Retrieval and freshness thresholds in hierarchical caching systems

To scale up with the explosive Web growth, caching systems have been proposed and deployed over the Internet in recent years. Among them, hierarchical caching systems employing expiration-based consistency control mechanisms have become a viable and efficient solution. In this paper, we first analyze the performance of such hierarchical caching systems from the perspectives of both cache servers and end users. Then, we examine retrieval and freshness threshold-based approaches and their impact on system performance and user-perceived QoS. We show that by setting these thresholds appropriately, it is possible that (1) users can impose a consistency QoS requirement on the object that they wish to obtain without too much trade-off in system performance, and (2) performance bias against leaf users due to their unfavorable locations in the hierarchical structure can be mitigated. (C) 2003 Elsevier B.V. All rights reserved.