Overview
A cluster is a group of independent servers, called nodes, that work together to increase the availability of applications and services that run on the cluster. One is identified as the active node, on which a SQL Server instance is running the production workload, and the other is a passive node, is installed but not running. If the SQL Server instance on the active node fails, the passive node becomes the active node and begins to run the SQL Server production workload with minimal failover downtime. This process is known as failover.
What Clustering Can Do
Clustering is designed to improve the availability of the physical server hardware, operating system, but excluding the shared storage. Should any of these aspects fail, the SQL Server instance fails over. The other node in a cluster automatically takes over the failed SQL Server instance to reduce downtime to a minimum.
Node: A Microsoft Windows Server system that is an active or inactive member of a server cluster.
Cluster resource: A physical or logical entity that can be owned by a node, brought online and taken offline, moved between nodes, and managed as a cluster object. A cluster resource can be owned by only a single node at any point in time.
Resource group: A collection of cluster resources managed as a single cluster object. Typically, a resource group contains all of the cluster resources that are required to run a specific application or service. Failover and failback always act on resource groups.
Resource dependency: A resource on which another resource depends. If resource A depends on resource B, then B is a dependency of A.
Network name resource: A logical server name that is managed as a cluster resource. A network name resource must be used with an IP address resource.
Preferred owner: A node on which a resource group prefers to run. Each resource group is associated with a list of preferred owners sorted in order of preference. During automatic failover, the resource group is moved to the next preferred node in the preferred owner list.
Possible owner: A secondary node on which a resource can run. Each resource group is associated with a list of possible owners. Resource groups can fail over only to nodes that are listed as possible owners.
Quorum mode: The quorum configuration in a failover cluster that determines the number of node failures that the cluster can sustain.
Forced quorum: The process to start the cluster even though only a minority of the elements that are required for quorum are in communication.
