The queue servers in a distributed system have one of the following roles:
Master queue server
Backup queue server
The master queue server is the primary server for managing the queues.
A backup queue server can replace a master queue server if it becomes unavailable. The backup queue server is inactive if the master queue server is available. When the master queue server restarts after becoming unavailable, it takes over its tasks from the backup queue server again.
You implement backup queue servers in order to make indexing highly available. The queues must be stored centrally, so that both the master and the backup queue servers can have write-access to them.