Caching under Content Freshness Constraints

TL;DR

This paper investigates cache replacement policies that account for content freshness demands, providing theoretical bounds, analytical models, and a new policy that improves cache performance in delay-sensitive applications.

Contribution

It introduces a new cache policy that explicitly considers user freshness requirements and offers analytical models and bounds for existing policies under freshness constraints.

Findings

Universal upper bound on cache performance.

Asymptotic optimality conditions for LP policy.

Approximate LRU hit-rates using Coupon Collector's model.

Abstract

Several real-time delay-sensitive applications pose varying degrees of freshness demands on the requested content. The performance of cache replacement policies that are agnostic to these demands is likely to be sub-optimal. Motivated by this concern, in this paper, we study caching policies under a request arrival process which incorporates user freshness demands. We consider the performance metric to be the steady-state cache hit probability. We first provide a universal upper bound on the performance of any caching policy. We then analytically obtain the content-wise hit-rates for the Least Popular (LP) policy and provide sufficient conditions for the asymptotic optimality of cache performance under this policy. Next, we obtain an accurate approximation for the LRU hit-rates in the regime of large content population. To this end, we map the characteristic time of a content in the LRU policy to the classical Coupon Collector's Problem and show that it sharply concentrates around its mean. Further, we develop modified versions of these policies which eject cache redundancies present in the form of stale contents. Finally, we propose a new policy which outperforms the above policies by explicitly using freshness specifications of user requests to prioritize among the cached contents. We corroborate our analytical insights with extensive simulations.