Available since: 3.0.0
Time complexity: O(1)
CLUSTER MEET is used in order to connect different Redis nodes with cluster
support enabled, into a working cluster.
The basic idea is that nodes by default don't trust each other, and are considered unknown, so that it is unlikely that different cluster nodes will mix into a single one because of system administration errors or network addresses modifications.
So in order for a given node to accept another one into the list of nodes composing a Redis Cluster, there are only two ways:
CLUSTER MEETcommand to force a node to meet another one.
Note that Redis Cluster needs to form a full mesh (each node is connected with each other node), but in order to create a cluster, there is no need to send all the
CLUSTER MEET commands needed to form the full mesh. What matter is to send enough
CLUSTER MEET messages so that each node can reach each other node through a chain of known nodes. Thanks to the exchange of gossip information in heartbeat packets, the missing links will be created.
So, if we link node A with node B via
CLUSTER MEET, and B with C, A and C will find their ways to handshake and create a link.
Another example: if we imagine a cluster formed of the following four nodes called A, B, C and D, we may send just the following set of commands to A:
CLUSTER MEET B-ip B-port
CLUSTER MEET C-ip C-port
CLUSTER MEET D-ip D-port
As a side effect of
A knowing and being known by all the other nodes, it will send gossip sections in the heartbeat packets that will allow each other node to create a link with each other one, forming a full mesh in a matter of seconds, even if the cluster is large.
CLUSTER MEET does not need to be reciprocal. If I send the command to A in order to join B, I don't need to also send it to B in order to join A.
If the optional
cluster_bus_port argument is not provided, the default of port + 10000 will be used.
When a given node receives a
CLUSTER MEET message, the node specified in the
command still does not know the node we sent the command to. So in order for
the node to force the receiver to accept it as a trusted node, it sends a
MEET packet instead of a
PING packet. The two packets have exactly the
same format, but the former forces the receiver to acknowledge the node as
Simple string reply:
OK if the command was successful. If the address or port specified are invalid an error is returned.