Technical Report CS0964

Title: Group Communication Specifications: A Comprehensive Study
Authors: Roman Vitenberg, Idit Keidar, Gregory V. Chockler and Danny Dolev
Abstract: View-oriented group communication is an important and widely used building block for many distributed applications. Much current research has been dedicated to specifying the semantics and services of {\em view-oriented Group Communication Systems (GCSs)}. However, the guarantees of different GCSs are formulated using varying terminologies and modeling techniques, and the specifications vary in their rigor. This makes it difficult to analyze and compare the different systems. This paper provides a comprehensive set of clear and rigorous specifications, which may be combined to represent the guarantees of most existing GCSs. In the light of these specifications, over thirty published GCS specifications are surveyed. Thus, the specifications serve as a unifying framework for the classification, analysis and comparison of group communication systems. The survey also discusses over a dozen different applications of group communication systems, shedding light on the usefulness of the presented specifications. Defining meaningful GCSs is challenging; such systems typically run in asynchronous environments in which agreement problems that resemble the services provided by group communication services are not solvable. Therefore, many of the suggested specifications turned out to be too trivial, and in particular, solvable by weaker algorithms than the actual implementations. In this paper, the non-triviality issues are addressed by guaranteeing conditional liveness and by using external failure detectors. The resulting specifications are non-trivial on one hand, and allow implementation on the other. This paper is aimed at both system builders and theoretical researchers. The specification framework presented in this paper will help builders of group communication systems understand and specify their service semantics; the extensive survey will allow them to compare their service to others. Application builders will find in this paper a guide to the services provided by a large variety of GCSs, which would help them chose the GCS appropriate for their needs. The formal framework may provide a basis for interesting theoretical work, for example, analyzing relative strengths of different properties and the costs of implementing them.
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