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Written by thoughtbot

Redis Pub/Sub... How Does it Work?

Redis is a key/value store, but it’s jam-packed with a ton of other little utilities that make it a joy to explore and implement. Two of these are the PUBLISH and SUBSCRIBE commands, which enable you to do quick messaging and communication between processes. Granted, there’s plenty of other messaging systems out there (AMQP and ØMQ come to mind), but Redis is worth a look too.

The way it works is simple:

  • SUBSCRIBE will listen to a channel
  • PUBLISH allows you to push a message into a channel

Those two commands are all you need to build a messaging system with Redis. But, what should you build?

Demo

I tend to build quick little games to learn new ideas, frameworks, languages, etc. For Redis pub/sub I chose to emulate IRC, since “channels” are essentially the same concept for an IRC server. A user connects, talks into a channel, and if others are there, they get the message. This is the basic concept, we’re not going to re-implement the IRC protocol here.

I scraped together two tiny little Ruby scripts for this. The repo is on GitHub, if you want to play with it. Make sure you’re running Redis locally first! Install Redis via redis.io if not.

PUBLISH

First up, pub.rb publishes messages to a channel. We’re going to bring in the Redis gem to use for making a client, and JSON in order to have an easy transport format. We could have used Ruby’s Marshal class instead to serialize, but this works fine and is human readable.

# usage:
# ruby pub.rb channel username

require 'rubygems'
require 'redis'
require 'json'

$redis = Redis.new

data = {"user" => ARGV[1]}

loop do
  msg = STDIN.gets
  $redis.publish ARGV[0], data.merge('msg' => msg.strip).to_json
end

This script will run interactively, once provided a channel and username from the command line arguments. It then fires off the PUBLISH command every time the user types a message and hits Enter. It publishes the message to a channel (in ARGV[0], the first command line argument.

So if we were to run:

% ruby pub.rb rubyonrails qrush
Hello world

Our Redis client would then send a PUBLISH command down the “rubyonrails” channel with the given message. The message itself is JSON and looks like:

{
  "msg": "Hello world",
  "user": "qrush"
}

We can actually verify this with the MONITOR command, which will spit out all commands the Redis server has processed. If we had MONITOR running before sending the above hello world snippet, it shows:

% redis-cli
redis> MONITOR
OK
1306462616.036890 "MONITOR"
1306462620.482914 "publish" "rubyonrails" "{\"user\":\"qrush\",\"msg\":\"Hello world\"}"

Currently this simple script doesn’t support publishing under more than one channel. Opening up more than one pub.rb process will let you do that.

SUBSCRIBE

Woot! Now that we have messages being sent we have to listen to them. Enter sub.rb:

require 'rubygems'
require 'redis'
require 'json'

$redis = Redis.new(:timeout => 0)

$redis.subscribe('rubyonrails', 'ruby-lang') do |on|
  on.message do |channel, msg|
    data = JSON.parse(msg)
    puts "##{channel} - [#{data['user']}]: #{data['msg']}"
  end
end

Once again we’ll need Redis and JSON to connect and parse messages. The initialization process for Redis is different this time: it’s using a new :timeout option. This will force the Redis client to never timeout when waiting a response, so we’ll wait forever for messages to come in. Perfect!

This script subscribes to two different channels: rubyonrails and ruby-lang. Basically, once the interpreter reaches the subscribe block, it will never exit and continue to wait for messages.

When a message comes in, the message block is fired, yielding two arguments: the channel the message was on, and the actual data sent down the pipe. Parsing that JSON chunk then allows us to spit out who said it, where they said it, and what was actually said. Here’s what it looks like if some other clients are publishing messages after we run our sub.rb file. (The published messages arrive in the order they are sent, but that’s hard to display in text):

% ruby pub.rb rubyonrails qrush
Whoa!
`rake routes` right?

% ruby pub.rb rubyonrails turbage
How do I list routes?
Oh, duh. thanks bro.

% ruby pub.rb ruby-lang qrush  
I think it's Array#include? you really want.

% ruby sub.rb 
#rubyonrails - [qrush]: Whoa!
#rubyonrails - [turbage]: How do I list routes?
#ruby-lang - [qrush]: I think it's Array#include? you really want.
#rubyonrails - [qrush]: `rake routes` right?
#rubyonrails - [turbage]: Oh, duh. thanks bro.

Whoa! How does it work!? Under the hood in the redis-rb client, the subscribe block is actually stuck in a Ruby loop (called from Redis::SubscribedClient#subscription). The client is going to continually attempt to read from the socket for messages, until there’s an error of some kind (but not a Timeout!). The redis-server then keeps a list of channels and patterns for each connected client, and publishes messages to them when a PUBLISH command is sent.

Usage

Although this is a simple example of how Redis pub/sub works, it’s pretty cool to see what others have done with it. Some examples include Convore, which is used as a pretty central part of their infrastructure, and Realie, a real-time code editor like Etherpad. Another good link to check out is Salvatore Sanfilippo’s recent interview on The Changelog (around ~24:00-27:00) where he discusses that developers are switching from other MQs to Redis due to its simplicity and performance.

If you’re using Redis' Pub/Sub within your infrastructure, we’d love to hear your feedback on how Radish can provide more visibility to what your messaging system is up to.

Next Steps & Related Reading

Redis sets - The intersection of space and time

Redis partial word match — you (auto)complete me