CLUSTER MEET
CLUSTER MEET ip port [cluster-bus-port]
- Available since:
- 3.0.0
- Time complexity:
- O(1)
- ACL categories:
-
@admin
,@slow
,@dangerous
,
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:
- The system administrator sends a
CLUSTER MEET
command to force a node to meet another one. - An already known node sends a list of nodes in the gossip section that we are not aware of. If the receiving node trusts the sending node as a known node, it will process the gossip section and send a handshake to the nodes that are still not known.
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.
Moreover 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.
Implementation details: MEET and PING packets
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
trusted.
RESP2/RESP3 Reply
Simple string reply:OK
if the command was successful. If the address or port specified are invalid an error is returned.
History
- Starting with Redis version 4.0.0: Added the optional
cluster_bus_port
argument.