Strong and Weak Virtual Synchrony in Horus
نویسندگان
چکیده
A formal de nition of strong virtual synchrony capturing the semantics of virtual syn chrony as implemented in Horus is presented This de nition has the nice property that every message is delivered within the view in which it was sent However it is shown that in order to implement strong virtual synchrony the application program has to block messages during view changes An alternative de nition called weak virtual synchrony which can be implemented without blocking messages is then presented This de nition 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 is developed and it is shown how to use a system that provides weak virtual synchrony even when strong virtual synchrony is actually needed To capture additional ordering requirements the de nition of ordered virtual synchrony is presented Finally it is discussed how to extend the de nitions in order to cope with the fact that a process can become a member of more than one group This work was supported by ARPA ONR grant N J
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