redis-py now supports cluster mode and provides a client for Redis Cluster.

The cluster client is based on Grokzen’s redis-py-cluster, has added bug fixes, and now supersedes that library. Support for these changes is thanks to his contributions.

To learn more about Redis Cluster, see Redis Cluster specifications.

Creating clusters | Specifying Target Nodes | Multi-key Commands | Known PubSub Limitations

Creating clusters#

Connecting redis-py to a Redis Cluster instance(s) requires at a minimum a single node for cluster discovery. There are multiple ways in which a cluster instance can be created:

  • Using ‘host’ and ‘port’ arguments:

>>> from redis.cluster import RedisCluster as Redis
>>> rc = Redis(host='localhost', port=6379)
>>> print(rc.get_nodes())
    [[host=,port=6379,name=,server_type=primary,redis_connection=Redis<ConnectionPool<Connection<host=,port=6379,db=0>>>], [host=,port=6378,name=,server_type=primary,redis_connection=Redis<ConnectionPool<Connection<host=,port=6378,db=0>>>], [host=,port=6377,name=,server_type=replica,redis_connection=Redis<ConnectionPool<Connection<host=,port=6377,db=0>>>]]
  • Using the Redis URL specification:

>>> from redis.cluster import RedisCluster as Redis
>>> rc = Redis.from_url("redis://localhost:6379/0")
  • Directly, via the ClusterNode class:

>>> from redis.cluster import RedisCluster as Redis
>>> from redis.cluster import ClusterNode
>>> nodes = [ClusterNode('localhost', 6379), ClusterNode('localhost', 6378)]
>>> rc = Redis(startup_nodes=nodes)

When a RedisCluster instance is being created it first attempts to establish a connection to one of the provided startup nodes. If none of the startup nodes are reachable, a ‘RedisClusterException’ will be thrown. After a connection to the one of the cluster’s nodes is established, the RedisCluster instance will be initialized with 3 caches: a slots cache which maps each of the 16384 slots to the node/s handling them, a nodes cache that contains ClusterNode objects (name, host, port, redis connection) for all of the cluster’s nodes, and a commands cache contains all the server supported commands that were retrieved using the Redis ‘COMMAND’ output. See RedisCluster specific options below for more.

RedisCluster instance can be directly used to execute Redis commands. When a command is being executed through the cluster instance, the target node(s) will be internally determined. When using a key-based command, the target node will be the node that holds the key’s slot. Cluster management commands and other commands that are not key-based have a parameter called ‘target_nodes’ where you can specify which nodes to execute the command on. In the absence of target_nodes, the command will be executed on the default cluster node. As part of cluster instance initialization, the cluster’s default node is randomly selected from the cluster’s primaries, and will be updated upon reinitialization. Using r.get_default_node(), you can get the cluster’s default node, or you can change it using the ‘set_default_node’ method.

The ‘target_nodes’ parameter is explained in the following section, ‘Specifying Target Nodes’.

>>> # target-nodes: the node that holds 'foo1's key slot
>>> rc.set('foo1', 'bar1')
>>> # target-nodes: the node that holds 'foo2's key slot
>>> rc.set('foo2', 'bar2')
>>> # target-nodes: the node that holds 'foo1's key slot
>>> print(rc.get('foo1'))
>>> # target-node: default-node
>>> print(rc.keys())
>>> # target-node: default-node

Specfiying Target Nodes#

As mentioned above, all non key-based RedisCluster commands accept the kwarg parameter ‘target_nodes’ that specifies the node/nodes that the command should be executed on. The best practice is to specify target nodes using RedisCluster class’s node flags: PRIMARIES, REPLICAS, ALL_NODES, RANDOM. When a nodes flag is passed along with a command, it will be internally resolved to the relevant node/s. If the nodes topology of the cluster changes during the execution of a command, the client will be able to resolve the nodes flag again with the new topology and attempt to retry executing the command.

>>> from redis.cluster import RedisCluster as Redis
>>> # run cluster-meet command on all of the cluster's nodes
>>> rc.cluster_meet('', 6379, target_nodes=Redis.ALL_NODES)
>>> # ping all replicas
>>> # ping a random node
>>> # get the keys from all cluster nodes
>>> rc.keys(target_nodes=Redis.ALL_NODES)
[b'foo1', b'foo2']
>>> # execute bgsave in all primaries
>>> rc.bgsave(Redis.PRIMARIES)

You could also pass ClusterNodes directly if you want to execute a command on a specific node / node group that isn’t addressed by the nodes flag. However, if the command execution fails due to cluster topology changes, a retry attempt will not be made, since the passed target node/s may no longer be valid, and the relevant cluster or connection error will be returned.

>>> node = rc.get_node('localhost', 6379)
>>> # Get the keys only for that specific node
>>> rc.keys(target_nodes=node)
>>> # get Redis info from a subset of primaries
>>> subset_primaries = [node for node in rc.get_primaries() if node.port > 6378]

In addition, the RedisCluster instance can query the Redis instance of a specific node and execute commands on that node directly. The Redis client, however, does not handle cluster failures and retries.

>>> cluster_node = rc.get_node(host='localhost', port=6379)
>>> print(cluster_node)
>>> r = cluster_node.redis_connection
>>> r.client_list()
[{'id': '276', 'addr': '', 'fd': '16', 'name': '', 'age': '0', 'idle': '0', 'flags': 'N', 'db': '0', 'sub': '0', 'psub': '0', 'multi': '-1', 'qbuf': '26', 'qbuf-free': '32742', 'argv-mem': '10', 'obl': '0', 'oll': '0', 'omem': '0', 'tot-mem': '54298', 'events': 'r', 'cmd': 'client', 'user': 'default'}]
>>> # Get the keys only for that specific node
>>> r.keys()

Multi-key Commands#

Redis supports multi-key commands in Cluster Mode, such as Set type unions or intersections, mset and mget, as long as the keys all hash to the same slot. By using RedisCluster client, you can use the known functions (e.g. mget, mset) to perform an atomic multi-key operation. However, you must ensure all keys are mapped to the same slot, otherwise a RedisClusterException will be thrown. Redis Cluster implements a concept called hash tags that can be used in order to force certain keys to be stored in the same hash slot, see Keys hash tag. You can also use nonatomic for some of the multikey operations, and pass keys that aren’t mapped to the same slot. The client will then map the keys to the relevant slots, sending the commands to the slots’ node owners. Non-atomic operations batch the keys according to their hash value, and then each batch is sent separately to the slot’s owner.

# Atomic operations can be used when all keys are mapped to the same slot
>>> rc.mset({'{foo}1': 'bar1', '{foo}2': 'bar2'})
>>> rc.mget('{foo}1', '{foo}2')
[b'bar1', b'bar2']
# Non-atomic multi-key operations splits the keys into different slots
>>> rc.mset_nonatomic({'foo': 'value1', 'bar': 'value2', 'zzz': 'value3')
>>> rc.mget_nonatomic('foo', 'bar', 'zzz')
[b'value1', b'value2', b'value3']

Cluster PubSub:

When a ClusterPubSub instance is created without specifying a node, a single node will be transparently chosen for the pubsub connection on the first command execution. The node will be determined by: 1. Hashing the channel name in the request to find its keyslot 2. Selecting a node that handles the keyslot: If read_from_replicas is set to true, a replica can be selected.

Known PubSub Limitations#

Pattern subscribe and publish do not currently work properly due to key slots. If we hash a pattern like fo* we will receive a keyslot for that string but there are endless possibilities for channel names based on this pattern - unknowable in advance. This feature is not disabled but the commands are not currently recommended for use. See redis-py-cluster documentation for more.

>>> p1 = rc.pubsub()
# p1 connection will be set to the node that holds 'foo' keyslot
>>> p1.subscribe('foo')
# p2 connection will be set to node 'localhost:6379'
>>> p2 = rc.pubsub(rc.get_node('localhost', 6379))

Read Only Mode

By default, Redis Cluster always returns MOVE redirection response on accessing a replica node. You can overcome this limitation and scale read commands by triggering READONLY mode.

To enable READONLY mode pass read_from_replicas=True to RedisCluster constructor. When set to true, read commands will be assigned between the primary and its replications in a Round-Robin manner.

READONLY mode can be set at runtime by calling the readonly() method with target_nodes=‘replicas’, and read-write access can be restored by calling the readwrite() method.

>>> from cluster import RedisCluster as Redis
# Use 'debug' log level to print the node that the command is executed on
>>> rc_readonly = Redis(startup_nodes=startup_nodes,
...                     read_from_replicas=True)
>>> rc_readonly.set('{foo}1', 'bar1')
>>> for i in range(0, 4):
...     # Assigns read command to the slot's hosts in a Round-Robin manner
...     rc_readonly.get('{foo}1')
# set command would be directed only to the slot's primary node
>>> rc_readonly.set('{foo}2', 'bar2')
# reset READONLY flag
>>> rc_readonly.readwrite(target_nodes='replicas')
# now the get command would be directed only to the slot's primary node
>>> rc_readonly.get('{foo}1')