What is high availability?
High availability is a system design protocol and associated implementation that ensures a certain absolute degree of operational continuity during a given measurement period.
Today's real-time enterprise solutions often deploy a messaging middleware that enables communication between various sub-components. This middleware is entrusted with important data that should be delivered reliably and as fast as possible to the recipient application. The middleware server might also be required to store this data in its data store until it is picked up. A failure of this middleware message bus might bring the entire system down within seconds. Hence, it is absolutely imperative for the messaging backbone to provide its backup, which allows messaging operations to resume quickly in the event of a failure of the running server. This backup server should restore the state prior to failure of the original message server. Any data that was stored previously in the server's data store should be accessible through this backup server and most importantly this operation of shifting from one server to its backup should be automatic and transparent to the client application.

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