The database and the message bus are not necessarily OpenStack services, but they are services that OpenStack depends on and that you want to be sure are highly available too. One option to make the database highly available is to add it to Pacemaker with a shared storage device. If the database were to fail on a node, then Pacemaker would move the shared storage to another node and start the database on a different node. There are also active/passive and active/active replication scenarios that can be configured for the database. Active/passive means that there is more than one instance of the database running, but only one of them is used as the active writable instance. The other instance(s) are there as passive backups and only become active if the current active instance needs to failover for some reason. Active/active means that there is more than one active writable instance of the database. These are running on different nodes and each can...