Synchronous and Asynchronous Group Communication.

The article describes an approach to integrate synchronous and asynchronous group communication services. It also highlights similarities between synchronous and asynchronous programming by discussing only strict agreement--the kind of asynchronous agreement closest to synchronous agreement. Asynchronous system allows timely communication most of the time. Synchronous systems rely on real-time diffusion to make communication between correct processors certain. Processor p diffuses a message to processor q by sending message copies in parallel on all paths between p and q. In a synchronous context, time means Clock time, while in an asynchronous context, time means real time. Synchronous group communication simplifies replicated programming considerably, since each replica has the same, accurate up to date knowledge of the system state. Synchronous and asynchronous programming are different system design philosophies, the first assuming communication is certain, the second assuming it is not.