Aging through cascaded caches: Performance issues in the distribution of web content

The Web is a distributed system, where data is stored and disseminated from both origin servers and caches. Origin servers provide the most up-to-date copy whereas caches store and serve copies that had been cached for a while. Origin servers do not maintain per-client state, and weak-consistency of cached copies is maintained by the origin server attaching to each copy an expiration time. Typically, the lifetime-duration of an object is fixed, and as a result, a copy fetched directly from its origin server has maximum time-to-live (TTL) whereas a copy obtained through a cache has a shorter TTL since its age (elapsed time since fetched from the origin) is deducted from its lifetime duration. Thus, a cache that is served from a cache would incur a higher miss-rate than a cache served from origin servers. Similarly, a high-level cache would receive more requests from the same client population than an origin server would have received. As Web caches are often served from other caches (e.g., proxy and reverse-proxy caches), age emerges as a performance factor. Guided by a formal model and analysis, we use different inter-request time distributions and trace-based simulations to explore the effect of age for different cache settings and configurations. We also evaluate the effectiveness of frequent pre-term refreshes by higher-level caches as a means to decrease client misses. Beyond Web content distribution, our conclusions generally apply to systems of caches deploying expiration-based consistency.