Master instance
===============

Luna-Video-Manager needs to a master instance for correct work
`several mechanics <./introduction.html#luna-video-manager-background-processes>`_.

All running service instances choose the `master instance` for the correct execution of these processes
(without race conditions and etc.). Only `master instance` can execute these processes.

Choice of master
----------------

Every service tries to get a database lock (update a field in the table `single_process_lock`).
If an instance gets a lock it saves the state `master` and sends heartbeat message to database further. Other instances
observe the heartbeat. If heartbeat is ended any service will try to be new master.