You can remove a master node in the same way as well, however in order to remove a master node it must be empty. If the master is not empty you need to reshard data away from it to all the other master nodes before.
An alternative to remove a master node is to perform a manual failover of it over one of its slaves and remove the node after it turned into a slave of the new master. Obviously this does not help when you want to reduce the actual number of masters in your cluster, in that case, a resharding is needed.
*Replicas migration
In Redis Cluster it is possible to reconfigure a slave to replicate with a different master at any time just using the following command:
CLUSTER REPLICATE
However there is a special scenario where you want replicas to move from one master to another one automatically, without the help of the system administrator. The automatic reconfiguration of replicas is calledreplicas migration and is able to improve the reliability of a Redis Cluster.
Note: you can read the details of replicas migration in the Redis Cluster Specification, here we'll only provide some information about the general idea and what you should do in order to benefit from it.
The reason why you may want to let your cluster replicas to move from one master to another under certain condition, is that usually the Redis Cluster is as resistant to failures as the number of replicas attached to a given slave.
For example a cluster where every master has a single replica can't continue operations if the master and its replica fail at the same time, simply because there is no other instance to have a copy of the hash slots the master was serving. However while netsplits are likely to isolate a number of nodes at the same time, many other kind of failures, like hardware or software failures local to a single node, are a very notable class of failures that are unlikely to happen at the same time, so it is possible that in your cluster where every master has a slave, the slave is killed at 4am, and the master is killed at 6am. This still will result in a cluster that can no longer operate.
To improve reliability of the system we have the option to add additional replicas to every master, but this is expensive. Replica migration allows to add more slaves to just a few masters. So you have 10 masters with 1 slave each, for a total of 20 instances. However you add, for example, 3 instances more as slaves of some of your masters, so certain masters will have more than a single slave.
With replicas migration what happens is that if a master is left without slaves, a replica from a master that has multiple slaves will migrate to theorphaned master. So after your slave goes down at 4am as in the example we made above, another slave will take its place, and when the master will fail as well at 5am, there is still a slave that can be elected so that the cluster can continue to operate.
So what you should know about replicas migration in short?
The cluster will try to migrate a replica from the master that has the greatest number of replicas in a given moment.To benefit from replica migration you have just to add a few more replicas to a single master in your cluster, it does not matter what master.There is a configuration parameter that controls the replica migration feature that is calledcluster-migration-barrier: you can read more about it in the exampleredis.conf file provided with Redis Cluster.
*Upgrading nodes in a Redis Cluster
Upgrading slave nodes is easy since you just need to stop the nod