Growing Half-Balls: Minimizing Storage and Communication Costs in Content Delivery Networks

The dynamic content distribution problem addresses the trade-off between storage and delivery costs in modern virtual content delivery networks (CDNs). That is, a video file can be stored in multiple places so that the request of each user is served from a location that is near the user. This minimizes the delivery costs, but is associated with a storage cost. This problem is NP-hard even in grid networks. In this paper, we present a constant factor approximation algorithm for grid networks. We also present an O(log delta)-competitive algorithm, where delta is the normalized diameter of the network, for general networks with general metrics. We show a matching lower bound by using a reduction from online undirected STEINER TREE. Our algorithms use a rather intuitive approach that has an elegant representation in geometric terms.