Maintaining Strong Cache Consistency in the World-Wide Web

Chengjie Liu and Pei Cao, Computer Sciences Department, University of Wisconsin-Madison, Madison, WI 53706

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As the Web continues to explode in size, caching becomes increasingly important. With caching comes the problem of cache consistency. Conventional wisdom holds that strong cache consistency is too expensive for the Web, and weak consistency methods such as Time-To-Live (TTL) are most appropriate. This study compares three consistency approaches: {\em adaptive TTL}, {\em polling-every-time} and {\em invalidation}, using prototype implementation and trace replay in a simulated environment. Our results show that {\em invalidation} generates less or a comparable amount of network traffic and server workload than {\em adaptive TTL} and has a slightly lower average client response time, while {\em polling-every-time} generates more network traffic and longer client response times. We show that, contrary to popular belief, strong cache consistency can be maintained for the Web with little or no extra cost than the current weak consistency approaches, and it should be maintained using an invalidation-based protocol.