On adding replies to publish-subscribe

Recently, the publish-subscribe communication model has attracted the attention of developers as a viable alternative to traditional communication schemas, like request/reply, for the flexibility it brings to the architecture of distributed applications, by allowing components to be easily added or removed at run-time. At the same time, first experiences in building complex distributed applications using such model point out how it is often hard to live without a request/reply facility.

We started from this consideration to introduce replies into the publish-subscribe model in a way that could minimize the impact on the positive characteristics of the model. In this paper we describe the resulting model and present four protocols to implement it, comparing them through the analysis of the results we gathered in running a large testbed on the PlanetLab network.