This topic introduces several SQL Server high-availability solutions that improve the availability of servers or databases. A high-availability solution masks the effects of a hardware or software failure and maintains the availability of applications so that the perceived downtime for users is minimized.
SQL Server provides several options for creating high availability for a server or database. The high-availability options include the following:
AlwaysOn Failover Cluster Instances
As part of the SQL Server AlwaysOn offering, AlwaysOn Failover Cluster Instances leverages Windows Server Failover Clustering (WSFC) functionality to provide local high availability through redundancy at the server-instance level—a failover cluster instance (FCI). An FCI is a single instance of SQL Server that is installed across Windows Server Failover Clustering (WSFC) nodes and, possibly, across multiple subnets. On the network, an FCI appears to be an instance of SQL Server running on a single computer, but the FCI provides failover from one WSFC node to another if the current node becomes unavailable.
AlwaysOn Availability Groups is an enterprise-level high-availability and disaster recovery solution introduced in SQL Server 2012 to enable you to maximize availability for one or more user databases. AlwaysOn Availability Groups requires that the SQL Server instances reside on Windows Server Failover Clustering (WSFC) nodes. For more information, see AlwaysOn Availability Groups (SQL Server).
This feature will be removed in a future version of Microsoft SQL Server. Avoid using this feature in new development work, and plan to modify applications that currently use this feature. We recommend that you use AlwaysOn Availability Groups instead.
Database mirroring is a solution to increase database availability by supporting almost instantaneous failover. Database mirroring can be used to maintain a single standby database, or mirror database, for a corresponding production database that is referred to as the principal database. For more information, see Database Mirroring (SQL Server).
Like AlwaysOn Availability Groups and database mirroring, log shipping operates at the database level. You can use log shipping to maintain one or more warm standby databases (referred to as secondary databases) for a single production database that is referred to as the primary database. For more information about log shipping, see About Log Shipping (SQL Server).
Our recommendation for providing data protection for your SQL Server environment are as follows:
For data protection through a third-party shared disk solution (a SAN), we recommend that you use AlwaysOn Failover Cluster Instances.
For data protection through SQL Server, we recommend that you use AlwaysOn Availability Groups.
If you are running an edition of SQL Server that does not support AlwaysOn Availability Groups, we recommend log shipping. For information about which editions of SQL Server support AlwaysOn Availability Groups, see the "High Availability (AlwaysOn)" section of Features Supported by the Editions of SQL Server 2014.