Microsoft SQL Server gains high availability by integrating with Windows Server Failover Cluster (WSFC), and Always On Failover Cluster Instances (FCI).

WSFC is a group of independent servers working together to increase the availability of cluster roles, which are typically applications (such as SQL Server) and services (such as networking and storage). In the case of a node failure, WSFC automatically makes the cluster roles available on a healthy node in the cluster.

WSFC also provides the Cluster Shared Volume (CSV) functionality, allowing access to shared storage from any node in the cluster, therefore improving the speed in which cluster roles are moved between nodes as the storage is already available on all nodes.

FCI leverages WSFC in such a way that a SQL Server instance can be intentionally moved, or automatically failed over between nodes in WSFC. As a result, a failure of one of the resources required for the SQL Server instance on one node in the cluster causes the instance to failover to a healthy node in which the resources and database can resume operations.

While WSFC and FCI provide high-availability for SQL Server instances within the data center across a cluster of servers, PowerFlex replication provides the ability to failover across PowerFlex systems located at greater distances, such as two data centers. In such a case a WSFC and FCI are configured at each site on a different PowerFlex system, and user databases can be replicated between the sites with PowerFlex replication.

As explained in the next section, all the databases can be replicated from a single source site to a target site, or some databases can be active on the first site, and replicated to the second, while other databases can be active on the second site, and replicated to first, therefore, allowing resources on both sites to be used and protected simultaneously.