Redis::Cluster

Getting started

Install with:

$ gem install redis-clustering

You can connect to Redis by instantiating the Redis::Cluster class:

require "redis-clustering"

redis = Redis::Cluster.new(nodes: (7000..7005).map { |port| "redis://127.0.0.1:#{port}" })

NB: Both redis_cluster and redis-cluster are unrelated and abandoned gems.

# Nodes can be passed to the client as an array of connection URLs.
nodes = (7000..7005).map { |port| "redis://127.0.0.1:#{port}" }
redis = Redis::Cluster.new(nodes: nodes)

# You can also specify the options as a Hash. The options are the same as for a single server connection.
(7000..7005).map { |port| { host: '127.0.0.1', port: port } }

You can also specify only a subset of the nodes, and the client will discover the missing ones using the CLUSTER NODES command.

Redis::Cluster.new(nodes: %w[redis://127.0.0.1:7000])

If you want the connection to be able to read from any replica, you must pass the replica: true. Note that this connection won't be usable to write keys.

Redis::Cluster.new(nodes: nodes, replica: true)

Also, you can specify the :replica_affinity option if you want to prevent accessing cross availability zones.

Redis::Cluster.new(nodes: nodes, replica: true, replica_affinity: :latency)

The calling code is responsible for avoiding cross slot commands.

redis = Redis::Cluster.new(nodes: %w[redis://127.0.0.1:7000])

redis.mget('key1', 'key2')
#=> Redis::CommandError (CROSSSLOT Keys in request don't hash to the same slot)

redis.mget('{key}1', '{key}2')
#=> [nil, nil]

• The client automatically reconnects after a failover occurred, but the caller is responsible for handling errors while it is happening.
• The client support permanent node failures, and will reroute requests to promoted slaves.
• The client supports MOVED and ASK redirections transparently.

Cluster mode with SSL/TLS

Since Redis can return FQDN of nodes in reply to client since 7.* with CLUSTER commands, we can use cluster feature with SSL/TLS connection like this:

Redis::Cluster.new(nodes: %w[rediss://foo.example.com:6379])

On the other hand, in Redis versions prior to 6.*, you can specify options like the following if cluster mode is enabled and client has to connect to nodes via single endpoint with SSL/TLS.

Redis::Cluster.new(nodes: %w[rediss://foo-endpoint.example.com:6379], fixed_hostname: 'foo-endpoint.example.com')

In case of the above architecture, if you don't pass the fixed_hostname option to the client and servers return IP addresses of nodes, the client may fail to verify certificates.

Transaction with an optimistic locking

Since Redis cluster is a distributed system, several behaviors are different from a standalone server. Client libraries can make them compatible up to a point, but a part of features needs some restrictions. Especially, some cautions are needed to use the transaction feature with an optimistic locking.

# The client is an instance of the internal adapter for the optimistic locking
redis.watch("{my}key") do |client|
  if client.get("{my}key") == "some value"
    # The tx is an instance of the internal adapter for the transaction
    client.multi do |tx|
      tx.set("{my}key", "other value")
      tx.incr("{my}counter")
    end
  else
    client.unwatch
  end
end

In a cluster mode client, you need to pass a block if you call the watch method and you need to specify an argument to the block. Also, you should use the block argument as a receiver to call commands in the block. Although the above restrictions are needed, this implementations is compatible with a standalone client.