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Continuous availability
Asynchronous replication software is one way to ensure Microsoft Exchange remains continuously available in the event the primary Exchange server goes offline. New features in asynchronous replication software products provide direct integration with Exchange to allow Exchange server failovers to occur in seconds or minutes with minimal or no disruption to enterprise users and even without administrator intervention.

EMC's RepliStor software gives admins a couple of different ways to deliver continuous availability. One is to configure two servers in the same domain at the same site with the same software and logical configuration, but to use unique hostnames on the production and standby servers. RepliStor is then installed on both servers and replicates and synchronizes the data between the production and standby servers. When a failure occurs on the primary server (an interruption of more than 120 seconds of the heartbeat between the two servers), RepliStor initiates the failover and starts Exchange services on the target server.

For offsite recoveries where servers may be in different domains, admins may configure RepliStor to work with Microsoft's VSS framework. RepliStor still replicates the primary Exchange database to the remote site, but it only initiates and retains snapshots on the standby server using Microsoft's VSS provider that Replistor calls through its built-in scheduler and snapshot policy manager. Exchange

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failovers are performed manually with admins selecting the most recent good point-in-time snapshot and then applying database and transaction logs to roll forward to the most current point-in-time.

CA's XOsoft WANSyncHA goes one step further and lets admins choose whether they want failovers at another site to occur automatically or if they want to be notified so the failover sequence can be initiated manually. Though automatic failover is a nice-to-have, there are many reasons servers at two different sites may lose communication, but these issues don't mean the production Exchange server is down and that the remote site should take over.

XOsoft WANSyncHA uses continuous data protection (CDP) to replicate data, remains aware of the state of the Exchange database during replication and supports cross-network failover. The use of CDP permits automated recovery up to the point of failure of the primary Exchange server.

This was first published in September 2007

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