Subscription propagation in highly-available publish/subscribe middleware

Achieving availability and scalability while providing service guarantees such as in-order, gapless delivery is essential for deploying publish/subscribe messaging middleware in wide area networks. Scalability often requires a publish/subscribe system to propagate subscription information and perform content matching across the network. Existing subscription propagation algorithms do not support in-order, gapless delivery in a redundant overlay network.This paper presents a novel approach that utilizes virtual time (VT) vectors to convey temporal consistency in propagating incremental and consolidated subscription information. The VT vectors provide a means of testing sufficiency of filtering information, by comparing a broker's VT vector with that of a message. When the test fails, indicating insufficient broker subscription information, safety may be preserved by "flooding" the message to all neighbors on a routing tree. This approach does not require subscription state agreement across redundant paths and hence is highly available. We present a detailed evaluation of the approach.