Replacement policies for general caching applications andWeb caching in particular have been extensively addressed in the literature. Many policies that focus on document costs, size, probability of references and temporal locality of requested documents have been proposed.

In many cases these policies are ad-hoc attempts to take advantage of the statistical information contained in the stream of requests, and to address the factors above. However, since the introduction of optimal replacement policies for conventional caching, the problem of finding optimal replacement policies under the factors indicated has not been studied in any systematic manner.

In this paper, we take a step in that direction: We first show, still under the Independent Reference Model, that a simple Markov stationary replacement policy, called the policy $C_0$, minimizes the long-run average metric induced by non-uniform document costs when document eviction is optional.

We then use these results to propose a framework to operate caching systems with multiple performance metrics. We do so by solving a constrained caching problem with a single constraint.The resulting constrained optimal replacement policy is obtained by simple randomization between two Markov stationary optimal replacement policies $C_0$ but induced by different costs.