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This is Part II of the article “SQL Server High Availability Choices.” Click here for Part I.

In Part 1 of this article we covered failover clustering. Here in Part II will continue the high availability discussion with database mirroring to provide the necessary information to ensure that you are choosing the right solution for your environment.

Database mirroring requires that your application is mirror aware. Making an application mirror aware could be as simple as modifying a connection string to be aware of the failover partner, or it could involve more complex changes.

The configuration for database mirroring is much less complex than with clustering. There is still the need for at least two servers but there is no requirement for an Active Directory domain, shared storage device, and multiple networks (a backend network is recommended). To provide the automatic failover functionality, you will need to have a third SQL Server instance known as the witness.

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When thinking of high availability in SQL Server, there are two choices that come to mind: Database Mirroring and Failover Clustering.

Choosing the right high availability solution will come down to cost, complexity of implementation, and scope of availability. Both Database Mirroring and Failover Clustering accomplish a similar goal in very different ways. The details covered in this post are based on a two node failover cluster for Clustering and high-safety mode with automatic failover for Database Mirroring.

The additional expense that goes along with Clustering is associated to the need of a shared storage device, an enterprise edition Windows Server. Implementation is more complex than Database Mirroring and has additional requirements which add to the complexity such as the need for an Active Directory domain and multiple networks. Since Clustering works at the server level, a failover will apply to the SQL Server Service and the configured Disk resources which contain all of your databases and logs.

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