Optimal Caching and Delivery of Dynamic Contents in a Wireless Edge System

We consider an edge caching system with a finite capacity edge-cache connected to a backend server via a wireless channel. The backend server stores the latest versions of dynamic contents. Users request the edge server for the contents, which can either fetch fresh versions from backend and serve or can serve locally cached versions or can even deny service. The edge server must decide which items to cache due to limited capacity. Fetching from the backend server incurs a fetching cost, serving a stale version incurs an ageing cost proportional to the content’s age-of-version (AoV), and denying service incurs a missing cost. We address optimal content fetching, caching and delivery problem to minimize the expected time average cost. The optimal control problem, a Markov decision process (MDP), suffers from curse-of-dimensionality. We frame the problem as a restless multiarmed bandit (RMAB), show that it is indexable, and design a Whittle index based joint policy for content fetching, caching and delivery. We provide explicit expressions for the Whittle indices. Finally, we demonstrate that our proposed policy performs very close to optimal.