Eliminating Tight Coupling using Subscriptions Subgrouping in Structured Overlays

Cyclic overlays may provide multiple paths between a publisher and a subscriber, an advertisement-tree and a matching subscription enables only one path for notifications routing in Publish/Subscribe (PS) systems. This poses serious challenges in offering dynamic routing when network conditions like congestion, and link or broker failures are detected. We argue that the tight coupling between an advertisement and matching subscriptions is the main bottleneck in providing multiple routing paths. This paper introduces subscription subgrouping that divides subscriptions into multiple sets, called subscription subgroups, to eliminate tight coupling. We use the structuredness of a new Structured Cyclic Overlay Topology (SCOT) to create multiple routing paths between a publisher and a subscriber without saving link information in routing tables. A homogeneous clustering technique with a bit-vector scheme is developed to realize subscription subgrouping and offer inter-cluster dynamic routing. The advertisement and cluster-level subscription broadcast algorithms implemented in a prototype tool, called OctopiA, which creates routing paths of shortestlengths. Experiments on a cluster testbed show that OctopiA reduces size of advertisement routing tables by 93%, subscription delay by 33%, static and dynamic notification delivery delays by 25% and 54%, respectively.