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The Distributed Object Consistency Protocol

Dilley, John; Arlitt, Martin; Perret, Stephane; Jin, Tai


Keyword(s): World Wide Web; cache consistency; cache invalidation; publish/subscribe protocol; proxy cache; HTTP cache control; strong consistency; delta consistency

Abstract: This report describes a protocol for improving content consistency in web proxy cache servers, which leads to reduced response time and server load. The Distributed Object Consistency Protocol (DOCP) is an extension to HTTP, replacing some of HTTP's current cache control mechanism. It supports incremental evolution: proxy servers that implement the portocol can interoperate with non-DOCP proxy servers and gradually learn about peers as they are deployed in the network. The DOCP uses a publish/subscribe mechanism with server invalidation when objects change instead of client validation to test for changes. Stronger semantic consistency assures content providers that pages served from cache are the same as are on the master origin server, and assures end users that the information they are getting is fresh, limiting the need to "Reload". Strong consistency is difficult to achieve in wide area networks, so the DOCP provides "delta consistency", where an object is consistent for all but a short, bounded time after a modification. This allows the DOCP to meet web user expectations for content availability and responsiveness. Protocol simulation shows that DOCP consumes fewer network and server resources than the current HTTP cache consistency protocol does, while providing greatly improved consistency. DOCP eliminates the need for users and content providers to guess at correct values for HTTP cache control headers by providing a clean framework for wide area web object replication.

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