Note that since this node is already connected to the cluster it is already able to redirect client queries correctly and is generally speaking part of the cluster. However it has two peculiarities compared to the other masters:
It holds no data as it has no assigned hash slots. Because it is a master without assigned slots, it does not participate in the election process when a slave wants to become a master.
Now it is possible to assign hash slots to this node using the resharding feature ofredis-trib. It is basically useless to show this as we already did in a previous section, there is no difference, it is just a resharding having as a target the empty node.
*Adding a new node as a replica
Adding a new Replica can be performed in two ways. The obvious one is to use redis-trib again, but with the --slave option, like this:
./redis-trib.rb add-node --slave 127.0.0.1:7006 127.0.0.1:7000
Note that the command line here is exactly like the one we used to add a new master, so we are not specifying to which master we want to add the replica. In this case what happens is that redis-trib will add the new node as replica of a random master among the masters with less replicas.
However you can specify exactly what master you want to target with your new replica with the following command line:
./redis-trib.rb add-node --slave --master-id 3c3a0c74aae0b56170ccb03a76b60cfe7dc1912e 127.0.0.1:7006 127.0.0.1:7000
This way we assign the new replica to a specific master.
A more manual way to add a replica to a specific master is to add the new node as an empty master, and then turn it into a replica using theCLUSTER REPLICATE command. This also works if the node was added as a slave but you want to move it as a replica of a different master.
For example in order to add a replica for the node 127.0.0.1:7005 that is currently serving hash slots in the range 11423-16383, that has a Node ID 3c3a0c74aae0b56170ccb03a76b60cfe7dc1912e, all I need to do is to connect with the new node (already added as empty master) and send the command:
redis 127.0.0.1:7006> cluster replicate 3c3a0c74aae0b56170ccb03a76b60cfe7dc1912e
That's it. Now we have a new replica for this set of hash slots, and all the other nodes in the cluster already know (after a few seconds needed to update their config). We can verify with the following command:
$ redis-cli -p 7000 cluster nodes | grep slave | grep 3c3a0c74aae0b56170ccb03a76b60cfe7dc1912e f093c80dde814da99c5cf72a7dd01590792b783b 127.0.0.1:7006 slave 3c3a0c74aae0b56170ccb03a76b60cfe7dc1912e 0 1385543617702 3 connected 2938205e12de373867bf38f1ca29d31d0ddb3e46 127.0.0.1:7002 slave 3c3a0c74aae0b56170ccb03a76b60cfe7dc1912e 0 1385543617198 3 connected
The node 3c3a0c... now has two slaves, running on ports 7002 (the existing one) and 7006 (the new one).
*Removing a node
To remove a slave node just use the del-node command of redis-trib:
./redis-trib del-node 127.0.0.1:7000 ``
The first argument is just a