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Also, https://github.com/madsimian/em-redis is no longer maintained.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://:secretpassword@example.com: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*") { |channel, 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.

Inactivity checks

Sometimes a network connection may hang in ways which are difficult to detect or involve very long timeouts before they can be detected from the application layer. This is especially true of Redis Pubsub connections, as they are not request-response driven. It is very difficult for a listening client to descern between a hung connection and a server with nothing to say.

To start an application layer ping-pong mechanism for testing connection liveness, call the following at any time on a client:

redis.configure_inactivity_check(5, 3)

This configures a PING command to be sent if 5 seconds elapse without receiving any data from the server, and a reconnection to be triggered if a futher 3 seconds elapse after the PING is submitted.

This configuration is per client, you may choose different value for clients with different expected traffic patterns, or activate it on some and not at all on others.

PING and Pubsub

Because the Redis Pubsub protocol limits the set of valid commands on a connection once it is in "Pubsub" mode, PING is not supported in this case (though it may be in future, see redis/redis#420). In order to create some valid request-response traffic on the connection, a Pubsub connection will issue SUBSCRIBE "__em-hiredis-ping", followed by a corresponding UNSUBSCRIBE immediately on success of the subscribe. While less than ideal, this is the case where an application layer inactivity check is most valuable, and so the trade off is reasonable until PING is supported correctly on Pubsub connections.

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.