Consuming XMPP PubSub in Ruby

By on November 10, 2009

XMPP is a very versatile protocol with well over several hundred proposed and working extensions, which has also proven itself in production (ex: Google Talk). Presence, roster management, federated and server to server (S2S) messaging are all examples of features that you get for free, which make it a very appealing platform for messaging applications. Combine it with extensions such as XEP-0060 (PubSub), and we have all the relevant buzzwords: pubsub, real-time, federated, and presence.

The PubSub specification within XMPP, as defined in XEP-0060, is definitely not as flexible as that of AMQP, but it is often times enough to cover the most popular use cases. However, technical merits aside, one of the key missing components, especially in Ruby, has been the historical lack of functioning libraries - xmpp4r claims to support it, but examples are lacking. Thankfully, after test driving the latest batch of gems, it looks like we're finally there.

Getting off the ground with XMPP

Without a good toolkit XMPP can be a gnarly protocol to get started with - Pidgin IM client has some great tools for spying on the exchange, but monitoring pages of XML scroll by can only get you so far. Thankfully, Seth Fitzsimmons has built switchboard ("curl for XMPP"), which offers a powerful command line tool to greatly simplify the process. Make sure to read the full tutorial, or jump right into it by testing it with the Wordpress XMPP stream:

# list available options, subscribe to a blog, list subscriptions and then open the stream
$ switchboard disco --target pubsub.im.wordpress.com info
$ switchboard pubsub --server pubsub.im.wordpress.com --node /blog/icanhazcheesburger.com subscribe
$ switchboard pubsub --server pubsub.im.wordpress.com subscriptions
$ switchboard pubsub --server pubsub.im.wordpress.com listen

Based on xmpp4r, switchboard is also a toolkit for assembling your own XMPP clients, which means that it can be easily customized to power a PubSub consumer. From start to finish, and since examples are still hard to come by:

require 'rubygems'
require 'switchboard'

class WordpressJack
  def self.connect(switchboard, settings)
    switchboard.plug!(PubSubJack)
    switchboard.hook(:post)

    switchboard.on_pubsub_event do |event|
      event.payload.each do |payload|
        payload.elements.each do |item|
          on(:post, item)
        end
      end
    end
  end
end

settings = Switchboard::Settings.new
settings['pubsub.server'] = 'pubsub.im.wordpress.com'
settings['jid'] = 'user@im.wordpress.com'
settings['password'] = 'password'

switchboard = Switchboard::Client.new(settings)
switchboard.plug!(WordpressJack)

switchboard.on_post do |post|
  puts "A new post was received:"
  puts post.methods.sort.uniq
  exit
end

switchboard.run!

XMPP with EventMachine and Nokogiri

If you have an EventMachine stack, or looking for a high performance library, Jeff Smick's blather is definitely a gem to investigate. The combination of the asynchronous nature of EventMachine, a SAX parser within Nokogiri, and a great DSL make it very fast and a pleasure to work with:

require 'rubygems'
require 'blather/client/client'
require 'blather/client/dsl/pubsub'
require 'blather'

EventMachine.run {
  host = 'pubsub.im.wordpress.com'
  node = 'blog/icanhazcheesburger.com'
  user = 'user@im.wordpress.com'
  pass = 'pass'

  jid = Blather::JID.new(user)
  client = Blather::Client.setup(jid, pass)
  client.register_handler(:ready) {
    puts "Connected. Send messages to #{client.jid.inspect}."
    pub = Blather::DSL::PubSub.new(client, host)
  }

  client.register_handler(:pubsub_event) { |event|
    puts event
  }

  client.connect
}

PubSub & Event-Driven Architecture

Having personally struggled in the past with XMPP PubSub and Ruby, it's been great to revisit the use case and find a new set of fully functional libraries. The event driven architecture which is enabled by technologies such as XMPP, AMQP, Comet, Webhooks and PubsubHubbub are increasingly becoming the staple of many web applications, and for a good reason. If you haven't already, grab switchboard or blather and take XMPP for a test drive.

Ilya GrigorikIlya Grigorik is a web ecosystem engineer, author of High Performance Browser Networking (O'Reilly), and Principal Engineer at Shopify — follow on Twitter.