Abstract:
In accordance with the present invention a process is provided for allocating and deallocating resources in a distributed processing system having a requester platform and a server platform. The process involves receiving a request from the requestor platform referring to a system resource and specifying a requested lease period, permitting shared access to the system resource for a lease period, sending a return call to the requestor platform advising of the lease period, and deallocating the system resource when the lease period expires.

Description:
This is a continuation of application Ser. No. 09/853,644, filed May 14, 2001, which is a continuation of application Ser. No. 09/152,062, filed Sep. 11, 1998, which is a continuation of application Ser. No. 08/729,421, filed Oct. 11, 1996, now U.S. Pat. No. 5,832,529, all of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     This invention generally relates to garbage collection for computer systems and, more particularly to a fault-tolerant distributed garbage collection method for collecting resources bound to or associated with references. 
     B. Description of the Related Art 
     Proper resource management is an important aspect to efficient and effective use of computers. In general, resource management involves allocating resources (e.g., memory) in response to requests as well as deallocating resources at appropriate times, for example, when the requesters no longer require the resources. In general, the resources contain data referenced by computational entities (e.g., applications, programs, applets, etc.) executing in the computers. 
     In practice, when applications executing on computers seek to refer to resources, the computers must first allocate or designate resources so that the applications can properly refer to them. When the applications no longer refer to a resource, the computers can deallocate or reclaim the resource for reuse. In computers each resource has a unique “handle” by which the resource can be referenced. The handle may be implemented in various ways, such as an address, array index, unique value, pointer, etc. 
     Resource management is relatively simple for a single computer because the events indicating when resources can be reclaimed, such as when applications no longer refer to them or after a power failure, are easy to determine. Resource management for distributed systems connecting multiple computers is more difficult because applications in several different computers may be using the same resource. 
     Disconnects in distributed systems can lead to the improper and premature reclamation of resources or to the failure to reclaim resources. For example, multiple applications operating on different computers in a distributed system may refer to resources located on other machines. If connections between the computers on which resources are located and the applications referring to those resources are interrupted, then the computers may reclaim the resources prematurely. Alternatively, the computers may maintain the resources in perpetuity, despite the extended period of time that applications failed to access the resources. 
     These difficulties have led to the development of systems to manage network resources, one of which is known as “distributed garbage collection.” That term describes a facility provided by a language or runtime system for distributed systems that automatically manages resources used by an application or group of applications running on different computers in a network. 
     In general, garbage collection uses the notion that resources can be freed for future use when they are no longer referenced by any part of an application. Distributed garbage collection extends this notion to the realm of distributed computing, reclaiming resources when no application on any computer refers to them. 
     Distributed garbage collection must maintain integrity between allocated resources and the references to those resources. In other words, the system must not be permitted to deallocate or free a resource when an application running on any computer in the network continues to refer to that resource. This reference-to-resource binding, referred to as “referential integrity,” does not guarantee that the reference will always grant access to the resource to which it refers. For example, network failures can make such access impossible. The integrity, however, guarantees that if the reference can be used to gain access to any resource, it will be the same resource to which the reference was first given. 
     Distributed systems using garbage collection must also reclaim resources no longer being referenced at some time in the finite future. In other words, the system must provide a guarantee against “memory leaks.” A memory leak can occur when all applications drop references to a resource, but the system fails to reclaim the resource for reuse because, for example, of an incorrect determination that some application still refers to the resource. 
     Referential integrity failures and memory leaks often result from disconnections between applications referencing the resources and the garbage collection system managing the allocation and deallocation of those resources. For example, a disconnection in a network connection between an application referring to a resource and a garbage collection system managing that resource may prevent the garbage collection system from determining whether and when to reclaim the resource. Alternatively, the garbage collection system might mistakenly determine that, since an application has not accessed a resource within a predetermined time, it may collect that resource. A number of techniques have been used to improve the distributed garbage collection mechanism by attempting to ensure that such mechanisms maintain referential integrity without memory leaks. One conventional approach uses a form of reference counting, in which a count is maintained of the number of applications referring to each resource. When a resource&#39;s count goes to zero, the garbage collection system may reclaim the resource. Such a reference counting scheme only works, however, if the resource is created with a corresponding reference counter. The garbage collection system in this case increments the resource&#39;s reference count as additional applications refer to the resource, and decrements the count when an application no longer refers to the resource. 
     Reference counting schemes, however, especially encounter problems in the face of failures that can occur in distributed systems. Such failures can take the form of a computer or application failure or network failure that prevent the delivery of messages notifying the garbage collection system that a resource is no longer being referenced. If messages go undelivered because of a network disconnect, the garbage collection system does not know when to reclaim the resource. 
     To prevent such failures, some conventional reference counting schemes include “keep-alive” messages, which are also referred to as “ping back.” According to this scheme, applications in the network send messages to the garbage collection system overseeing resources and indicate that the applications can still communicate. These messages prevent the garbage collection system from dropping references to resources. Failure to receive such a “keep-alive” message indicates that the garbage collection system can decrement the reference count for a resource and, thus, when the count reaches zero, the garbage collection system may reclaim the resource. This, however, can still result in the premature reclamation of resources following reference counts reaching zero from a failure to receive “keep-alive” messages because of network failures. This violates the referential integrity requirement. 
     Another proposed method for resolving referential integrity problems in garbage collection systems is to maintain not only a reference count but also an identifier corresponding to each computational entity referring to a resource. See A. Birrell, et al., “Distributed Garbage Collection for Network Objects,” No. 116, digital Systems Research Center, Dec. 15, 1993. This method suffers from the same problems as the reference counting schemes. Further, this method requires the addition of unique identifiers for each computational entity referring to each resource, adding overhead that would unnecessarily increase communication within distributed systems and add storage requirements (i.e., the list of identifiers corresponding to applications referring to each resource). 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, referential integrity is guaranteed without costly memory leaks by leasing resources for a period of time during which the parties in a distributed system, for example, an application holding a reference to a resource and the garbage collection system managing that resource, agree that the resource and a reference to that resource will be guaranteed. At the end of the lease period, the guarantee that the reference to the resource will continue lapses, allowing the garbage collection system to reclaim the resource. Because the application holding the reference to the resource and the garbage collection system managing the resource agree to a finite guaranteed lease period, both can know when the lease and, therefore, the guarantee, expires. This guarantees referential integrity for the duration of a reference lease and avoids the concern of failing to free the resource because of network errors. 
     In accordance with the present invention, as embodied and broadly described herein, a method for managing resources comprises the steps of receiving a request from a process referring to a resource and specifying a requested lease period, permitting shared access to the resource for a granted lease period, advising the process of the granted lease period, and deallocating the resource when the granted lease period expires. In accordance with another aspect of the present invention, as embodied and broadly described herein, a method for managing resources comprises the steps of requesting from a process access to a resource for a lease period, receiving from the process a granted lease period during which shared access to the resource is permitted, and sending a request to the process for a new lease period upon a determination that the granted lease period is about to expire but access to the resource has not completed. 
     In accordance with the present invention, as embodied and broadly described herein, an apparatus comprises a receiving module configured to receive a request from a process referring to a resource and specifying a requested lease period, a resource allocator configured to permit shared access to the resource for a granted lease period, an advising module configured to advise the process of the granted lease period, and a resource deallocator configured to deallocate the resource when the granted lease period expires. In accordance with another aspect of the present invention, as embodied and broadly described herein, an apparatus comprises a requesting module configured to request from a process access to a resource for a lease period. a receiving module configured to receive from the process a granted lease period during which shared access to the resource is permitted, and a second sending module configured to send another request to the process for a new lease period upon a determination that the granted lease period is about to expire but access to the resource has not completed. 
     In accordance with yet another aspect of the present invention, as embodied and broadly described herein, a computer program product comprises a computer usable medium having computable readable code embodied therein for managing resources. The code comprises a receiving module configured to receive a request from a process referring to a resource and specifying a requested lease period, a resource allocator configured to permit shared access to the resource for a granted lease period, an advising module configured to advise of the granted lease period, and a resource deallocator configured to deallocate the resource when the granted lease period expires. In accordance with another aspect of the present invention, as embodied and broadly described herein, a computer program product comprises a computer usable medium having computable readable code embodied therein for managing resources. The code comprises a requesting module configured to request from a process access to a resource for a lease period, a receiving module configured to receive from the process a granted lease period during which the process permits shared access to the resource, and a sending module configured to send another request to the process for a new lease period upon a determination that the granted lease period is about to expire. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, serve to explain the advantages and principles of the invention. In the drawings, 
     FIG. 1 is a flow diagram of the steps performed by the application call processor according to an implementation of the present invention; 
     FIG. 2 is a flow diagram of the steps performed by the server call processor to process dirty calls according to the implementation of the present invention; 
     FIG. 3 is a flow diagram of the steps performed by the server call processor to process clean calls according to the implementation of the present invention; 
     FIG. 4 is a flow diagram of the steps performed by the server call processor to initiate a garbage collection process according to the implementation of the present invention; 
     FIG. 5 is a diagram of a preferred flow of calls within a distributed processing system; 
     FIG. 6 is a block diagram of the components of the implementation of a method invocation service according to the present invention; 
     FIG. 7 is a diagram of a distributed processing system that can be used in an implementation of the present invention; and 
     FIG. 8 is a diagram of the individual software components in the platforms of the distributed processing system according to the implementation of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to an implementation of the present invention as illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings and the following description to refer to the same or like parts. 
     The present invention may be implemented by computers organized in a conventional distributed processing system architecture. The architecture for and procedures to implement this invention, however, are not conventional, because they provide a distributed garbage collection scheme that ensures referential integrity and eliminates memory leaks. 
     A. Overview 
     A method invocation (MI) component located in each of the computers in the distributed processing system implements the distributed garbage collection scheme of this invention. The MI component may consist of a number of software modules preferably written in the JAVA™ programming language. 
     In general, whenever an application in the distributed processing system obtains a reference to a distributed resource, by a name lookup, as a return value to some other call, or another method, and seeks to access the resource, the application makes a call to the resource or to an MI component managing the resource. That MI component, called a managing MI component, keeps track of the number of outstanding references to the resource. When the number of references to a reference is zero, the managing MI component can reclaim the resource. The count of the number of references to a resource is generally called the “reference count” and the call that increments the reference count may be referred to as a “dirty call.” 
     When an application no longer requires a distributed resource, it sends a different call to the resource or the managing MI component. Upon receipt of this call, the managing MI component decrements the reference count for the resource. This call to drop a reference may be referred to as a “clean call.” 
     In accordance with an implementation of the present invention, a dirty call can include a requested time interval, called a lease period, for the reference to the resource. Upon receipt of the dirty call, the managing MI component sends a return call indicating a period for which the lease was granted. The managing MI component thus tracks the lease period for those references as well as the number of outstanding references. Consequently, when the reference count for a resource goes to zero or when the lease period for the resource expires, the managing MI component can reclaim the resource. 
     B. Procedure 
     An application call processor in an MI component performs the steps of the application call procedure  100  illustrated,in FIG.  1 . The server call processor in the managing MI component performs the steps of the procedures  200 ,  300 , and  400  illustrated in FIGS. 2-4, respectively. The managing MI component&#39;s garbage collector performs conventional procedures to reclaim resources previously bound to references in accordance with instructions from the server call processor. Accordingly, the conventional procedures of the garbage collector will not be explained. 
     1. Application Call Processor 
     FIG. 1 is a flow diagram of the procedure  100  that the application call processor of the MI component uses to handle application requests for references to resources managed by the same or another MI component located in the distributed processing system. 
     After an application has obtained a reference to a resource, the application call processor sends a dirty call, including the resource&#39;s reference and a requested lease period to the managing MI component for the resource (step  110 ). The dirty call may be directed to the resource itself or to the managing MI component. 
     The application call processor then waits for and receives a return call from the managing MI component (step  120 ). The return call includes a granted lease period during which the managing MI component guarantees that the reference of the dirty call will be bound to its resource. In other words, the managing MI component agrees not to collect the resource corresponding to the reference of a dirty call for the grant period. If the managing MI component does not provide a grant period, or rejects the request for a lease, then the application call processor will have to send another dirty call until it receives a grant period. 
     The application call processor monitors the application&#39;s use of the reference and, either when the application explicitly informs the application call processor that the reference is no longer required or when the application call processor makes this determination on its own (step  130 ), the application call processor sends a clean call to the managing MI component (step  140 ). In a manner similar to the method used for dirty calls, the clean call may be directed to the referenced resource and the managing MI component will process the clean call. Subsequently, the application call processor eliminates the reference from a list of references being used by the application (step  150 ). 
     If the application is not yet done with the reference (step  130 ), but the application call processor determines that the grant period for the reference is about to expire (step  160 ), then the application call processor repeats steps  110  and  120  to ensure that the reference to the resource is maintained by the managing MI component on behalf of the application. 
     2. Server Call Processor 
     The MI component&#39;s server call processor performs three main procedures: (1) handling dirty calls; (2) handling incoming clean calls; and (3) initiating a garbage collection cycle to reclaim resources at the appropriate time. 
     (i) Dirty Calls 
     FIG. 2 is a flow diagram of the procedure  200  that the MI component&#39;s server call processor uses to handle requests to reference resources, i.e., dirty calls, that the MI software component manages. These requests come from application call processors of MI components in the distributed processing system, including the application call processor of the same MI component as the server call processor handling requests. 
     First, the server call processor receives a dirty call (step  210 ). The server call processor then determines an acceptable grant period (step  220 ). The grant period may be the same as the requested lease period or some other time period. The server call processor determines the appropriate grant period based on a number of conditions including the amount of resource required and the number of other grant periods previously granted for the same resource. 
     When the server call processor determines that a resource has not yet been allocated for the reference of a dirty call (step  230 ), the server call processor allocates the required resource (step  240 ). 
     The server call processor then increments a reference count corresponding to the reference of a dirty call (step  250 ), sets the acceptable grant period for the reference-to-resource binding (step  260 ), and sends a return call to an application call processor with the grant period (step  270 ). In this way, the server call processor controls incoming dirty calls regarding references to resources under its control. 
     Applications can extend leases by sending dirty calls with an extension request before current leases expire. As shown in procedure  200 , a request to extend a lease is treated just like an initial request for a lease. An extension simply means that the resource will not be reclaimed for some additional interval of time, unless the reference count goes to zero. 
     (ii) Clean Calls 
     The MI component&#39;s server call processor also handles incoming clean calls from application call processors. When an application in the distributed processing system no longer requires a reference to a resource, it informs the MI component managing the resource for that reference so that the resource may be reclaimed for reuse. FIG. 3 is a flow diagram of the procedure  300  with the steps that the MI component&#39;s server call processor uses to handle clean calls. 
     When the server call processor receives a clean call with a reference to a resource that the MI component manages (step  310 ), the server call processor decrements a corresponding reference count (step  320 ). The clean call may be sent to the resource, with the server call processor monitoring the resource and executing the procedure  300  to process the call. Subsequently, the server call processor sends a return call to the MI component that sent the clean call to acknowledge receipt (step  330 ). In accordance with this implementation of the present invention, a clean call to drop a reference may not be refused, but it must be acknowledged. 
     (iii) Garbage Collection 
     The server call processor also initiates a garbage collection cycle to reclaim resources for which it determines that either no more references are being made to the resource or that the agreed lease period for the resource has expired. The procedure  400  shown in FIG. 4 includes a flow diagram of the steps that the server call processor uses to initiate a garbage collection cycle. 
     The server call processor monitors reference counts and granted lease periods and determines whether a reference count is zero for a resource managed by the MI component or the grant period for a reference has expired (step  410 ). When either condition exists, the server call processor initiates garbage collection (step  420 ) of that resource. Otherwise, the server call processor continues monitoring the reference counts and granted lease periods. 
     C. Call Flow 
     FIG. 5 is a diagram illustrating the flow of calls among MI components within the distributed processing system. Managing MI component  525  manages the resources  530  by monitoring the references to those resources  530  (see garbage collect  505 ). Because the managing MI components  525  manages the resources, the server call processor of managing MI component  525  performs the operations of this call flow description. 
     FIG. 5 also shows that applications  510  and  540  have corresponding MI components  515  and  545 , respectively. Each of the applications  510  and  540  obtains a reference to one of the resources  530  and seeks to obtain access to one of the resources  530  such that a reference is bound to the corresponding resource. To obtain access, applications  510  and  540  invoke their corresponding MI components  515  and  545 , respectively, to send dirty calls  551  and  571 , respectively, to the MI component  525 . Because the MI components  515  and  525  handle application requests for access to resources  530  managed by another MI component, such as managing MI component  525 , the application call processors of MI components  515  and  545  perform the operations of this call flow description. 
     In response to the dirty calls  551  and  571 , managing MI component  525  sends return calls  552  and  572 , respectively, to each of the MI components  515  and  545 , respectively. The dirty calls include granted lease periods for the references of the dirty calls  551  and  571 . 
     Similarly, FIG. 5 also shows MI components  515  and  545  sending clean calls  561  and  581 , respectively, to managing MI component  525 . Clean calls  561  and  581  inform managing MI component  525  that applications  510  and  540 , respectively, no longer require access to the resource specified in the clean calls  561  and  581 . Managing MI component  525  responds to clean calls  561  and  581  with return calls  562  and  582 , respectively. Return calls  562  and  582  differ from return calls  552  and  572  in that return calls  562  and  582  are simply acknowledgments from MI component  525  of the received clean calls  561  and  581 . 
     Both applications  510  and  540  may request access to the same resource. For example, application  510  may request access to “RESOURCE(1)” while application  540  was previously granted access to that resource. MI component  525  handles this situation by making the resource available to both applications  510  and  540  for agreed lease periods. Thus, MI component  525  will not initiate a garbage collection cycle to reclaim the “RESOURCE(1)” until either applications  510  and  540  have both dropped their references to that resource or the latest agreed periods has expired, whichever event occurs first. 
     By permitting more than one application to access the same resource simultaneously, the present invention also permits an application to access a resource after it sent a clean call to the managing MI component dropping the reference to the resource. This occurs because the resource is still referenced by another application or the reference&#39;s lease has not yet expired so the managing MI component  525  has not yet reclaimed the resource. The resource, however, will be reclaimed after a finite period, either when no more applications have leases or when the last lease expires. 
     D. MI Components 
     FIG. 6 is a block diagram of the modules of an MI component  600  according to an implementation of the present invention. MI component  600  can include a reference component  605  for each reference monitored, application call processor  640 , server call processor  650 , and garbage collector  660 . 
     Reference component  605  preferably constitutes a table or comparable structure with reference data portions  610 , reference count  620 , and grant period register  630 . MI component  600  uses the reference count  620  and grant period  630  for each reference specified in a corresponding reference data portion  610  to determine when to initiate garbage collector  660  to reclaim the corresponding resource. 
     Application call processor  640  is the software module that performs the steps of procedure  100  in FIG.  1 . Server call processor  650  is the software module that performs the steps of procedures  200 ,  300 , and  400  in FIGS. 2-4. Garbage collector  660  is the software module that reclaims resources in response to instructions from the server call processor  650 , as explained above. 
     E. Distributed Processing System 
     FIG. 7 illustrates a distributed processing system  50  which can be used to implement the present invention. In FIG. 7, distributed processing system  50  contains three independent and heterogeneous platforms  100 ,  200 , and  300  connected in a network configuration represented by the network cloud  55 . The composition and protocol of the network configuration represented in FIG. 7 by the cloud  55  is not important as long as it allows for communication of the information between platforms  700 ,  800  and  900 . In addition, the use of just three platforms is merely for illustration and does not limit the present invention to the use of a particular number of platforms. Further, the specific network architecture is not crucial to this invention. For example, another network architecture that could be used in accordance 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  50 , platforms  700 ,  800  and  900  each include a processor  710 ,  810 , and  910  respectively, and a memory,  750 ,  850 , and  950 , respectively. Included within each processor  710 ,  810 , and  910 , are applications  720 ,  820 , and  920 , respectively, operating systems  740 ,  840 , and  940 , respectively, and MI components  730 ,  830 , and  930 , respectively. 
     Applications  720 ,  820 , and  920  can be programs that are either previously written and modified to work with the present invention, or that are specially written to take advantage of the services offered by the present invention. Applications  720 ,  820 , and  920  invoke operations to be performed in accordance with this invention. 
     MI components  730 ,  830 , and  930  correspond to the MI component  600  discussed above with reference to FIG.  6 . 
     Operating systems  740 ,  840 , and  940  are standard operating systems tied to the corresponding processors  710 ,  810 , and  910 , respectively. The platforms  700 ,  800 , and  900  can be heterogenous. For example, platform  700  has an UltraSparc® microprocessor manufactured by Sun Microsystems Corp. as processor  710  and uses a Solaris® operating system  740 . Platform  800  has a MIPS microprocessor manufactured by Silicon Graphics Corp. as processor  810  and uses a Unix operating system  840 . Finally, platform  900  has a Pentium microprocessor manufactured by Intel Corp. as processor  910  and uses a Microsoft Windows 95 operating system  940 . 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  750 ,  850 , and  950  serve several functions, such as general storage for the associated platform. Another function is to store applications  720 ,  820 , and  920 , MI components  730 ,  830 , and  930 , and operating systems  740 ,  840 , and  940  before execution by the respective processor  710 ,  810 , and  910 . In addition, portions of memories  750 ,  850 , and  950  may constitute shared memory available to all of the platforms  700 ,  800 , and  900  in network  50 . 
     E. MI Services 
     The present invention may be implemented using a client/server model. The client generates requests, such as the dirty calls and clean calls, and the server responds to requests. 
     Each of the MI components  730 ,  830  and  930  shown in FIG. 7 preferably includes both client components and server components. FIG. 8, which is a block diagram of a client platform  1000  and a server platform  1100 , applies to any two of the platforms  700 ,  800 , and  900  in FIG.  7 . 
     Platforms  1000  and  1100  contain memories  1050  and  1150 , respectively, and processors  1010  and  1110 , respectively. The elements in the platforms  1000  and  1100  function in the same manner as similar elements described above with reference to FIG.  7 . In this example, processor  1010  executes a client application  1020  and processor  1110  executes a server application  1120 . Processors  1010  and  1110  also execute operating systems  1040  and  1140 , respectively, and MI components  1030  and  1130 , respectively. 
     MI components  1030  and  1130  each include a server call processor  1031  and  1131 , respectively, an application call processor  1032  and  1132 , respectively, and a garbage collector  1033  and  1133 , respectively. Each of the MI components  1030  and  1130  also contains reference components, including reference data portions  1034  and  1134 , respectively, reference counts  1035  and  1135 , respectively, and grant period registers  1036  and  1136 , respectively, for each reference that the respective MI component  1030  or  1130  monitors. 
     Application call processors  1032  and  1132  represent the client service and communicate with server call processors  1031  and  1131 , respectively, which represent the server service. Because platforms  1000  and  1100  contain a server call processor, an application call processor, a garbage collector, and reference components, either platform can act as a client or a server. 
     For purposes of the discussion that follows, however, platform  1000  is designated the client platform and platform  1100  is designated as the server platform. In this example, client application  1020  obtains references to distributed resources and uses MI component  1030  to send dirty calls to the resources managed by MI component  1130  of server platform  1100 . 
     Additionally, server platform  1100  may be executing a server application  1120 . Server application  1120  may also use MI component  1130  to send dirty calls, which may be handled by MI component  1130  when the resources of those dirty calls are managed by MI component  1130 . Alternatively, server application  1120  may use MI component  1130  to send dirty calls to resources managed by MI component  1030 . 
     Accordingly, server call processor  1031 , garbage collector  1033 , and reference count  1035  for MI component  1030  of client platform  1000  are not active and are therefore presented in FIG. 8 as shaded. Likewise, application call processor  1132  of MI component  1130  of the server platform  1100  is shaded because it is also dormant. 
     When client application  1020  obtains a reference corresponding to a resource, application call processor  1032  sends a dirty call, which server call processor  1131  receives. The dirty call includes a requested lease period. Server call processor  1131  increments the reference count  1135  for the reference in the dirty call and determines a grant period. In response, server call processor  1131  sends a return call to application call processor  1030  with the grant period. Application call processor  1032  uses the grant period to update recorded grant period  1035 , and to determine when the resource corresponding to the reference of its dirty call may be reclaimed. 
     Server call processor  1131  also monitors the reference counts and grant periods corresponding to references for resources that it manages. When one of its reference counts  1135  is zero, or when the grant period  1135  for a reference has expired, whichever event occurs first, server call processor  1131  may initiate the garbage collector  1133  to reclaim the resource corresponding to the reference that has a reference count of zero or an expired grant period. 
     The leased-reference scheme according to the implementation of the present invention does not require that the clocks on the platforms  1000  and  1100  involved in the protocol be synchronized. The scheme merely requires that they have comparable periods of increase. Leases do not expire at a particular time, but rather expire after a specific time interval. As long as there is approximate agreement on the interval, platforms  1000  and  1100  will have approximate agreement on the granted lease period. Further, since the timing for the lease is, in computer terms, fairly long, minor differences in clock rate will have little or no effect. 
     The transmission time of the dirty call can affect the protocol. If MI component  1030  holds a lease to reference and waits until just before the lease expires to request a renewal, the lease may expire before the MI component  1130  receives the request. If so, MI component  1130  may reclaim the resource before receiving the renewal request. Thus, when sending dirty calls, the sender should add a time factor to the requested lease period in consideration of transmission time to the platform handling the resource of a dirty call so that renewal dirty calls may be made before the lease period for the resource expires. 
     F. Conclusion 
     In accordance with the present invention a distributed garbage collection scheme ensures referential integrity and eliminates memory leaks by providing granted lease periods corresponding to references to resources in the distributed processing system such that when the granted lease periods expire, so do the references to the resources. The resources may then be collected. Resources may also be collected when they are no longer being referenced by processes in the distributed processing system with reference to counters assigned to the references for the resources. 
     The foregoing description of an implementation of the invention has been presented for purposes of illustration and description. It is not exhaustive and does not limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practicing of the invention. For example, the described implementation includes software but the present invention may be implemented as a combination of hardware and software or in hardware alone. The scope of the invention is defined by the claims and their equivalents.