Programming Erlang

My current interests in XMPP leaded me to ejabberd (XMPP server) and this to some XEPs PubSub and POSH, and this to wondering about using XMPP as a queue system. Then I discovered some discussions comparing AMQP vs XMPP and realized that both RabbitMQ (an implementation of AMQP) and ejabberd were programmed in Erlang. I was astonished that three of the currently most interesting projects were implemented in the same language (if we include couchDB).

This is why I decided to read “Programming Erlang”. I’ve nearly read half of the book and my expectations has been really exceeded:

• smooth pace of the book goes from basic knowledge to the hardest part (sequential programming -> advanced sequential programming -> concurrent programming -> distributed programming -> programming multicore CPUs).
• clear and easy to understand examples.
• some complete examples such as an IRC Lite implementation gives you the concepts in action.
• interesting concepts: functional programming, concurrency, distributed programming, fault-tolerant programs.

As missing parts (not sure if they are cover at the end of the book), I found out the following:

• a bit of history of Erlang. Why erlang?
• Comparison between Erlang and other concurrency oriented languages such as Scala, Clojure, Stackless Python, …

To sum up, really a MUST READ if you are curious about other programming languages apart from Java, Python or Ruby.

My interests with Wordle

Below you can get an idea of my interests. The image is generated through my delicious tags with the wordle service.

XMPP PubSub with ejabberd and XMPP4R

After reading “Beyond REST? Building Data Services with XMPP PubSub” and other articles about PubSub with XMPP, I decided it would be worth to test it. However, I didn’t find any complete step by step guide in how to test it with ejabberd as XMPP server and XMPP4R as XMPP client.

Below are the steps I followed to test a subscriber (user “sub”) waiting for items published to the node (”home/localhost/pub/updates”) for the publisher (user “pub”). Note you can add as many subscribers/publishers as you want.

1. install ejabberd (XMPP server). I followed instructions of this article (spanish) or article (english) without any surprise.
2. create two ejabberd users: “pub” (the publisher) and “sub” (the subscriber).

   sudo ejabberdctl register pub localhost pub
sudo ejabberdctl register sub localhost sub

3. install XMPP4R Ruby gem.
   sudo gem install xmpp4r
4. create file nodecreator.rb. See code below.

   
   #! /usr/bin/ruby
   require "rubygems"
   require "xmpp4r"
   
   require "xmpp4r/pubsub"
   require "xmpp4r/pubsub/helper/servicehelper.rb"
   require "xmpp4r/pubsub/helper/nodebrowser.rb"
   require "xmpp4r/pubsub/helper/nodehelper.rb"
   
   include Jabber
   Jabber::debug = true
   
   service = 'pubsub.localhost'
   jid = 'pub@localhost/laptop'
   
   password = 'pub'
   client = Client.new(JID.new(jid))
   client.connect
   client.auth(password)
   
   client.send(Jabber::Presence.new.set_type(:available))
   pubsub = PubSub::ServiceHelper.new(client, service)
   pubsub.create_node('home/localhost/pub/')
   pubsub.create_node('home/localhost/pub/updates')
   

5. create file publisher.rb. See code below.

#! /usr/bin/ruby
require "rubygems"

require "xmpp4r"
require "xmpp4r/pubsub"
require "xmpp4r/pubsub/helper/servicehelper.rb"
require "xmpp4r/pubsub/helper/nodebrowser.rb"
require "xmpp4r/pubsub/helper/nodehelper.rb"
include Jabber
Jabber::debug = true
jid = 'pub@localhost/laptop'

password = 'pub'
service = 'pubsub.localhost'
node = 'home/localhost/pub/updates'
# connect XMPP client
client = Client.new(JID.new(jid))
# remove "127.0.0.1" if you are not using a local ejabberd
client.connect("127.0.0.1")
client.auth(password)
client.send(Jabber::Presence.new.set_type(:available))
# create item
pubsub = PubSub::ServiceHelper.new(client, service)
item = Jabber::PubSub::Item.new
xml = REXML::Element.new("greeting")
xml.text = 'hello world!'

item.add(xml);
# publish item
pubsub.publish_item_to(node, item)

6. create file subscriber.rb. See code below.

   
   #! /usr/bin/ruby
   require "rubygems"
   require "xmpp4r"
   require "xmpp4r/pubsub"
   require "xmpp4r/pubsub/helper/servicehelper.rb"
   
   require "xmpp4r/pubsub/helper/nodebrowser.rb"
   require "xmpp4r/pubsub/helper/nodehelper.rb"include Jabber
   #Jabber::debug = true
   jid = 'sub@localhost/laptop'
   password = 'sub'
   node = 'home/localhost/pub/updates'
   service = 'pubsub.localhost'
   
   # connect XMPP client
   client = Client.new(JID.new(jid))
   # remove "127.0.0.1" if you are not using a local ejabberd
   client.connect("127.0.0.1")
   client.auth(password)
   client.send(Jabber::Presence.new.set_type(:available))
   sleep(1)
   # subscribe to the node
   pubsub = PubSub::ServiceHelper.new(client, service)
   pubsub.subscribe_to(node)
   subscriptions = pubsub.get_subscriptions_from_all_nodes()
   puts "subscriptions: #{subscriptions}\n\n"
   puts "events:\n"
   
   # set callback for new events
   
   
   pubsub.add_event_callback do |event|
   begin
   event.payload.each do |e|
   puts e,"----\n"
   end
   rescue
   puts "Error : #{$!} \n #{event}"
   
   end
   # infinite loop
   loop do
   sleep 1
   end
   

7. run nodecreator.rb. It creates the XMPP node “home/localhost/pub/updates”. It creates first the node “home/localhost/pub” and then the “home/localhost/pub/updates”. Seems quite obvious but I spent some hours after I got it.
8. check the nodes have been created. I used the discovery service functionality of Psi client.
9. run subscriber.rb file. The “sub” user subscribes to the node ‘updates’ and waits for items in the “updates” node. Be aware you should close any XMPP connection with users “pub” and “sub” in case you are using any XMPP client such as Pidgin, Psi,… otherwise it won’t work.
10. run publisher.rb file. It will send a message “<greetings>hello world!”</greetings> to the subscriber. Run it as many times as you want.
11. If everything goes well yo will see in the subscriber screen a message like this.

    
    subscriptions: <subscription node='home/localhost/pub/updates' jid='sub@localhost' subscription='subscribed' xmlns='http://jabber.org/protocol/pubsub'/>
    
    events:
    <items node='home/localhost/pub/updates'><item id='3376'><greeting>hello world!</greeting></item></items>
    ----
    

Good Luck!

For this test, I used the following versions:

• Operating System: Ubuntu 8.04
• Ruby: 1.8.6
• XMPP4R: 0.4
• ejabberd: 1.1.4