Available since: 3.0.0
Time complexity: O(1)
This command, that can only be sent to a Redis Cluster replica node, forces the replica to start a manual failover of its master instance.
A manual failover is a special kind of failover that is usually executed when there are no actual failures, but we wish to swap the current master with one of its replicas (which is the node we send the command to), in a safe way, without any window for data loss. It works in the following way:
This way clients are moved away from the old master to the new master atomically and only when the replica that is turning into the new master has processed all of the replication stream from the old master.
The command behavior can be modified by two options: FORCE and TAKEOVER.
If the FORCE option is given, the replica does not perform any handshake with the master, that may be not reachable, but instead just starts a failover ASAP starting from point 4. This is useful when we want to start a manual failover while the master is no longer reachable.
However using FORCE we still need the majority of masters to be available in order to authorize the failover and generate a new configuration epoch for the replica that is going to become master.
There are situations where this is not enough, and we want a replica to failover without any agreement with the rest of the cluster. A real world use case for this is to mass promote replicas in a different data center to masters in order to perform a data center switch, while all the masters are down or partitioned away.
The TAKEOVER option implies everything FORCE implies, but also does
not uses any cluster authorization in order to failover. A replica receiving
CLUSTER FAILOVER TAKEOVER will instead:
configEpochunilaterally, just taking the current greatest epoch available and incrementing it if its local configuration epoch is not already the greatest.
Note that TAKEOVER violates the last-failover-wins principle of Redis Cluster, since the configuration epoch generated by the replica violates the normal generation of configuration epochs in several ways:
Because of this the TAKEOVER option should be used with care.
CLUSTER FAILOVER, unless the TAKEOVER option is specified, does not execute a failover synchronously. It only schedules a manual failover, bypassing the failure detection stage.
OKreply is no guarantee that the failover will succeed.
CLUSTER REPLICASto each of the master nodes and check that it appears as a replica, before sending
CLUSTER FAILOVERto the replica.
INFO REPLICATION(which indicates "role:master" after successful failover), or
CLUSTER NODESto verify that the state of the cluster has changed sometime after the command was sent.
Simple string reply:
OK if the command was accepted and a manual failover is going to be attempted. An error if the operation cannot be executed, for example if we are talking with a node which is already a master.