Pacemaker is a cluster management tool meant to orchestrate services or, more generally, resources. The idea is to be able to provide HA for applications that do not natively support it by running a pacemaker cluster managing the services. It is part of the Linux HA stack.
It is used extensively to provide HA to Openstack services and other solutions, so it is backed by big enterprises such as Red Hat.
Pacemaker has a holistic view of the applications it manages. It manages not only the services themselves, but also load balancing and shared storage. It also has support for fencing, which means it can detect faulty nodes and avoid that they corrupt cluster state by either isolating it on the network or shutting them down directly (STONITH - Shoot The Other Node In The Head).
It’s worth mentioning that Pacemaker provides resource management for the cluster, but it still depends on a cluster engine like Corosync, Heartbeat or CMAN to provide reliable messaging, quorum and low-level cluster management facilities.
A resource is anything that can be managed by Pacemaker. Resources can be services, DRBD devices, floating ips etc.
Resource agents are “drivers” that provide a standard interface for Pacemaker to manage services from a wide range of implementations. These are usually implemented as shell scripts. There are resource agents for Systemd, OCF (Open Cluster Framework), LSB (Linux Standard Base) etc. This architecture makes Pacemaker extensible and able to manage heterogeneous resources while keeping a consistent interface to the operator.
Anything that can be scripted can be clustered
The main way to interact with Pacemaker clusters is via the crm command. With it we can create and manage resources, as well as migrating resources between nodes or put a node into maintenance mode so we can restart the services manually if needed.
References
https://www.youtube.com/watch?v=yq5nYPKxBCo
https://www.linuxjournal.com/content/ahead-pack-pacemaker-high-availability-stack?page=0,0
https://clusterlabs.org/pacemaker/doc/deprecated/en-US/Pacemaker/1.1/html/Clusters_from_Scratch/
http://www.formilux.org/archives/haproxy/1003/3259.html