em-hiredis

What

A Redis client for EventMachine designed to be fast and simple.

Why

I wanted a client which:

used the C hiredis library to parse redis replies
had a convenient API for pubsub
exposed the state of the underlying redis connections so that custom failover logic could be written outside the library

Also, https://github.com/madsimian/em-redis is no longer maintained.

Getting started

Connect to redis:

require 'em-hiredis'
redis = EM::Hiredis.connect

Or, connect to redis with a redis URL (for a different host, port, password, DB)

redis = EM::Hiredis.connect("redis://:[email protected]:9000/4")

Commands may be sent immediately. Any commands sent while connecting to redis will be queued.

All redis commands are available without any remapping of names, and return a deferrable

redis.set('foo', 'bar').callback {
  redis.get('foo').callback { |value|
    p [:returned, value]
  }
}

If redis replies with an error (for example you called a hash operation against a set or the database is full), or if the redis connection disconnects before the command returns, the deferrable will fail.

redis.sadd('aset', 'member').callback {
  response_deferrable = redis.hget('aset', 'member')
  response_deferrable.errback { |e|
    p e # => #<EventMachine::Hiredis::RedisError: Error reply from redis (wrapped in redis_error)>
    p e.redis_error # => #<RuntimeError: ERR Operation against a key holding the wrong kind of value>
  }
}

As a shortcut, if you're only interested in binding to the success case you can simply provide a block to any command

redis.get('foo') { |value|
  p [:returned, value]
}

Understanding the state of the connection

When a connection to redis server closes, a :disconnected event will be emitted and the connection will be immediately reconnect. If the connection reconnects a :connected event will be emitted.

If a reconnect fails to connect, a :reconnect_failed event will be emitted (rather than :disconnected) with the number of consecutive failures, and the connection will be retried after a timeout (defaults to 0.5s, can be set via EM::Hiredis.reconnect_timeout=).

If a client fails to reconnect 4 consecutive times then a :failed event will be emitted, and any queued redis commands will be failed (otherwise they would be queued forever waiting for a reconnect).

Pubsub

The way pubsub works in redis is that once a subscribe has been made on a connection, it's only possible to send (p)subscribe or (p)unsubscribe commands on that connection. The connection will also receive messages which are not replies to commands.

The regular EM::Hiredis::Client no longer understands pubsub messages - this logic has been moved to EM::Hiredis::PubsubClient. The pubsub client can either be initialized directly (see code) or you can get one connected to the same redis server by calling #pubsub on an existing EM::Hiredis::Client instance.

Pubsub can either be used in em-hiredis in a close-to-the-metal fashion, or you can use the convenience functionality for binding blocks to subscriptions if you prefer (recommended).

Close to the metal pubsub interface

Basically just bind to :message and :pmessage events:

# Create two connections, one will be used for subscribing
redis = EM::Hiredis.connect
pubsub = redis.pubsub

pubsub.subscribe('bar.0').callback { puts "Subscribed" }
pubsub.psubscribe('bar.*')

pubsub.on(:message) { |channel, message|
  p [:message, channel, message]
}

pubsub.on(:pmessage) { |key, channel, message|
  p [:pmessage, key, channel, message]
}

EM.add_periodic_timer(1) {
  redis.publish("bar.#{rand(2)}", "hello").errback { |e|
    p [:publisherror, e]
  }
}

Richer pubsub interface

If you pass a block to subscribe or psubscribe, the passed block will be called whenever a message arrives on that subscription:

redis = EM::Hiredis.connect

puts "Subscribing"
redis.pubsub.subscribe("foo") { |msg|
  p [:sub1, msg]
}

redis.pubsub.psubscribe("f*") { |msg|
  p [:sub2, msg]
}

EM.add_periodic_timer(1) {
  redis.publish("foo", "Hello")
}

EM.add_timer(5) {
  puts "Unsubscribing sub1"
  redis.pubsub.unsubscribe("foo")
}

It's possible to subscribe to the same channel multiple time and just unsubscribe a single callback using unsubscribe_proc or punsubscribe_proc.

Lua

You can of course call EVAL or EVALSHA directly; the following is a higher-level API.

Registering a named command on a redis client defines a ruby method with the given name on the client:

redis.register_script(:multiply, <<-END)
  return redis.call('get', KEYS[1]) * ARGV[1]
END

The method can be called in a very similar way to any other redis command; the only difference is that the first argument must be an array of keys, and the second (optional) an array of values.

# Multiplies the value at key foo by 2
redis.multiply(['foo'], [2]).callback { ... }

Lua commands are submitted to redis using EVALSHA for efficiency. If redis replies with a NOSCRIPT error, the command is automatically re-submitted with EVAL; this is totally transparent to your code and the intermediate 'failure' will not be passed to your errback.

You may register scripts globally, in which case they will be available to all clients:

EM::Hiredis::Client.register_script(:multiply, <<-END)
  return redis.call('get', KEYS[1]) * ARGV[1]
END

As a final convenience, it is possible to load all lua scripts from a directory automatically. All .lua files in the directory will be registered, and named according to filename (so a file called sum.lua becomes available as redis.sum(...)).

EM::Hiredis::Client.load_scripts_from('./lua_scripts')

For examples see examples/lua.rb or lib/em-hiredis/lock_lua.

Developing

You need bundler and a local redis server running on port 6379 to run the test suite.

# WARNING: The tests call flushdb on db 9 - this clears all keys!
bundle exec rake

Run an individual spec:

bundle exec rspec spec/redis_commands_spec.rb

Many thanks to the em-redis gem for getting this gem bootstrapped with some tests.