Always On Failover Cluster instances leverage on Windows Server Failover Cluster (WSFC) feature, and provide high availability locally through redundancy at the Server-instance leave – as failover cluster instance (FCI).
An FCI consists of physical servers (nodes) containing similar hardware and configuration and identical software configuration, including Operating Systems and patch level, SQL Server version, components and instance name. The similarity would ensure that the FCI can be fully functional as it fails over between nodes.
An FCI is a single instance of SQL Server installed across a Windows Server Failover Cluster (WSFC) node and multiple subnets. An FCI appears as a single instance of SQL Server running on a single computer on the network; it provides failover from one WSFC node to another if the current node becomes unavailable.
AN FCI runs in a WSFC resource group within one or more WSFC nodes. When FCI starts up, one of the nodes brings the SQL Server instance online and assumes ownership of the resource group, including the following resources:
•Network name
•IP address
•Shared disks
•SQL Server Database Engine service
•SQL Server Agent service
•SQL Server Analysis Services service, if installed
•One file share resource, if the FILESTREAM feature is installed
The following would take place when failover occurs:
1. The dirty pages in the buffer cache would be written to disk unless hardware or system failure occurs.
2. all SQL Server services would be stopped on the active node.
3. The resource group ownership would be transferred to another node in the FCI.
4. The new resource group owner starts its SQL Server services.
5. All user connection requests would be automatically directed to the new active node using the same virtual network name (VNN).
The key benefits of a Failover Cluster Instance (FCI) include the following:
•Data are protected at the instance level through redundancy
•Support automatic failover *
•Support disaster recovery within a multi-subnet FCI or running an FCI-hosted database inside an availability group. A multi-subnet FCI does not require a virtual LAN; it increases the manageability and security of a multi-subnet FCI.
•Reconfiguration of applications and users is not required.
•Flexible failover policy for granular trigger events for automatic failover.
•Reliable failover through periodic and detailed health detection using dedicated and persisted connections.
Network Recommendation:
In a Production environment, it is recommended to use static IP addresses in conjunction with the virtual IP addresses of a Failover Cluster Instance (FCI). Please note that if DHCP (Dynamic Host Configuration Protocol) is used and the DHCP IP lease expires, additional recovery time would be required to re-register the new DHCP IP address associated with the DNS name.
* NB: In an availability group, automatic failover from an FCI to other nodes within the availability group is not supported.