HAclusters

HA clusters, short for high-availability clusters, are groups of computers that work together to keep services running despite failures. In an HA cluster, several servers share control of a service; if one node fails, another takes over automatically to minimize downtime. Clustering software manages resource allocation, health monitoring, failover, and recovery, while storage and networking support data integrity and connectivity.

A typical HA cluster includes: multiple nodes connected by a private network, shared or distributed storage, and a cluster manager that enforces a desired state. Key concepts are quorum to avoid split-brain, fencing (STONITH) to isolate malfunctioning nodes, and resource agents that manage individual services such as databases, web servers, or virtual machines.

How it works: nodes monitor each other and the services they host. When a problem is detected, resources are relocated to healthy nodes. Planned maintenance can be performed with resource relocation to spare nodes. Quorum ensures the cluster operates only with a majority of nodes online.

Architectures vary: active-passive (one active node, others in standby) or active-active (multiple nodes serving); shared storage or shared-nothing with data replication. Common use cases include database services, file and mail servers, virtualization platforms, and container orchestration backends. Notable implementations include Pacemaker with Corosync, Windows Server Failover Clustering, and commercial HA offerings from vendors.

Limitations include added complexity, fencing requirements, and the need for thorough testing. When properly implemented, HA clusters reduce downtime and help meet availability targets.