed systems. As a result, program developers must include in programs developed for such heterogeneous distributed systems the capability of dealing with ensuring that information is handled and interpreted consistently on different machines. 
   However, one simplification is afforded by noting that a large proportion of programs use a request and response interaction between processes where the initiator (i.e., program initiating a communication) is blocked waiting until the response is returned and is thus idle during this time. This can be modeled by a procedure call mechanism between processes. One such mechanism is referred to as the remote procedure call (RPC). 
   RPC is a mechanism for providing synchronized communication between two processes (e.g., program, applet, etc.) running on the same machine or different machines. In a simple case, one process, e.g., a client program, sends a message to another process, e.g., a server program. In this case, it is not necessary for the processes to be synchronized either when the message is sent or received. It is possible for the client program to transmit the message and then begin a new activity, or for the server program&#39;s environment to buffer the incoming message until the server program is ready to process a new message. 
   RPC, however, imposes constraints on synchronism because it closely models the local procedure call, which requires passing parameters in one direction, blocking the calling process (i.e., the client program) until the called procedure of the server program is complete, and then returning a response. RPC thus involves two message transfers, and the synchronization of the two processes for the duration of the call. 
   The RPC mechanism is usually implemented in two processing parts using the local procedure call paradigm, one part being on the client side and the other part being on the server side. Both of these parts will be described below with reference to FIG.  1 . 
     FIG. 1  is a diagram illustrating the flow of call information using an RPC mechanism. As shown in  FIG. 1 , a client program  100  issues a call (step  102 ). The RPC mechanism  101  then packs the call as arguments of a call packet (step  103 ), which the RPC mechanism  101  then transmits to a server program  109  (step  104 ). The call packet also contains information to identify the client program  100  that first sent the call. After the call packet is transmitted (step  104 ), the RPC mechanism  101  enters a wait state during which it waits for a response from the server program  109 . 
   The RPC mechanism  108  for the server program  109  (which may be the same RPC mechanism as the RPC mechanism  101  when the server program  109  is on the same platform as the client program  100 ) receives the call packet (step  110 ), unpacks the arguments of the call from the call packet (step  111 ), identifies, using the call information, the server program  109  to which the call was addressed, and provides the call arguments to the server program  109 . 
   The server program receives the call (step  112 ), processes the call by invoking the appropriate procedure (step  115 ), and returns a response to the RPC mechanism  108  (step  116 ). The RPC  108  then packs the response in a response packet (step  114 ) and transmits it to the client program  100  (step  113 ). 
   Receiving the response packet (step  107 ) triggers the RPC mechanism  101  to exit the wait state and unpack the response from the response packet (step  106 ). RPC  101  then provides the response to the client program  100  in response to the call (step  105 ). This is the process flow of the typical RPC mechanism modeled after the local procedure call paradigm. Since the RPC mechanism uses the local procedure call paradigm, the client program  100  is blocked at the call until a response is received. Thus, the client program  100  does not continue with its own processing after sending the call; rather, it waits for a response from the server program  109 . 
   The Java™ programming language is an object-oriented programming language that is typically compiled into a platform-independent format, using a bytecode instruction set, which can be executed on any platform supporting the Java virtual machine (JVM). This language is described, for example, in a text entitled “The Java Language Specification” by James Gosling, Bill Joy, and Guy Steele, Addison-Wesley, 1996, which is incorporated herein by reference. The JVM is described, for example, in a text entitled “The Java Virtual Machine Specification,” by Tim Lindholm and Frank Yellin, Addison Wesley, 1996, which is incorporated herein by reference. 
   Because the JVM may be implemented on any type of platform, implementing distributed programs using the JVM significantly reduces the difficulties associated with developing programs for heterogenous distributed systems. Moreover, the JVM uses a Java remote method invocation system (RMI) that enables communication among programs of the system. RMI is explained in, for example, the following document, which is incorporated herein by reference: Remote Method Invocation Specification, Sun Microsystems, Inc. (1997), which is available via universal resource locator (URL) http://wwwjavasoft.com/products/jdk/1.1/docs/guide/rmi/spec/rmiTOC.doc.html. 
     FIG. 2  is a diagram illustrating the flow of objects in an object-oriented distributed system  200  including machines  201  and  202  for transmitting and receiving method invocations using the JVM. In system  200 , machine  201  uses RMI  205  for responding to a call for object  203  by converting the object into a byte stream  207  including an identification of the type of object transmitted and data constituting the object. While machine  201  is responding to the call for object  203 , a process running on the same or another machine in system  200  may continue operation without waiting for a response to its request. 
   Machine  202  receives the byte stream  207 . Using RMI  206 , machine  202  automatically converts it into the corresponding object  204 , which is a copy of object  203  and which makes the object available for use by an program executing on machine  202 . Machine  202  may also transmit the object to another machine by first converting the object into a byte stream and then sending it to the third machine, which also automatically converts the byte stream into the corresponding object. 
   The communication among the machines may include verification of data or other information. Such verification typically requires multiple calls for verification of particular data or other information, which may result in a large volume of calls and potentially increased expense for the verification. Accordingly, a need exists for efficient verification of data or other information in a distributed system. 
   SUMMARY OF THE INVENTION 
   A first method consistent with the present invention transmits a request for a verification object. A response to the request is received, including an indication of a first code corresponding to the verification object and an indication of a second code for processing associated with verification. The verification object is constructed using the indicated first code and information is verified using the indicated second code. 
   A second method consistent with the present invention transmits a request for a verification object. A response to the request is received, including an indication of a code corresponding to the verification object. The verification object is constructed using the indicated code and information is verified based on the constructed object. 
   A third method consistent with the present invention receives at a machine a request for an object for use in verification. A response to the request is transmitted, including an indication of a first code for constructing the verification object and including an indication of a second code for processing associated with the verification. 
   A first apparatus consistent with the present invention transmits a request for a verification object. The apparatus receives a response to the request, including an indication of a first code corresponding to the verification object and an indication of a second code for processing associated with verification. The apparatus constructs the verification object using the indicated first code and verifies information using the indicated second code. 
   A second apparatus consistent with the present invention transmits a request for a verification object and receives a response to the request, including an indication of a code corresponding to the verification object. The apparatus constructs the verification object using the indicated code and verifies information based on the constructed object. 
   A third apparatus consistent with the present invention receives at a machine a request for an object for use in verification. The apparatus transmits a response to the request, including an indication of a first code for constructing the verification object and including an indication of a second code for processing associated with the verification. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are incorporated in and constitute a part of this specification and, together with the description, explain the advantages and principles of the invention. In the drawings, 
       FIG. 1  is a diagram illustrating the flow of call information using an RPC mechanism; 
       FIG. 2  is a diagram illustrating the transmission of objects in an object-oriented distributed system; 
       FIG. 3  is a diagram of an exemplary distributed processing system that can be used in an implementation consistent with the present invention; 
       FIG. 4  is a diagram of an exemplary distributed system infrastructure; 
       FIG. 5  is a diagram of a computer in a distributed system infrastructure shown in  FIG. 4 ; 
       FIG. 6  is a diagram of an exemplary distributed network for use in transmission of a policy object; and 
       FIG. 7  is a flow chart of a process for data validation using a policy object. 
   

   DETAILED DESCRIPTION 
   Overview 
   Machines consistent with the present invention may use a policy object, also referred to as a verification object, in a distributed system, the policy object performing processing when verification is needed. A machine downloads a policy object containing a reference to code governing verification of data or other information. The machine uses the reference to obtain the code and locally verify, for example, data constraints among items, data items, or objects. A verification object may also be used to verify other types of information. As particular rules for the data or information change, the policy object may be updated to provide a reference to the code for the new rules when it is downloaded. 
   Systems consistent with the present invention may efficiently transfer policy objects using a variant of an RPC or RMI, passing arguments and return values from one process to another process each of which may be on different machines. The term “machines” is used in this context to refer to a physical machine or a virtual machine. Multiple virtual machines may exist on the same physical machine. Examples of RPC systems include distributed computed environment (DCE) RPC and Microsoft distributed common object model (DCOM) RPC. 
   Distributed Processing Systems 
     FIG. 3  illustrates an exemplary distributed processing system  300  which can be used in an implementation consistent with the present invention. In  FIG. 3 , distributed processing system  300  contains three independent and heterogeneous platforms  301 ,  302 , and  303  connected in a network configuration represented by network cloud  319 . The composition and protocol of the network configuration represented by cloud  319  is not important as long as it allows for communication of the information between platforms  301 ,  302  and  303 . In addition, the use of just three platforms is merely for illustration and does not limit an implementation consistent with the present invention to the use of a particular number of platforms. Further, the specific network architecture is not crucial to embodiments consistent with this invention. For example, another network architecture that could be used in an implementation consistent with this invention would employ one platform as a network controller to which all the other platforms would be connected. 
   In the implementation of distributed processing system  300 , platforms  301 ,  302  and  303  each include a processor  316 ,  317 , and  318  respectively, and a memory,  304 ,  305 , and  306 , respectively. Included within each memory  304 ,  305 , and  306 , are applications  307 ,  308 , and  309 , respectively, operating systems  310 ,  311 , and  312 , respectively, and RMI components  313 ,  314 , and  315 , respectively. 
   Applications  307 ,  308 , and  309  can be programs that are either previously written and modified to work with, or that are specially written to take advantage of, the services offered by an implementation consistent with the present invention. Applications  307 ,  308 , and  309  invoke operations to be performed in accordance with an implementation consistent with this invention. 
   Operating systems  310 ,  311 , and  312  are typically standard operating systems tied to the corresponding processors  316 ,  317 , and  318 , respectively. The platforms  301 ,  302 , and  303  can be heterogenous. For example, platform  301  has an UltraSparc® microprocessor manufactured by Sun Microsystems, Inc. as processor  316  and uses a Solaris® operating system  310 . Platform  302  has a MIPS microprocessor manufactured by Silicon Graphics Corp. as processor  317  and uses a Unix operating system  311 . Finally, platform  303  has a Pentium microprocessor manufactured by Intel Corp. as processor  318  and uses a Microsoft Windows 95 operating system  312 . An implementation consistent with the present invention is not so limited and could accommodate homogenous platforms as well. 
   Sun, Sun Microsystems, Solaris, Java, and the Sun Logo are trademarks or registered trademarks of Sun Microsystems, Inc. in the United States and other countries. UltraSparc and all other SPARC trademarks are used under license and are trademarks of SPARC International, Inc. in the United States and other countries. Products bearing SPARC trademarks are based upon an architecture developed by Sun Microsystems, Inc. 
   Memories  304 ,  305 , and  306  serve several functions, such as general storage for the associated platform. Another function is to store applications  307 ,  308 , and  309 , RMI components  313 ,  314 , and  315 , and operating systems  310 ,  311 , and  312  during execution by the respective processor  316 ,  317 , and  318 . In addition, portions of memories  304 ,  305 , and  306  may constitute shared memory available to all of the platforms  301 ,  302 , and  303  in network  319 . Note that RMI components  313 ,  314 , and  315  operate in conjunction with a JVM, which is not shown for the purpose of simplifying the figure. 
   Distributed System Infrastructure 
   Systems and methods consistent with the present invention may also operate within a particular distributed system  400 , which will be described with reference to  FIGS. 4 and 5 . This distributed system  400  is comprised of various components, including hardware and software, to (1) allow users of the system to share services and resources over a network of many devices; (2) provide programmers with tools and programming patterns that allow development of robust, secured distributed systems; and (3) simplify the task of administering the distributed system. To accomplish these goals, distributed system  400  utilizes the Java programming environment to allow both code and data to be moved from device to device in a seamless manner. Accordingly, distributed system  400  is layered on top of the Java programming environment and exploits the characteristics of this environment, including the security offered by it and the strong typing provided by it. 
   In distributed system  400  of  FIGS. 4 and 5 , different computers and devices are federated into what appears to the user to be a single system. By appearing as a single system, distributed system  400  provides the simplicity of access and the power of sharing that can be provided by a single system without giving up the flexibility and personalized response of a personal computer or workstation. Distributed system  400  may contain thousands of devices operated by users who are geographically disperse, but who agree on basic notions of trust, administration, and policy. 
   Within an exemplary distributed system are various logical groupings of services provided by one or more devices, and each such logical grouping is known as a Djinn. A “service” refers to a resource, data, or functionality that can be accessed by a user, program, device, or another service and that can be computational, storage related, communication related, or related to providing access to another user. Examples of services provided as part of a Djinn include devices, such as printers, displays, and disks; software, such as programs or utilities; information, such as databases and files; and users of the system. 
   Both users and devices may join a Djinn. When joining a Djinn, the user or device adds zero or more services to the Djinn and may access, subject to security constraints, any one of the services it contains. Thus, devices and users federate into a Djinn to share access to its services. The services of the Djinn appear programmatically as objects of the Java programming environment, which may include other objects, software components written in different programming languages, or hardware devices. A service has an interface defining the operations that can be requested of that service, and the type of the service determines the interfaces that make up that service. 
   Distributed system  400  is comprised of computer  402 , a computer  404 , and a device  406  interconnected by a network  408 . Device  406  may be any of a number of devices, such as a printer, fax machine, storage device, computer, or other devices. Network  408  may be a local area network, wide area network, or the Internet. Although only two computers and one device are depicted as comprising distributed system  400 , one skilled in the art will appreciate that distributed system  400  may include additional computers or devices. 
     FIG. 5  depicts computer  402  in greater detail to show a number of the software components of distributed system  400 . One skilled in the art will appreciate that computer  404  or device  406  may be similarly configured. Computer  402  includes a memory  502 , a secondary storage device  504 , a central processing unit (CPU)  506 , an input device  508 , and a video display  510 . Memory  502  includes a lookup service  512 , a discovery server  514 , and a Java runtime system  516 . The Java runtime system  516  includes the Java RMI system  518  and a JVM  520 . Secondary storage device  504  includes a Java space  522 . 
   As mentioned above, distributed system  400  is based on the Java programming environment and thus makes use of the Java runtime system  516 . The Java runtime system  516  includes the Java API libraries, allowing programs running on top of the Java runtime system to access, in a platform-independent manner, various system functions, including windowing capabilities and networking capabilities of the host operating system. Since the Java API libraries provides a single common API across all operating systems to which the Java runtime system is ported, the programs running on top of a Java runtime system run in a platform-independent manner, regardless of the operating system or hardware configuration of the host platform. The Java runtime system  516  is provided as part of the Java software development kit available from Sun Microsystems, Inc. of Mountain View, Calif. 
   JVM  520  also facilitates platform independence. JVM  520  acts like an abstract computing machine, receiving instructions from programs in the form of bytecodes and interpreting these bytecodes by dynamically converting them into a form for execution, such as object code, and executing them. RMI  518  facilitates remote method invocation by allowing objects executing on one computer or device to invoke methods of an object on another computer or device. Both RMI and the JVM are also provided as part of the Java software development kit. 
   Lookup service  512  defines the services that are available for a particular Djinn. That is, there may be more than one Djinn and, consequently, more than one lookup service within distributed system  400 . Lookup service  512  contains one object for each service within the Djinn, and each object contains various methods that facilitate access to the corresponding service. Lookup service  512  is described in U.S. patent application entitled “Method and System for Facilitating Access to a Lookup Service,” which was previously incorporated herein by reference. 
   Discovery server  514  detects when a new device is added to distributed system  400 , during a process known as boot and join (or discovery), and when such a new device is detected, the discovery server passes a reference to lookup service  512  to the new device so that the new device may register its services with the lookup service and become a member of the Djinn. After registration, the new device becomes a member of the Djinn, and as a result, it may access all the services contained in lookup service  512 . The process of boot and join is described in U.S. patent application entitled “Apparatus and Method for providing Downloadable Code for Use in Communicating with a Device in a Distributed System,” which was previously incorporated herein by reference. 
   A Java space  522  is an object repository used by programs within distributed system  400  to store objects. Programs use a Java space  522  to store objects persistently as well as to make them accessible to other devices within distributed system  400 . Java spaces are described in U.S. patent application Ser. No. 08/971,529, entitled “Database System Employing Polymorphic Entry and Entry Matching,” assigned to a common assignee, and filed on Nov. 17, 1997, which is incorporated herein by reference. One skilled in the art will appreciate that an exemplary distributed system  400  may contain many lookup services, discovery servers, and Java spaces. 
   Data Flow in a Distributed Processing System 
     FIG. 6  is a diagram of an object-oriented distributed network  600  for use in transmission of a policy object for use in verification. Network  600  includes client machine  601  and server machine  604 , which may be implemented with computers or virtual machines executing on one or more computers, or the machines described with reference to  FIGS. 3 ,  4 , and  5 . Client machine  601  includes RMI  602  and associated object  603 . Server machine  604  includes RMI  605  and associated policy object  606 . 
   Client machine  601 , using RMI  602 , transmits a call or request  609  to RMI  605  requesting a policy object  606 . In response, RMI  605  transmits in call  610  policy object  606  or a reference to it. RMI  602  and client machine  601  use the policy object, represented as object  603 , for verification of data or other information. If necessary, RMI  602  may access code server  607  to obtain code  608  used by the policy object. A code server is an entity and process that has access to code and responds to requests for a particular type or class of object and returns code for that object. A code server may be located within machine  604  or on another machine. Also, the code may be resident on the same platform as the code server or on a separate platform. 
   RMI  602  may access such code by using a network-accessible location in the form of a URL for code that is associated with the object. URLs are known in the art and an explanation, which is incorporated herein by reference, appears in, for example, a text entitled “The Java Tutorial: Object-Oriented Programming for the Internet,” pp. 494-507, by Mary Campione and Kathy Walrath, Addison-Wesley, 1996. 
   The objects may be transmitted as object streams as described in The Object Serialization Specification or The RMI Specification, both available from Sun Microsystems, Inc. Streams, including input and output streams, are also described in, for example, the following text, which is incorporated herein by reference: “The Java Tutorial: Object-Oriented Programming for the Internet,” pp. 325-53, by Mary Campione and Kathy Walrath, Addison-Wesley, 1996. 
   Transmission of a Policy Object 
     FIG. 7  is a flow chart of a process  700  for verification using a policy object, also referred to as a verification object. A machine first determines if verification is requested (step  701 ). If so, it requests a policy object from a server (step  702 ) and receives the policy object including a reference to code for use in verification of data or other information (step  703 ). Using the reference, it downloads code for the verification (step  704 ). The downloading of code may occur using the methods described in U.S. patent application Ser. No. 08/950,756, filed on Oct. 15, 1997, and entitled “Deferred Reconstruction of Objects and Remote Loading in a Distributed System,” which is incorporated herein by reference. 
   The machine then obtains data or other information (step  705 ). It determines if the data or information is valid using the policy object (step  706 ), which may occur through local processing. Validation may be based on particular predefined criteria. If the data or information was not valid, it obtains new data or information; for example, it provides a message to the user requesting re-entry of the data (step  707 ). The machine then determines if there is more data or information to process (step  708 ). If so, it returns to step  705  to receive and verify additional data. Otherwise, it makes use of the verified data or other information (step  709 ); for example, it may submit data to the server. During these steps, the server may send an indication of code, such as a reference to the code or the code itself, for updating the policy or rules concerning verification. Thus, the policy or rules may be dynamically updated so that client machines receive and maintain code or a reference to code for the current policy or rules. 
   Machines implementing the steps shown in  FIG. 7  may include computer processors for performing the functions, as shown in  FIGS. 3 ,  4 ,  5 , and  6 . They may include modules or programs configured to cause the processors to perform the above functions. They may also include computer program products stored in a memory. The computer program products may include a computer-readable medium or media having computer-readable code embodied therein for causing the machines to perform functions described in this specification. The media may include a computer data signal embodied in a carrier wave and representing sequences of instructions which, when executed by a processor, cause the processor to securely address a peripheral device at an absolute address by performing the method described in this specification. The media may also include data structures for use in performing the method described in this specification. In addition, the processing shown in  FIG. 7  may occur through the use of smart proxies, which are described in U.S. patent application filed on even date herewith, assigned to a common assignee, and entitled “Downloadable Smart Proxies for Performing Processing Associated with a Remote Procedure Call in a Distributed System,” which is incorporated herein by reference. 
   Example of a Policy Object 
   The following provides an example of using a policy object to verify data in an expense report consistent with the present invention. Table 1 provides an example of a remote policy interface written in the Java programming language defining methods a client can invoke on a server for this expense report example. 
                           TABLE 1                          public interface ExpenseServer extends Remote {                         Policy getPolicy () throws RemoteException;           void submitReport (ExpenseReport report)                         throws RemoteException, InvalidReportException;                         }                        
The interface shown in Table 1 supports two methods. A get policy (“getPolicy”) method returns an object that implements the interface. A submit report (“submitReport”) method submits a completed expense request, throwing (generating) an exception if the report is malformed for any reason. The policy interface declares a method, shown in Table 2, informing a client whether it is acceptable to add an entry to the expense report.
 
                           TABLE 2                          public interface Policy {                         void checkValid (ExpenseEntry entry)                         throws Policy ViolationException;                         }                        
If an expense report entry is valid, meaning that it matches current policy, the method returns normally. Otherwise it throws an exception that describes the error. The exemplary policy interface may be local (not remote) and thus may be implemented by an object local to a client. Table 3 illustrates how the client may operate for this example.
 
                           TABLE 3                          Policy curPolicy = server.getPolicy ();           start a new expense report           show the GUI to the user           while (user keeps adding entries) {                         try {                         curPolicy.checkValid(entry); //throws exception if not OK                         add the entry to the expense report                         } catch (policy ViolationException e) {                         show the error to the user                         }                         }           server.submitReport (report);                        
When a user requests the client software to start up a new expense report, the client invokes “server.getPolicy” method to ask the server to return an object that embodies the current expense policy. Each entry that is added is first submitted to that policy object for approval. If the policy object reports no error, the entry is added to the report; otherwise the error is displayed to the user for corrective action. When the user is finished adding entries to the report, the entire report is submitted.
 
   Table 4 illustrates how the server may operate for this example. 
                           TABLE 4                          import java.rmi.*;           import java.rmi.server.*;           class ExpenseServerImpl                         extends UnicastRemoteObject           implements ExpenseServer                         {                         ExpenseServerImpl () throws RemoteException {                         // . . . set up server state . . .                         }           public Policy getPolicy () {                         return new TodaysPolicy();                         }           public void submitReport (ExpenseReport report) {                         // . . . write the report into the db . . .                         }                         }                        
The type “UnicastRemoteObject” defines the kind of remote object corresponding to this server, in this example a single server as opposed to a replicated service. The Java programming language class “ExpenseServerImpl” implements the methods of the remote interface “ExpenseServer.” Clients on remote hosts can use RMI to send messages to “ExpenseServerlmpl” objects.
 
   Table 5 provides an example of an implementation of a policy for this expense report example. 
                   TABLE 5                  public class TodaysPolicy implements Policy {                         public void checkValid(ExpenseEntry entry)                         throws Policy ViolationException                         {                         if (entry.dollars() &lt;20) {                         return; // no receipt required                         } else if (entry.haveReceipt() == false) {                         throw new PolicyViolationException (“receipt required”);                         }                         }                 }                    
The policy defined in Table 5 checks to ensure that any entry without a receipt is less than twenty dollars. If the policy changes tomorrow so that only meals under twenty dollars are exempt from the “receipts required” policy, an implementation of new policy may be provided as shown in Table 6.
 
                   TABLE 6                  public class TomorrowsPolicy implements Policy {                         public void checkValid(ExpenseEntry entry)                         throws Policy ViolationException                         {                         if (entry.isMeal( ) &amp;&amp; entry.dollars( ) &lt;20) {                         return; // no receipt required                         } else if (entry.haveReceipt( ) == false) {                         throw new PolicyViolationException (“receipt required”);                            }                           }                 }                    
The new policy (“TomorrowsPolicy”) defined in Table 6 may be installed on a server, and the server may then deliver the new policy objects instead of the current (“TodaysPolicy”) objects. When a client invokes the server&#39;s get policy method, RMI on the client verifies whether the returned object is of a known type. The first time each client encounters a “TomorrowsPolicy” object, the client&#39;s RMI downloads the implementation for the policy before “getPolicy” method returns, and the client thus begins enforcing the new policy. This expense report example is only one example of use of a policy object for verification, and many other applications of a policy object are possible.
 
   Although the illustrative embodiments of the systems consistent with the present invention are described with reference to a computer system implementing the Java programming language on the JVM specification, the invention is equally applicable to other computer systems processing code from different programming languages. Specifically, the invention may be implemented with both object-oriented and nonobject-oriented programming systems. In addition, although an embodiment consistent with the present invention has been described as operating in the Java programming environment, one skilled in the art will appreciate that the present invention can be used in other programming environments as well. 
   While the present invention has been described in connection with an exemplary embodiment, it will be understood that many modifications will be readily apparent to those skilled in the art, and this application is intended to cover any adaptations or variations thereof. For example, different labels or definitions for the policy object may be used without departing from the scope of the invention. This invention should be limited only by the claims and equivalents thereof.