A note about the word slave used in this man page and command name: Starting with Redis 5 this command: starting with Redis version 5, if not for backward compatibility, the Redis project no longer uses the word slave. Please use the new command REPLICAOF. The command SLAVEOF will continue to work for backward compatibility.
The SLAVEOF command can change the replication settings of a replica on the fly. If a Redis server is already acting as replica, the command SLAVEOF NO ONE will turn off the replication, turning the Redis server into a MASTER. In the proper form SLAVEOF hostname port will make the server a replica of another server listening at the specified hostname and port.
If a server is already a replica of some master, SLAVEOF hostname port will stop the replication against the old server and start the synchronization against the new one, discarding the old dataset.
The form SLAVEOF NO ONE will stop replication, turning the server into a MASTER, but will not discard the replication. So, if the old master stops working, it is possible to turn the replica into a master and set the application to use this new master in read/write. Later when the other Redis server is fixed, it can be reconfigured to work as a replica.