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The Effect of Consistency on Cache Response Time

Dilley, John


Keyword(s): World Wide Web; proxy cache; response time; consistency validation; distributed object consistency protocol

Abstract: Caching in the World Wide Web improves response time, reduces network bandwidth demand and reduces load on origin web servers. Caches achieve these benefits by storing copies of recently requested objects near end users, avoiding future need to transfer those objects. Cached objects are usually served more quickly and do not consume external network or server resources. Before returning an object, a cache must guess if the object it holds is still consistent with the original copy of the object. The cache may choose to validate the object's consistency with the origin server or may serve it directly to the user. If the cache must communicate with the origin server the response will take longer than one directly from cache. This report analyzes the impact of cache consistency on the response time of client requests. The analysis divides cache responses into classes according to whether or not the cache communicated with a remote server and whether or not object data was served from the cache. Analysis of traces from deployed proxy cache servers demonstrates that a round trip to a remote server is the dominant factor for response time. This study concludes that improving cache consistency will reduce response time and allow a cache to serve more user requests.

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