XenServer Pool, Master host failure

Every member of a resource pool contains all the information necessary to take over the role of master if required. When a master node fails, the following sequence of events occurs:

1. The members realize that communication has been lost and each tries to reconnect for sixty seconds.

2. Each member then puts itself into emergency mode, whereby the member XenServer hosts will only accept the pool-emergency commands:

xe pool-emergency-reset-master

and

xe pool-emergency-transition-to-master

If the master comes back up at this point, it re-establishes communication with its members, the members leave emergency mode, and operation returns to normal.
However if the master is really dead, choose one of the remaining members and run the command:

xe pool-emergency-transition-to-master

on it. Once it has become the master, issue the command:

xe pool-recover-slaves

and the members will now point to the new master.
If you repair or replace the server that was the original master, you can simply bring it up, install the XenServer host software, and add it to the pool.