Methods and apparatus for efficient caching in a distributed environment

• US 5,864,837 A
• Filed: 06/12/1996
• Issued: 01/26/1999
• Est. Priority Date: 06/12/1996
• Status: Expired due to Term

• Current Assignee: Unisys Corporation
• Original Assignee: Unisys Corporation
• Inventors: Maimone, William D.
• Primary Examiner(s): Black, Thomas G.
• Assistant Examiner(s): HO, RUAY L
• Application Number:
  US08/662,305
• Time in Patent Office: 958 Days
• Field of Search: 395/601, 395/607, 395/608, 395/609, 395/610, 395/611, 395/616, 395/701, 395/702, 395/703, 395/704, 395/705, 395/712, 395/716, 395/683, 395/828, 341/51
• US Class Current: 1/1
• CPC Class Codes: G06F 12/0813 with a network or matrix configuration
• CPC Class Codes: G06F 12/0815 Cache consistency protocols
• CPC Class Codes: G06F 16/10 File systems; File servers
• CPC Class Codes: Y10S 707/99931 Database or file accessing
• CPC Class Codes: Y10S 707/99938 Concurrency, e.g. lock management in shared database

Abstract

Disclosed are a method and system for transferring data objects from a server to a client in a distributed computing system. The disclosed method comprises the steps of: receiving, at the server, a request from the client for the data object; determining whether a cached copy of the requested object retained by the client is the same as a current version of the requested object retained by the server, wherein the cached copy and the current version are stored in a compressed form; and transmitting the current version of the requested object from the server to the client in the compressed form if the cached copy of the requested object is not the same as the current version of the requested object. The step of determining whether a cached copy of the requested object retained by the client is the same as a current version of the requested object comprises comparing a first relatively unique number computed on the basis of the current version of the requested object with a second relatively unique number computed on the basis of the cached copy of the requested object. The cached copy of the requested object is assumed to be the same as the current version of the requested object if the first relatively unique number is equal to the second relatively unique number.

Claims

1. A method for use in a distributed computing system in which a client worksite requests an updated data object from a remote server having access to the latest version of said object, comprising the steps of:

a. generating at the client worksite a first unique number which is indicative of the content of an object previously stored at the client worksite;

b. generating at the client worksite a request for the latest version of the object presently stored on a network;

c. transmitting the unique number and a request to a server having access to the latest version of said object, over said network;

d. receiving the request and the unique number at the server;

e. computing, at the server, a second unique number indicative of the latest version of the object stored at the server;

f. comparing, at the server, said first unique number with said second unique number; and

g. transmitting encoded data from said server to said client worksite which informs the client worksite whether or not the object stored at the client worksite is the latest updated version of the object requested.

2. A method as recited in claim 1, wherein said first and second unique numbers comprise a checksum of the requested data object.

3. A method as recited in claim 1, wherein said first and second unique numbers comprise a message digest of the requested data object.

4. A method as recited in claim 1, wherein said first and second relatively unique numbers comprise a hash code.

5. A method as recited in claim 1, wherein said first unique number is computed on the basis of a compressed form of the latest version of the requested object, and the second unique number is computed on the basis of a compressed form of the cached copy of the requested object.

6. A method as recited in claim 1, further comprising the step of transmitting, from said server to said client, said latest version of said object.

7. A method as recited in claim 1, wherein said latest version of the requested object and said cached copy are stored in an LZW compressed form.

8. A method as recited in claim 1, wherein said first and second unique numbers are computed by the server prior to the comparison step.

9. A method as recited in claim 1, wherein said first unique number is computed by the server, and the second unique number is computed by the client.

10. A method as recited in claim 1, wherein said first and second unique numbers comprise a checksum;

wherein said latest version of the requested object and said cached copy are stored in a compressed form; and

wherein said first unique number is computed on the basis of said compressed form of the latest version of the requested object, and the second unique number is computed on the basis of said compressed form of the cached copy of the requested object.

11. A method for transferring a data object from a server to a client in a distributed computing system, comprising the steps of:

a. computing a first unique number representing the version of said data object that is retained by said client;

b. transmitting a request for a latest version of said data object and said first unique number to said server;

c. computing a second unique number representing the latest version of said data object that is retained by said server;

d. comparing said first unique number with said second unique number; and

e. transmitting said latest version of said data object that is retained by said server to said client if said first unique number is equal to said second unique number.

12. A method as recited in claim 11, wherein said first and second unique numbers comprise a checksum.

13. A method as recited in claim 11, wherein said first and second unique numbers comprise a message digest.

14. A method as recited in claim 11, wherein said first and second unique numbers comprise a hash code.

15. A method as recited in claim 11, wherein said first unique number is computed on the basis of a compressed form of the current version of the requested object, and the second relatively unique number is precomputed, when it is first provided to the client, on the basis of a compressed form of the cached copy of the requested object.

17. A method as recited in claim 11, wherein said first and second unique numbers comprise a checksum.

18. A method as recited in claim 11, wherein said first and second unique numbers comprise a message digest.

19. A method as recited in claim 11, wherein said first and second unique numbers comprise a hash code.

16. A computer system comprising:

a. a server connected via a communications link to at least one client;

b. a client memory coupled to said client;

c. a server memory coupled to said server;

d. a first data object stored in said client memory;

e. a second data object, corresponding to said first data object, stored in said server memory;

f. first computing means, coupled to said client, for computing a first unique number representing the version of said first data object stored in said client memory;

g. second computing means, coupled to said server, for computing a second unique number representing the version of said second data object stored in said server memory;

h. comparison means, coupled to said server, for comparing said first unique number with said second unique number, wherein said first data object is assumed to be the same version as said second data object if said first unique number is the same as said second unique number; and

i. means for transmitting said second data object to said client when said first unique number is not the same as said second unique number.