Each node in a Redis Cluster has its view of the current cluster configuration, given by the set of known nodes, the state of the connection we have with such nodes, their flags, properties and assigned slots, and so forth.
CLUSTER NODES provides all this information, that is, the current cluster configuration of the node we are contacting, in a serialization format which happens to be exactly the same as the one used by Redis Cluster itself in order to store on disk the cluster state (however the on disk cluster state has a few additional info appended at the end).
Note that normally clients willing to fetch the map between Cluster hash slots and node addresses should use CLUSTER SLOTS instead. CLUSTER NODES, that provides more information, should be used for administrative tasks, debugging, and configuration inspections. It is also used by
redis-trib in order to manage a cluster.
The output of the command is just a space-separated CSV string, where each line represents a node in the cluster. The following is an example of output:
07c37dfeb235213a872192d90877d0cd55635b91 127.0.0.1:30004 slave e7d1eecce10fd6bb5eb35b9f99a514335d9ba9ca 0 1426238317239 4 connected 67ed2db8d677e59ec4a4cefb06858cf2a1a89fa1 127.0.0.1:30002 master - 0 1426238316232 2 connected 5461-10922 292f8b365bb7edb5e285caf0b7e6ddc7265d2f4f 127.0.0.1:30003 master - 0 1426238318243 3 connected 10923-16383 6ec23923021cf3ffec47632106199cb7f496ce01 127.0.0.1:30005 slave 67ed2db8d677e59ec4a4cefb06858cf2a1a89fa1 0 1426238316232 5 connected 824fe116063bc5fcf9f4ffd895bc17aee7731ac3 127.0.0.1:30006 slave 292f8b365bb7edb5e285caf0b7e6ddc7265d2f4f 0 1426238317741 6 connected e7d1eecce10fd6bb5eb35b9f99a514335d9ba9ca 127.0.0.1:30001 myself,master - 0 0 1 connected 0-5460
Each line is composed of the following fields:
<id> <ip:port> <flags> <master> <ping-sent> <pong-recv> <config-epoch> <link-state> <slot> <slot> ... <slot>
The meaning of each filed is the following:
id: The node ID, a 40 characters random string generated when a node is created and never changed again (unless
CLUSTER RESET HARDis used).
ip:port: The node address where clients should contact the node to run queries.
flags: A list of comma separated flags:
noflags. Flags are explained in detail in the next section.
master: If the node is a slave, and the master is known, the master node ID, otherwise the “-” character.
ping-sent: Milliseconds unix time at which the currently active ping was sent, or zero if there are no pending pings.
pong-recv: Milliseconds unix time the last pong was received.
config-epoch: The configuration epoch (or version) of the current node (or of the current master if the node is a slave). Each time there is a failover, a new, unique, monotonically increasing configuration epoch is created. If multiple nodes claim to serve the same hash slots, the one with higher configuration epoch wins.
link-state: The state of the link used for the node-to-node cluster bus. We use this link to communicate with the node. Can be
slot: A hash slot number or range. Starting from argument number 9, but there may be up to 16384 entries in total (limit never reached). This is the list of hash slots served by this node. If the entry is just a number, is parsed as such. If it is a range, it is in the form
start-end, and means that the node is responsible for all the hash slots from
endincluding the start and end values.
Meaning of the flags (field number 3):
myself: The node you are contacting.
master: Node is a master.
slave: Node is a slave.
fail?: Node is in
PFAILstate. Not reachable for the node you are contacting, but still logically reachable (not in
fail: Node is in
FAILstate. It was not reachable for multiple nodes that promoted the
handshake: Untrusted node, we are handshaking.
noaddr: No address known for this node.
noflags: No flags at all.
*Notes on published config epochs
Slaves broadcast their master’s config epochs (in order to get an
UPDATE message if they are found to be stale), so the real config epoch of the slave (which is meaningless more or less, since they don’t serve hash slots) can be only obtained checking the node flagged as
myself, which is the entry of the node we are asking to generate CLUSTER NODES output. The other slaves epochs reflect what they publish in heartbeat packets, which is, the configuration epoch of the masters they are currently replicating.
*Special slot entries
Normally hash slots associated to a given node are in one of the following formats, as already explained above:
- Single number: 3894
- Range: 3900-4000
However node hash slots can be in a special state, used in order to communicate errors after a node restart (mismatch between the keys in the AOF/RDB file, and the node hash slots configuration), or when there is a resharding operation in progress. This two states are importing and migrating.
The meaning of the two states is explained in the Redis Specification, however the gist of the two states is the following:
- Importing slots are yet not part of the nodes hash slot, there is a migration in progress. The node will accept queries about these slots only if the
ASKcommand is used.
- Migrating slots are assigned to the node, but are being migrated to some other node. The node will accept queries if all the keys in the command exist already, otherwise it will emit what is called an ASK redirection, to force new keys creation directly in the importing node.
Importing and migrating slots are emitted in the CLUSTER NODES output as follows:
- Importing slot:
- Migrating slot:
The following are a few examples of importing and migrating slots:
Note that the format does not have any space, so CLUSTER NODES output format is plain CSV with space as separator even when this special slots are emitted. However a complete parser for the format should be able to handle them.
- Migration and importing slots are only added to the node flagged as
myself. This information is local to a node, for its own slots.
- Importing and migrating slots are provided as additional info. If the node has a given hash slot assigned, it will be also a plain number in the list of hash slots, so clients that don’t have a clue about hash slots migrations can just skip this special fields.
Bulk string reply: The serialized cluster configuration.