Strong and weak virtual synchrony in Horus

This paper presents two variants of virtual synchrony, which are supported by Horus. The first variant, called strong virtual synchrony, includes the property that every message is delivered within the view in which it is sent. This property is very useful in developing applications, since it helps in minimizing the amount of context information that needs to be sent on messages, and the amount of computation which is required in order to process a message. However, it is shown that in order to support this property, the application program has to block messages during view changes. An alternative definition, called weak virtual synchrony, which can be implemented without blocking messages, is then presented. This definition still guarantees that messages will be delivered within the view in which they were sent, only that it uses a slightly weaker notion of what the view in which a message was sent is. An implementation of weak virtual synchrony that does not block messages during view changes as also developed in this paper.