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The Temenos T24 application can make use of many of the high availability features that come with SQL Server 2008 R2 Enterprise and Data Center editions.  In part 1 of this series about high availability options for Temenos T24, we took a look at Failover Clustering.  In this post, we'll discuss Database Mirroring.

Database mirroring transfers transaction log records directly from a principal server to a mirror server and can be configured to quickly "failover" to the standby server. You can also code client applications to automatically redirect their connection information, and in the event of a failover, automatically connect to the mirror server and database. Database mirroring does not require proprietary hardware and is easy to set up and manage. 

The above diagram shows a Database Mirroring topology.

There are 3 operating modes that we will go over in this article:

High Safety/Protection Mode:  This operating mode is also known as synchronous mirroring without a witness.  With synchronous mirroring a transaction is not committed on the principal until all transaction log records have been copied successfully to the mirror.  The benefit of this operating mode is that you are guaranteed that if your principal fails that your principal and mirror are synchronized.  This operating mode does not make use of a witness server so failover is a manual process.

High Availability Mode:  This operating mode is also known as synchronous mirroring with a witness.  High Availability Mode is similar to High Safety/Protection Mode with 1 major difference.  High Availability mode can be configured to provide automated failover with the use of another SQL Server instance called a "witness server".  The witness server is the deciding vote on whether or not the principal can be contacted from the mirror.  If the mirror and the witness agree and determine that the principal is not able to be contacted the mirror can initiate a failover automatically.

High Performance Mode:This operating mode is also known as asynchronous mirroring.  High Performance mode does not ensure that transactions are copied over to the mirror before committing them on the principal.  This can result in some data loss but it's not guaranteed.  In High Performance mode failover to the mirror is a manual process.

NOTES:  Because the mirror database is in a recovering state, it cannot be accessed directly. You can create database snapshots on the mirror to indirectly read the mirror database at a point in time.  Please review the How to: Configure a Database Mirroring Session article on MSDN for more information.

SQL Server 2008 added additional functionality to it's Database Mirroring feature:

  • Automatic Page Repair - If a corrupt page is found on the principal (or mirror) database, the database mirroring system will attempt to obtain the page image from the mirror (or principal) database and then repair the corruption.
  • Log Stream Compression - By compressing the log records before sending them to the mirror database less data needs to travel across your network which can improve the performance of a synchronous mirroring topology. 

Stay tuned for future blog posts on other High Availability options for Temenos T24

Please review the Best Practices for Running TEMENOS T24 on Microsoft SQL Server and Windows Serverwhitepaper for more information.