Method and system for reliable remote object reference management

• US 6,138,251 A
• Filed: 06/30/1997
• Issued: 10/24/2000
• Est. Priority Date: 06/30/1997
• Status: Expired due to Term

• Current Assignee: Oracle America, Inc. (Oracle Corporation)
• Original Assignee: Sun Microsystems Incorporated (Oracle Corporation)
• Inventors: Bernabeu-Auban, Jose, Khalidi, Yousef A., Murphy, Declan J., Talluri, Madhusudhan, Tucker, Andrew G.
• Primary Examiner(s): Sheikh, Ayaz R.
• Assistant Examiner(s): Backer, Firmin
• Application Number:
  US08/884,978
• Time in Patent Office: 1,212 Days
• Field of Search: 714/41, 714/18, 714/748, 714/749, 714/40, 714/42, 714/710, 709/303, 709/203, 709/224, 709/239, 709/223, 710/34, 711/1, 711/101, 711/170, 711/196, 712/28
• US Class Current: 714/41
• CPC Class Codes: G06F 12/0261 using reference counting
• CPC Class Codes: G06F 9/4493 Object persistence
• CPC Class Codes: G06F 9/465 Distributed object oriented systems remote method invocation [RMI] G06F9/548

Abstract

The present invention pertains to a system and method for tracking object references in an object-oriented computing system including a number of independent computing nodes interconnected by a communications link. The reference counting mechanism tracks references to an object through the use of a messaging protocol. A server node keeps a foreign reference count for each of its objects. The foreign reference count indicates the number of remote nodes having a reference to one of the servers objects. A server node increments the foreign reference count for each object reference that it exports prior to sending it to an intended client node. A client node will send a message to the server node when it has already received the exported object reference and in response to this message, the server node decrements the appropriate foreign reference count. When a client node receives an object reference from another client node, the receiving client node will message the appropriate server node that the receiving client node has a reference to the object. In response, the server node increments the appropriate foreign reference count. The reference counting mechanism utilizes a node failure protocol in order to accurately track the foreign reference counts in light of node failures.

Claims

1. A method for tracking references to an object in a computer system including at least one server node and at least one client node, said method comprising the steps of:

providing in each said server node a plurality of objects and object references, each object associated with a server-side foreign reference count indicating a number of client nodes having a reference to the object, each object reference associated with a select one of said objects resident in a particular server node;

incrementing a first server-side foreign reference count associated with a first object reference;

exporting from a first server node said first object reference to a first client node;

exporting from said first client node said first object reference to a second client node;

upon receiving said exported object reference, said second client node sending said first server node a first message indicating receipt of said first object reference; and

upon receipt of said first message, incrementing said first server-side foreign reference count.

2. The method of claim 1, further comprising the steps of:

acknowledging to said second client node, receipt of said first message;

decrementing in said second client node a second foreign reference count associated with said first object reference;

indicating to said first client node receipt of said acknowledgment; and

upon receipt of said indication, decrementing in said first client node a third foreign reference count associated with said first object reference.

3. The method of claim 1, further comprising the steps of:

upon receiving an object reference in a client node already having the received object reference, sending a second message to a server node associated with the received object reference, said second message indicates that the receiving client node already has the received object reference; and

upon receiving said second message, decrementing a server-side foreign reference count associated with the object of the second message.

4. The method of claim 1, further comprising the steps of:

providing in each client node, a client-side foreign reference count indicating a number of nodes to which the client node has exported an associated object reference;

exporting from a third client node an object reference to a third server node having the object corresponding to the exported object reference;

upon receiving said exported object reference, sending a third message to said third client node, said third message indicating that the third server node has said exported object reference; and

upon receiving said third message, decrementing the client-side foreign reference count associated with the exported object reference.

5. The method of claim 4,

said exporting step further comprising the step of:

incrementing a foreign reference count in said third client node associated with said object reference.

6. The method of claim 4, further comprising the steps of:

maintaining a client-side local reference count in each client node for each object reference, said client-side local reference count indicating a count of local references to an object within a client node;

comparing a client-side local reference count and a client-side foreign reference count for an object reference;

sending a fourth message responsive to the step of comparing to a server node associated with the object reference, said fourth message indicating that no outstanding references exist to the object reference by the client node; and

upon receiving said fourth message, decrementing a server-side foreign reference count associated with the object in the fourth message.

7. The method of claim 6, further comprising the steps of:

reclaiming resources associated with an object reference when the client-side local reference count of the object reference and client-side foreign reference count indicate no outstanding references to the object reference exist.

8. The method of claim 1, further comprising the steps of:

maintaining a server-side local reference count in each server node for each object, said server-side local reference count indicating a count of local references to an object within the server node; and

reclaiming resources associated with an object when the server-side local reference count of the object and server-side foreign reference count indicate no outstanding references to the object exist.

9. The method of claim 6,

wherein said first, third, and fourth messages are sent asynchronously.

10. The method of claim 3,

wherein said second message is sent asynchronously.

11. A method for tracking references to an object in a computer system including at least one server node and at least one client node, said method comprising the steps of:

a) providing in each said server node a plurality of objects and a plurality of object references, each said object reference associated with a select one of the objects resident in a particular server node;

b) exporting from one or more server nodes, object references to one or more client nodes;

c) maintaining in each server node a server-side foreign reference count for each object, each server-side foreign reference count indicating a number of client nodes having a reference to an associated object;

d) detecting a node failure in at least one client node; and

after the detecting of a node failure performing the steps of;

e) disabling temporarily each server node from exporting object references;

f) reinitializing each server-side foreign reference count to a fixed, predefined starting value;

g) waiting for all in transit object references to arrive at an intended client node;

h) sending a first message from each client node having an object reference to a server node having an object associated with the object reference;

i) upon receiving said first message, incrementing a server-side foreign reference count associated with the object reference in said first message; and

j) when each server node receives all intended first messages, enabling each server node to export object references to one or more client nodes.

12. The method of claim 11,

said step g further comprising the steps of:

maintaining in each node a first count representing a number of object references exported to other nodes and a second count representing a number of object references received from another node;

sending to each node a second message indicating a number of object references exported; and

upon receiving each said second message, waiting to receive the number of object references indicated in the second message.

13. The method of claim 11, further comprising the steps of:

said step b further comprising the step of exporting from one or more client nodes, object references to other client nodes;

said step c further comprising the step of maintaining in each client node a client-side foreign reference count for each object reference, said client-side foreign reference count indicating a number of receiving client nodes having received an object reference from an exporting client node; and

said step e further comprising the step of disabling temporarily each client node from exporting object references.

14. The method of claim 12, further comprising the steps of:

said step b further comprising the step of exporting from one or more client nodes, object references to other client nodes;

said step c further comprising the step of maintaining in each client node a client-side foreign reference count for each object reference, said client-side foreign reference count indicating a number of receiving client nodes having received an object reference from an exporting client node; and

said step e further comprising the step of disabling temporarily each client node from exporting object references.

15. The method of claim 11, further comprising the steps of:

providing a local reference count for each object in a server node, said local reference count tracking local references to an object within a server node; and

reclaiming resources associated with an object when an object'"'"'s associated local reference count and foreign reference count indicate there are no longer any outstanding references to an object.

16. A computer system including one or more client nodes and at least one server node connected via a communications link, said system comprising:

each said server node comprising;

a plurality of objects, each object having a server-side foreign reference count indicating a number of client nodes having a reference to a particular object;

a plurality of object references, each object reference associated with an object resident in a select one of said server nodes;

a server-side object reference export mechanism having a capability to export a reference to one of said objects to one or more client nodes;

a server-side reference counting mechanism having a capability to maintain each object'"'"'s server-side foreign reference count;

wherein said server-side reference counting mechanism increments a server-side foreign reference count associated with an object prior to the server-side object reference export mechanism exporting a reference to the object to a client node;

each said client node comprising;

a plurality of object references, each object reference associated with an object resident in a select one of said server nodes;

a client-side object reference export mechanism having a capability to export an object reference to one or more other client nodes;

a client-side message sending mechanism having a capability to send a first message to a server node associated with an exported object reference, wherein the exported object reference is received by a first client node and exported by a second client node, said first message indicating that said first client node has obtained the exported object reference;

wherein said server-side reference counting mechanism receives said first message and increments a server-side foreign reference count associated with the object in said first message.

17. The system of claim 16,

wherein said client-side message sending mechanism sends a second message to a server node associated with an exported object reference that was received by a client node from the server node, said second message indicating that the receiving client node already has the exported object reference;

and

wherein said server-side reference counting mechanism receives said second message and decrements a server-side foreign reference count associated with the object reference of the second message.

18. The system of claim 16, further comprising:

each said client node comprising a plurality of client-side foreign reference counts, each client-side foreign reference count associated with each object reference, each client-side foreign reference count indicating a number of nodes that a client node has exported an associated object reference to;

wherein said client-side object reference export mechanism increments a client-side foreign reference count associated with an exported object reference prior to exporting the object reference;

said server node comprising a server-side message sending mechanism having a capability to send a third message to each client node that exports to a server node an object reference whose associated object resides in the receiving server node;

wherein said client-side reference counting mechanism receives said third message and decrements a foreign reference count associated with the object reference of the third message.

19. The system of claim 18, further comprising:

each said object reference associated with a client-side local reference count, said client-side local reference count indicating a number of references to an object reference within a client node;

said client-side message sending mechanism sending a fourth message to a server node associated with an object reference whose associated client-side local reference count and client-side foreign reference count indicate that no outstanding references exist;

said server-side reference counting mechanism receiving said fourth message and decrementing a server-side foreign reference count associated with the object of the fourth message.

20. The system of claim 19, further comprising:

each said client node further comprising a client-side object reclamation mechanism having a capability to reclaim resources associated with an object that is no longer referenced.

21. The system of claim 16, further comprising:

each said object associated with a local reference count, said local reference count indicating a number of local references to an object within a server node;

a server-side object reclamation mechanism having a capability to reclaim resources associated with an object that is no longer referenced.

22. The system of claim 16,

wherein each said message sending mechanism sends each message asynchronously.

23. The system of claim 16, further comprising:

a node failure detection mechanism that determines when one or more of said nodes has failed;

wherein said server-side reference counting mechanism reinitializes each server-side foreign reference count to a fixed, predefined starting value upon notification of a node failure;

wherein said client-side message sending mechanism after detection of a node failure by the node failure detection mechanism, sends a fifth message to each server node whose object reference the client node has received;

wherein said server-side reference counting mechanism increments each server-side foreign reference count for each received fifth message.

24. The system of claim 23, further comprising:

each said node comprising an enablement mechanism having a capability to temporarily disable each respective object reference export mechanism upon notification of a node failure and having a capability to enable each respective object reference export mechanism when each node completes a node failure processing protocol.

25. A computer system having a plurality of computing nodes, said system comprising:

a plurality of objects, each said object stored in a select one of said computing nodes;

a plurality of object references, each object reference associated with an object stored in a first computing node and stored in a second computing node;

a plurality of foreign reference counts, each foreign reference count associated with an object and indicating a number of remote computing nodes referencing an object;

a reference counting mechanism having a capability to maintain each foreign reference count;

an object exportation mechanism having a capability to export object references to one or more remote computing nodes, wherein said object exportation mechanism increments a foreign reference count associated with an object corresponding to an exported object reference prior to exporting the exported object reference;

a node failure detection mechanism having a capability to detect when one or more of said computing nodes has failed;

an enablement mechanism having a capability to temporarily disable the object reference export mechanism upon notification of a node failure and having a capability to enable the object reference export mechanism when each node completes a node failure processing protocol; and

a message sending mechanism having a capability mechanism, after detection of a node failure by the node failure detection mechanism, to send a first message for each object reference resident in a node, said first message sent to a node storing an object associated with the object reference;

wherein said reference counting mechanism, upon notification of a node failure, reinitializes each foreign reference count to a fixed, predefined starting value and upon receiving each said first message increments a corresponding foreign reference count.

26. The system of claim 25,

wherein said message sending mechanism has the capability to send a second message for each object reference received in a first node where the first node already has the received object reference, said second message sent to a second node that sent the received object reference;

and

wherein said reference counting mechanism, upon receiving said second message, decrements the corresponding foreign reference count.

27. The system of claim 25,

wherein said message sending mechanism has the capability to send a third message when a second node receives an object reference from a first node that does not store an object associated with the received object reference, the third message sent to a third node that stores the associated object;

and

wherein said reference counting mechanism, upon receiving said third message, increments the corresponding foreign reference count.

28. The system of claim 25,

wherein said message sending mechanism sends each message asynchronously.

29. The system of claim 25, further comprising:

an object reclamation mechanism having the capability to reclaim resources associated with an object when said reference counting mechanism determines that no outstanding references remain to an object.

30. A computer program product for storing data for access by programs being executed on a data processing system including a plurality of nodes, the computer program product comprising a computer storage medium and a computer program mechanism embedded therein, the computer program mechanism comprising:

a plurality of objects, each said object stored in a select one of said computing nodes;

a plurality of object references, each object reference associated with an object stored in a first computing node and stored in a second computing node;

a plurality of foreign reference counts, each foreign reference count associated with an object and indicating a number of remote computing nodes referencing an object;

a reference counting mechanism having a capability to maintain each foreign reference count;

an object exportation mechanism having a capability to export object references to one or more remote computing nodes, wherein said object exportation mechanism increments a foreign reference count associated with an object corresponding to an exported object reference prior to exporting the exported object reference;

a node failure detection mechanism having a capability to detect when one or more of said computing nodes has failed;

an enablement mechanism having a capability to temporarily disable the object reference export mechanism upon notification of a node failure and having a capability to enable the object reference export mechanism when each node completes a node failure processing protocol; and

a message sending mechanism having a capability after detection of a node failure by the node failure detection mechanism, to send a first message for each object reference resident in a node, said first message sent to a node storing an object associated with the object reference;

wherein said reference counting mechanism, upon notification of a node failure, reinitializes each foreign reference count to a fixed, predefined starting value and upon receiving each said first message increments a corresponding foreign reference count.

31. The mechanism of claim 30,

wherein said message sending mechanism has the capability to send a second message for each object reference received in a first node where the first node already has the received object reference, said second message sent to a second node that sent the received object reference;

and

wherein said reference counting mechanism, upon receiving said second message, decrements the corresponding foreign reference count.

32. The mechanism of claim 30,

wherein said message sending mechanism has the capability to send a third message when a second node receives an object reference from a first node that does not store an object associated with the received object reference, the third message sent to a third node that stores the associated object;

and

wherein said reference counting mechanism, upon receiving said third message, increments the corresponding foreign reference count.

33. The mechanism of claim 30,

wherein said message sending mechanism sends each message asynchronously.

34. The mechanism of claim 30, further comprising:

an object reclamation mechanism having the capability to reclaim resources associated with an object when said reference counting mechanism determines that no outstanding references remain to an object.