Basic ConceptsΒΆ

Before we get any further, we should establish some basic concepts and terminology used in High Availability.

A cluster in the sense used in High Availability consists of one or more communicating computers, either virtual machines or physical hardware. It’s possible to mix and match virtual and physical machines.
A node is a single machine participating in a cluster. Nodes invariably fail or experience malfunction. The HA software provides reliable operations by connecting multiple nodes together, each monitoring the state of the others, coordinating the allocation of resources across all healthy nodes.
Anything that can be managed by the cluster is a resource. Pacemaker knows how to manage software using LSB init scripts, systemd service units or OCF resource agents. OCF is a common standard for HA clusters providing a configurable interface to many common applications. The OCF agents are adapted for running in a clustered environment, and provide configurable parameters and monitoring functionality.
Constraints are rules that Pacemaker uses to determine where and how to start and stop resources. Using constraints, you can limit a resource to run only on a certain subset of nodes or set a preference for where a resource should normally be running. You can also use more complex rule expressions to move resources between nodes according to the time of day or date, for example. This guide won’t go into all the details of what can be done with constraints, but later on we will create and test constraints using Hawk.
Cluster Information Base. This is the configuration of the cluster, which is configured in a single location and automatically synchronised across the cluster. The format of the configuration is XML, but usually there is no need to look at the XML directly. The CRM Shell provides a line-based syntax which is easier to work with from the command line, and Hawk provides a graphical interface with which to work with the configuration.
CRM Shell
Behind the scenes, Hawk uses the CRM command shell to interact with the cluster. The CRM shell can be used directly from the command line via the command crm.