A cluster is a named collection of server instances that share the same applications, resources, and configuration information. You can group server instances on different machines into one logical cluster and administer them as one unit. You can easily control the lifecycle of a multi-machine cluster with the DAS.
Instances can be grouped into clusters. You can distribute an application to all instances in the cluster with a single deployment. Clusters are dynamic. When an instance is added or removed, the changes are handled automatically.
Clusters enable horizontal scalability, load balancing, and failover protection. By definition, all the instances in a cluster have the same resource and application configuration. When a server instance or a machine in a cluster fails, the load balancer detects the failure, redirects traffic from the failed instance to other instances in the cluster, and recovers the user session state. Since the same applications and resources are on all instances in the cluster, an instance can failover to any other instance in the cluster.
Cluster instances are organized in a ring topology. Each member in the ring sends in-memory state data to the next member in the ring, its replica partner, and receives state data from the previous member. As state data is updated in any member, it is replicated around the ring. When a member fails in the ring topology, the ring is broken. Group Management Service (GMS) can recognize the failure of a member. In that event, the replication framework reshapes the topology of the cluster and notifies members of the changes. When a member learns that its replica partner has disappeared, it selects a new partner from in-service members.