Pacemaker (software)

Pacemaker is a high-availability resource manager for Linux clusters. It provides a framework for managing services and ensuring they remain available in the event of hardware or software failures. Specifically, Pacemaker orchestrates and manages the state of services across a cluster, moving them to healthy nodes when a failure is detected.

Pacemaker is a key component of the Corosync Cluster Engine, working in conjunction with Corosync for cluster membership and messaging. While Corosync handles communication and consensus between nodes, Pacemaker makes the decisions about which services should run where, and it manages the start, stop, and monitoring of these services.

The primary function of Pacemaker is to ensure continuous service availability. It achieves this by:

• Resource Management: Defining and managing resources, which are the services or applications being managed for high availability.
• Failure Detection: Monitoring resources and nodes within the cluster to detect failures. This can involve heartbeat mechanisms, application-specific probes, or other monitoring techniques.
• Resource Fencing (STONITH): Isolating failed nodes to prevent data corruption or other issues arising from conflicting access to shared resources. STONITH, or "Shoot The Other Node In The Head," ensures that a failing node cannot interfere with the rest of the cluster.
• Resource Recovery: Automatically relocating services from a failed node to a healthy node in the cluster.
• Constraint Management: Defining rules and policies that govern where services can run, based on factors such as node attributes, resource dependencies, and preferred locations.
• Policy Enforcement: Enforcing the configured policies to maintain the desired state of the cluster.

Pacemaker supports a variety of resource agents, which are scripts or executables that provide the necessary logic for managing specific services. These agents allow Pacemaker to interact with and control applications in a standardized way. Common resource agents exist for databases (e.g., MySQL, PostgreSQL), web servers (e.g., Apache), file systems, and network services.

Pacemaker is widely used in mission-critical environments where high availability is essential, such as database clusters, web hosting infrastructure, and other enterprise applications. It provides a robust and flexible framework for automating the management of highly available services.