Abstract
We investigate the problem of designing a scalable overlay network to support decentralized topic-based pub/sub communication. We introduce a new optimization problem, called Minimum Topic-Connected Overlay (Min-TCO), that captures the tradeoff between the scalability of the overlay (in terms of the nodes' fanout) and the message forwarding overhead incurred by the communicating parties. Roughly, the Min-TCO problem is as follows: Given a collection of nodes and their subscriptions, connect the nodes using the minimum possible number of edges so that for each topic t, a message published on t could reach all the nodes interested in t by being forwarded by onlythe nodes interested in t.
We show that the decision version of Min-TCO is NP-complete, and present a polynomial algorithm that approximates the optimal solution within a logarithmic factor with respect to the number of edges in theconstructed overlay. We further prove that this approximation ratio is almost tight by showing that no polynomial algorithm can approximate Min-TCO within a constant factor (unless P=NP). We show experimentally that on typical inputs, the fanout of the overlay constructed by our approximation algorithm is significantly lower thanthat of the overlays built by the existing algorithms, and that its running time is just a small fraction of the analytical worst case bound. As Min-TCO can be shown to capture several important aspects of most known overlay-based pub/sub implementations, our study sheds light on the inherent limitations of the existing systems as well asprovides an insight into the best possible feasible solution.
Finally, we introduce a flexible framework that generalizes Min-TCO and formalizes most similar overlay design problems that occur in scalable pub/sub systems. We also briefly discuss several examples of such problems, and show some results with respect to their complexity.