Abstract
All Web caches must try to keep cached pages up to date with the master copies of those pages, to avoid returning stale pages to users. In traditional distributed systems terminology, the problem of keeping cached pages up to date is called coherence. We discuss the coherence problem for Web caches, and argue that coherence techniques used for distributed file system caches may not be satisfactory for Web caches. We survey techniques used by popular Web caches for maintaining coherence, including the popular “expiration mechanism” which probably originated in CERN's proxy http server. We discuss a number of problems with the existing expiration mechanism, and present several extensions to it which solve these problems, reduce user wait times and decrease the staleness of returned Web pages. We also discuss pre-fetching and replication, more speculative techniques for keeping Web caches up to date.
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