In this paper, we model and analyze the effect of age on the performance of various cache configurations. We consider a low-level cache that fetches objects either from their origin servers or from other caches and analyze its miss-rate as function of its fetching policy. We distinguish between three basic fetching policies, namely, fetching always from the origin, fetching always from the same high-level cache, and fetching from a “random” high-level cache. We explore the relationships between these policies in terms of the miss-rate achieved by the low-level cache, both on worst-case sequences, and on sequences generated using particular probability distributions.

Guided by web caching practice, we consider two variations of the basic policies. In the first variation the high-level cache uses pre-term refreshes to keep a copy with lower age. In the second variation the low-level cache uses extended lifetime duration. We analyze how these variations affect the miss-rates. Our theoretical results help to understand how age may affect the miss-rate, and imply guidelines for improving performance of web caches.