Windows Server Failover Clustering (WSFC) -- a feature of Microsoft Windows Server operating system for fault tolerance and high availability (HA) of applications and services -- enables several computers to host a service, and if one has a fault, the remaining computers automatically take over the hosting of the service. It is included with Windows Server 2022, Windows Server 2019, Windows Server 2016 and Azure Stack HCI.
In WSFC, each individual server is called a node. The nodes can be physical computers or virtual machines, and are connected through physical connections and through software. Two or more nodes are combined to form a cluster, which hosts the service. The cluster and nodes are constantly monitored for faults. If a fault is detected, the nodes with issues are removed from the cluster and the services may be restarted or moved to another node.
Windows Server Failover Cluster performs several functions, including:
A number of different applications can use WSFC, including:
Every cluster network must account for the possibility of individual nodes losing communication to the cluster but still being able to serve requests or access resources. If this were to happen, the service could become corrupt and serve bad responses or cause data stores to become out of sync. This is known as split-brain condition.
WSFC uses a voting system with quorum to determine failover and to prevent a split-brain condition. In the cluster, the quorum is defined as half of the total nodes. After a fault, the nodes vote to stay online. If less than the quorum amount votes yes, those nodes are removed. For example, a cluster of five nodes has a fault, causing three to stay in communication in one segment and two in the other. The group of three will have the quorum and stay online, while the other two will not have a quorum and will go offline.
In small clusters, an extra witness vote should be added. The witness is an extra vote that is added as a tiebreaker in clusters with even numbers of nodes. Without a witness, if half of the nodes go offline at one time the whole service is stopped. A witness is required in clusters with only two nodes and recommended for three and four node clusters. In clusters of five or more nodes, a witness does not provide benefits and is not needed. The witness information is stored in a witness.log file. It can be hosted as a File Share Witness, an Azure Cloud Witness or as a Disk Witness (aka custom quorum disk).
A Dynamic Quorum allows the number of votes to constitute a quorum to adjust as faults occur. This way, as long as more than half of the nodes don't go offline at one time, the cluster will be able to continuously lose nodes without it going offline. This allows for a single node to run the services as the "last man standing."
SQL Server Always On is a high-availability and disaster recovery product for Microsoft SQL server that takes advantage of WSFC. SQL Server Always On has two configurations that can be used separately or in tandem. Failover Cluster Instance (FCI) is a SQL Server instance that is installed across several nodes in a WSFC. Availability Group (AG) is a one or more databases that fail over together to replicated copies. Both register components with WSFC as cluster resources.
See Microsoft for full documentation on how to deploy a failover cluster using WSFC.
See failover cluster quorum considerations for Windows admins, 10 top tips to maximize hyper-converged infrastructure benefits and how to build a Hyper-V home lab in Windows Server 2019.
08 Mar 2022