SQL Server Backend High-Availability

Lync Server relies on a SQL Server backend to support the environment. In previous versions, a SQL Enterprise Cluster was required to provide high-availability to SQL. However, Lync Server 2013 introduces support for SQL Server mirroring, and SQL Clustering is no longer supported. SQL mirroring introduces a primary and secondary model for the backend database. Essentially, the primary publish (SQL Server 1) will copy all databases to the secondary subscriber (SQL Server 2). This happens after every SQL transaction has been completed. Each transaction is then committed to the mirror, resulting in an always-up-to-date copy of the information.

In the event of a SQL Server failure, the mirror server takes over. This failover can be invoked by an administrator, or can happen automatically. For the failover to happen automatically, a third server must be deployed as a witness. The witness will monitor the availability of the SQL Servers, and in the event of a failure, it will invoke the failover to the mirror.

To deploy this functionality, the minimum supported SQL version is SQL Server 2008 R2; you must also deploy all SQL Servers in the mirroring configuration on the same SQL version.

Note

It is possible to deploy the primary and mirror with one SQL version, and deploy the witness with a separate SQL version. However, best practice is to keep the versions the same across the board.

For SQL best practices in terms of which SQL versions are supported for a witness role, see “Database Mirroring Witness” in the MSDN Library at http://go.microsoft.com/fwlink/?LinkId=247345.

The actual configuration of SQL mirroring is covered in the “Configuring SQL Server Mirroring” section. For an overview of how SQL Mirroring provides high-availability, see Figure 15.1.

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Figure 15.1. SQL Server mirroring in Lync Server 2013.

RTO and RPO for SQL Mirroring

Microsoft has tested and designed for a 5-minute RTO and 5-minute RPO for SQL Server mirroring. This means that the service can be brought back online within 5 minutes, and only up to 5 minutes of data could be lost.

User Impact During Failover

In the event of a SQL Server failure, very little use impact is expected. If a witness is configured, the failover will happen so quickly that users are not likely to notice the system disconnect. However, the Lync client will reregister in the event of a SQL mirror failover. If no witness is configured, users will enter resiliency mode. This will provide a notification that the system is down, and some features will not be available to the users until the administrator has initiated the failover.

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