SQL Server AlwaysOn

SQL Server AlwaysOn consists of two technologies:

       i.            AlwaysOn Failover Clustering Instances (AlwaysOn FCI)

     ii.            AlwaysOn Availability Groups (AlwaysOn AG)

AlwaysOn Failover Cluster Instance

As part of the SQL Server Always On offering, Always On 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.

WSFC Cluster is a group of independent servers that work together to increase the availability of applications or services. Each server is called a node in a WSFC cluster and if an application fails or services that were hosted on that node crash, then they are automatically (or manually) transferred to another node in the WSFC cluster in a process named Failover.

Before creating a WSFC cluster, we must enable WSFC on our server that provides infrastructure features that support high-availability and disaster recovery scenarios of hosted server applications or services such as SQL Server or Microsoft Exchange. This feature is turned off by default when installing the Windows Server Operating System

SQL Server AlwaysOn Failover Cluster Instance or FCI, a substantial offering of AlwaysOn, provides high availability at the server-instance level by making use of Windows Server Failover Clustering. It is a single instance of SQL, which is installed across Windows Server Failover Cluster nodes. In a network, FCI seems to be an instance, which runs on a single computer. However, in real it provides failover to the other node in the scenario if one WSFC node is unavailable.

An FCI is high-availability and disaster recovery solution at SQL Server Instance level, it means SQL Server instance installed across nodes in a WSFC cluster. It leverages WSFC feature to provide local high availability through redundancy at server-instance level.

In the instance of any hardware or software failure of the SQL Server, the clients and the applications connected to it, experience downtime. The high availability of the instances of SQL Server is safeguarded by the redundant node, whenever the instances are configured to be a Failover Cluster Instance.

AlwaysOn Availability Groups

AlwaysOn Availability Groups is a database mirroring technique for Microsoft SQL Server that allows administrators to pull together a group of user databases that can fail over together.

This feature provides high-availability and disaster recovery solution at user-database level. It is similar to the combination between Database Mirroring and WSFC solution. It requires that SQL Server Instance of user-databases resides on WSFC nodes.