Renames key to newkey if newkey does not yet exist.
It returns an error when key does not exist.
In Cluster mode, both key and newkey must be in the same hash slot, meaning that in practice only keys that have the same hash tag can be reliably renamed in cluster.
Examples
127.0.0.1:6379> SET mykey "Hello"
OK
127.0.0.1:6379> SET myotherkey "World"
OK
127.0.0.1:6379> RENAMENX mykey myotherkey
(integer) 0
127.0.0.1:6379> GET myotherkey
"World"