Bridging the HLA: A Case Study in Composing Publish-Subscribe Systems

The recent popularity of publish-subscribe (pub-sub) system architectures has led to a desire for a refined architecture that supports the composition of pub-sub systems. One proposed solution links such systems using a special bridge component that acts as a mediator, passing events between the systems. The bridge appears to be an ordinary pub-sub component to each system, effectively serving as a surrogate for the other system. Although attractive as a lightweight pub-sub combinator, the notion of a pub-sub bridge raises a number of questions, such as whether its use introduces new sources of deadlock or inconsistency. In this paper, we show that designing such a bridge is far from trivial, and indeed requires special treatment to achieve desired properties. To make these issues concrete, we describe our results in analyzing the feasibility of a bridge for the HLA, a standardized pub-sub framework designed for distributed simulation applications. We identify a small set of core problem classes for pub-sub bridge designs. Additionally, we also classify a set of generic solution paths and show how each applies to the problem classes. Although based on the HLA, we believe that these problems and solutions are applicable to many pub-sub systems as well as to other architectures for loosely coupled distributed systems.