A survey of web caching schemes for the Internet <s> Cache coherency <s> 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. <s> BIB001 </s> A survey of web caching schemes for the Internet <s> Cache coherency <s> The bandwidth demands of the World Wide Web continue to grow at a hyper-exponential rate. Given this rocketing growth, caching of web objects as a means to reduce network bandwidth consumption is likely to be a necessity in the very near future. Unfortunately, many Web caches do not satisfactorily maintain cache consistency. This paper presents a survey of contemporary cache consistency mechanisms in use on the Internet today and examines recent research in Web cache consistency. Using trace-driven simulation, we show that a weak cache consistency protocol (the one used in the Alex ftp cache) reduces network bandwidth consumption and server load more than either time-to-live fields or an invalidation protocol and can be tuned to return stale data less than 5% of the time. <s> BIB002
Caches provide lower access latency with a side effect: every cache sometimes provide users with stale pages -pages which are out of date with respect to their master copies on the Web servers where the pages originated BIB001 . Every Web cache must update pages in its cache so that it can give users pages which are as fresh as possible. Caching and the problem of cache coherency on the World Wide Web are similar to the problems of caching in distributed file systems. However, the Web is different than a distributed file system in its access patterns, its larger scale, and its single point of updates for Web objects BIB002 .