Microsoft Exchange works by storing all information in a users Mailbox, with a master copy saved on the server. Because the master copy is on the server, this allows a user to connect to their Mailbox using many different methods (such as Microsoft Outlook, with a web browser or a mobile device) and have each instance of the users mailbox always up to date.
High availability is a key design goal of Microsoft Exchange, and it has been designed with several important features to ensure a single server failure does not cause an outage of the whole messaging environment. Servers can be configured in clusters, so if one should fail the remaining members of the cluster can still service user requests.
Later versions of Microsoft Exchange (Exchange 2007 and greater) introduce the concept of separation of Exchange roles so each component can be made highly available as well as distributing the load between multiple servers.
Backup and Restore
The backup capabilities of Microsoft Exchange are designed so that it can remain online and continue servicing requests, while still generating a complete and consistent backup. It is important that Active Directory is also frequently backed up as a lot of configuration data for Microsoft Exchange is stored in Active Directory.
As part of the recovery strategy, Microsoft Exchange utilises a write-ahead transaction log for all operations it performs on an Exchange database. Write-ahead transaction logs work by writing all intended modifications to the Exchange database to a transaction log file before it performs the operation on the actual database. It is best practice to store transaction logs on a separate physical disk than the Exchange databases so that in the event of a failure, any transactions made since the last full backup can be rolled forward to restore the database to the point of failure.
Two types of online backup can be performed: a full backup and a transaction log backup. Both types of backup will flush the transaction logs when completed.
- A full backup will back up the Exchange databases. This may take many hours and is typically done over a weekend.
- A transaction log backup will back up only the transaction logs created since the last backup. This takes a relatively small amount of time and is typically done each night.
If the disk containing the transaction logs is filled to capacity, Exchange will dismount any associated databases and will not accept mail for Mailboxes in these databases until the problem is rectified.