# @title Ruby amqp gem: Getting Started with AMQP and Ruby

h1. Getting started with the Ruby amqp gem


h2. About this guide

This guide is a quick tutorial that helps you to get started with v0.9.1 of the AMQP specification in general and the "Ruby amqp gem":http://github.com/ruby-amqp/amqp in particular.
It should take about 20 minutes to read and study the provided code examples. This guide covers:

 * Installing RabbitMQ, a mature popular server implementation of the AMQP protocol.
 * Installing the amqp gem via "Rubygems":http://rubygems.org and "Bundler":http://gembundler.com.
 * Running a "Hello, world" messaging example that is a simple demonstration of 1:1 communication.
 * Creating a "Twitter-like" publish/subscribe example with 1 publisher and 4 subscribers that demonstrates 1:n communication.
 * Creating a topic routing example with 2 publishers and 8 subscribers showcasing n:m communication when subscribers only receive messages that they are interested in.
 * Learning how the amqp gem can be integrated with Ruby objects in a way that makes unit testing easy.

This work is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by/3.0/">Creative Commons Attribution 3.0 Unported License</a> (including images & stylesheets). The source is available "on Github":https://github.com/ruby-amqp/amqp/tree/master/docs.


h2. Which versions of the amqp gem does this guide cover?

This guide covers v0.8.0 and later of the "Ruby amqp gem":https://github.com/ruby-amqp/amqp.


h2. Installing RabbitMQ

The "RabbitMQ site":http://rabbitmq.com has a good "installation guide":http://www.rabbitmq.com/install.html that addresses many operating systems.
On Mac OS X, the fastest way to install RabbitMQ is with "Homebrew":http://mxcl.github.com/homebrew/:

<pre>
<code>
brew install rabbitmq
</code>
</pre>

then run it:

<pre>
<code>
rabbitmq-server
</code>
</pre>

On Debian and Ubuntu, you can either "download the RabbitMQ .deb package":http://www.rabbitmq.com/server.html and install it with
"dpkg":http://www.debian.org/doc/FAQ/ch-pkgtools.en.html or make use of the "apt repository":http://www.rabbitmq.com/debian.html#apt that the RabbitMQ team provides.
For RPM-based distributions like RedHat or CentOS, the RabbitMQ team provides an "RPM package":http://www.rabbitmq.com/install.html#rpm.

<span class="note">
The RabbitMQ package that ships with recent Ubuntu 10.10 versions is outdated and *will not work with v0.8.0 and later of the amqp gem* (we need at least RabbitMQ v2.0 for use with this guide).
</span>



h2. Installing the Ruby amqp gem

h3. Make sure that you have Ruby and "Rubygems":http://docs.rubygems.org/read/chapter/3 installed

This guide assumes that you have installed one of the following supported Ruby implementations:

 * Ruby v1.8.7
 * Ruby v1.9.2
 * JRuby (we recommend v1.6)
 * Rubinius v1.2 or higher
 * Ruby Enterprise Edition

h3. You can use Rubygems to install the amqp gem

h4. On Microsoft Windows 7:

<pre>
gem install eventmachine --pre
gem install amqp --pre
</pre>

h4. On other OSes or JRuby:

<pre>
gem install amqp --pre
</pre>

h3. You can also use Bundler to install the gem 

<pre>
<code>
source :rubygems

gem "amqp", "~> 0.8.0.RC15" # optionally: :git => "git://github.com/ruby-amqp/amqp.git", :branch => "master"
</code>
</pre>

h3. Verifying your installation

Let us verify your installation with this quick irb session:

<pre>
<code>
irb -rubygems

:001 > require "amqp"
=> true
:002 > AMQP::VERSION
=> "0.8.0.rc15"
</code>
</pre>


h2. "Hello, world" example

Let us begin with the classic "Hello, world" example. First, here is the code:

<script src="https://gist.github.com/998690.js"> </script>

(if the example above isn't displayed, see this "gist":https://gist.github.com/998690)

This example demonstrates a very common communication scenario: *application A* wants to publish a message that will end up in
a queue that *application B* listens on. In this case, the queue name is "amqpgem.examples.hello". Let us go through the code
step by step:

<pre>
<code>
require "rubygems"
require "amqp"
</code>
</pre>

is the simplest way to load the amqp gem if you have installed it with RubyGems. The following piece of code

<pre>
<code>
EventMachine.run do
  # ...
end
</code>
</pre>

runs what is called the "EventMachine":http://rubyeventmachine.com reactor. We will not go into what the term 'reactor' means here, but suffice it to say
that the amqp gem is asynchronous and is based on an asynchronous network I/O library called _EventMachine_.

The next line

<pre>
<code>
connection = AMQP.connect(:host => '127.0.0.1')
</code>
</pre>

connects to the server running on localhost, with the default port (5672), username (guest), password (guest) and virtual host ('/').

The next line

<pre>
<code>
channel  = AMQP::Channel.new(connection)
</code>
</pre>

opens a new _channel_. AMQP is a multi-channeled protocol that uses channels to multiplex a TCP connection.
Channels are opened on a connection, therefore the AMQP::Channel constructor takes a connection object as a parameter.

This line

<pre>
<code>
queue    = channel.queue("amqpgem.examples.helloworld", :auto_delete => true)
</code>
</pre>

declares a _queue_ on the channel that we have just opened. Consumer applications get messages from queues.
We declared this queue with the "auto-delete" parameter. Basically, this means that the queue will be deleted when there are no more processes
consuming messages from it.

The next line

<pre>
<code>
exchange = channel.direct("")
</code>
</pre>

instantiates an _exchange_. Exchanges receive messages that are sent by producers. Exchanges route messages to queues
according to rules called _bindings_. In this particular example, there are no explicitly defined bindings.
The exchange that we defined is known as the _default exchange_ and it has implied bindings to all queues. Before we get
into that, let us see how we define a handler for incoming messages

<pre>
<code>
queue.subscribe do |payload|
  puts "Received a message: #{payload}. Disconnecting..."
  connection.close { EventMachine.stop }
end
</code>
</pre>

{AMQP::Queue#subscribe} takes a block that will be called every time a message arrives. {AMQP::Session#close} closes the 
AMQP connection and runs a callback that stops the EventMachine reactor.

Finally, we publish our message

<pre>
<code>
exchange.publish "Hello, world!", :routing_key => queue.name
</code>
</pre>

Routing key is one of the _message attributes_. The default exchange will route the message to a queue that has the same name
as the message's routing key. This is how our message ends up in the "amqpgem.examples.helloworld" queue.

This first example can be modified to use the method chaining technique:

<script src="https://gist.github.com/998691.js"> </script>

This diagram demonstrates the "Hello, world" example data flow:

!https://github.com/ruby-amqp/amqp/raw/master/docs/diagrams/001_hello_world_example_routing.png!

For the sake of simplicity, both the message producer (App I) and the consumer (App II) are running in the same Ruby process.
Now let us move on to a little bit more sophisticated example.


h2. Blabblr: one-to-many publish/subscribe (pubsub) example

The previous example demonstrated how a connection to a broker is made and how to do 1:1 communication
using the default exchange. Now let us take a look at another common scenario: broadcast, or multiple consumers
and one producer.

A very well-known broadcast example is Twitter: every time a person tweets, followers receive a notification.
Blabbr, our imaginary information network, models this scenario: every network member has a separate
queue and publishes blabs to a separate exchange. 3 Blabbr members, Joe, Aaron and Bob, follow the official NBA
account on Blabbr to get updates about what is happening in the world of basketball. Here is the code:

<script src="https://gist.github.com/998692.js"> </script>

The first line has a few differences from the "Hello, world" example above:

 * We use {AMQP.start} instead of {AMQP.connect}
 * Instead of return values, we pass a block to the connection method and it yields a connection
   object back as soon as the connection is established.
 * Instead of passing connection parameters as a hash, we use a URI string.

{AMQP.start} is just a convenient way to do

<pre>
<code>
EventMachine.run do
  AMQP.connect(options) do |connection|
    # ...
  end
end
</code>
</pre>

The {AMQP.start} call blocks the current thread which means that its use is limited to scripts and small command
line applications. Blabbr is just that.

{AMQP.connect}, when invoked with a block, will yield a connection object as soon as the AMQP connection
is open. Finally, connection parameters may be supplied as a Hash or as a connection string. The {AMQP.connect}
method documentation contains all of the details.

In this example, opening a channel is no different to opening a channel in the previous example,
however, the exchange is declared differently

<pre>
<code>
exchange = channel.fanout("nba.scores")
</code>
</pre>

The exchange that we declare above using {AMQP::Channel#fanout} is a _fanout exchange_. A fanout exchange delivers messages to all of the queues that
 are bound to it: exactly what we want in the case of Blabbr!

This piece of code

<pre>
<code>
channel.queue("joe", :auto_delete => true).bind(exchange).subscribe do |payload|
  puts "#{payload} => joe"
end
</code>
</pre>

is similar to the subscription code that we used for message delivery previously, but what does that {AMQP::Queue#bind}
method do? It sets up a binding between the queue and the exchange that you pass to it. We need to do this
to make sure that our fanout exchange routes messages to the queues of any subscribed followers.

<pre>
<code>
exchange.publish("BOS 101, NYK 89").publish("ORL 85, ALT 88")
</code>
</pre>

demonstrates {AMQP::Exchange#publish} call chaining. Blabbr members use a fanout exchange
for publishing, so there is no need to specify a message routing key because every queue that is bound to the exchange will get its own copy of all messages, regardless of the queue name and routing key used.

A diagram for Blabbr looks like this:

!https://github.com/ruby-amqp/amqp/raw/master/docs/diagrams/002_blabbr_example_routing.png!


Next we use EventMachine's {http://eventmachine.rubyforge.org/EventMachine.html#M000466 add_timer} method to
run a piece of code in 1 second from now:

<pre>
<code>
EventMachine.add_timer(1) do
  exchange.delete

  connection.close { EventMachine.stop }
end
</code>
</pre>

The code that we want to run deletes the exchange that we declared earlier using {AMQP::Exchange#delete} and closes the AMQP
connection with {AMQP::Session#close}. Finally, we stop the EventMachine event loop and exit.

Blabbr is pretty unlikely to secure hundreds of millions of dollars in funding, but it does a pretty good job of
demonstrating how one can use AMQP fanout exchanges to do broadcasting.



h2. Weathr: many-to-many topic routing example

So far, we have seen point-to-point communication and broadcasting. Those two communication styles are possible with many protocols, for instance,
HTTP handles these scenarios just fine. You may ask "what differentiates AMQP?" Well, next we are going to introduce you to _topic
exchanges_ and routing with patterns, one of the features that makes AMQP very powerful.

Our third example involves weather condition updates. What makes it different from the previous two examples is that
not all of the consumers are interested in all of the messages. People who live in Portland usually do not care about
the weather in Hong Kong (unless they are visiting soon). They are much more interested in
weather conditions around Portland, possibly all of Oregon and sometimes a few neighbouring states.

Our example features multiple consumer applications monitoring updates for different regions. Some are
interested in updates for a specific city, others for a specific state and so on, all the way up to continents.
Updates may overlap so that an update for San Diego, CA appears as an update for California, but also should show up
on the North America updates list.

Here is the code:

<script src="https://gist.github.com/998694.js"> </script>

The first line that is different from the Blabbr example is

<pre>
<code>
exchange = channel.topic("pub/sub", :auto_delete => true)
</code>
</pre>

We use a topic exchange here. Topic exchanges are used for "multicast":http://en.wikipedia.org/wiki/Multicast messaging
where consumers indicate which topics they are interested in (think of it as subscribing to a feed for an individual tag
in your favourite blog as opposed to the full feed). Routing with a topic exchange is done by specifying a _routing pattern_ on binding, for example:

<pre>
<code>
channel.queue("americas.south").bind(exchange, :routing_key => "americas.south.#").subscribe do |headers, payload|
  puts "An update for South America: #{payload}, routing key is #{headers.routing_key}"
end
</code>
</pre>

Here we bind a queue with the name of "americas.south" to the topic exchange declared earlier using the {AMQP::Queue#bind} method.
This means that only messages with a routing key matching "americas.south.#" will be routed to that queue. A routing pattern consists of several words
separated by dots, in a similar way to URI path segments joined by slashes. Here are a few examples:

 * asia.southeast.thailand.bangkok
 * sports.basketball
 * usa.nasdaq.aapl
 * tasks.search.indexing.accounts

Now let us take a look at a few routing keys that match the "americas.south.#" pattern:

 * americas.south
 * americas.south.*brazil*
 * americas.south.*brazil.saopaolo*
 * americas.south.*chile.santiago*

In other words, the "#" part of the pattern matches 0 or more words.

For a pattern like "americas.south.*", some matching routing keys would be:

 * americas.south.*brazil*
 * americas.south.*chile*
 * americas.south.*peru*

but not

 * americas.south
 * americas.south.chile.santiago

so "*" only matches a single word. The AMQP v0.9.1 specification says that topic segments (words) may contain the letters A-Z and a-z
and digits 0-9.

A (very simplistic) diagram to demonstrate topic exchange in action:

!https://github.com/ruby-amqp/amqp/raw/master/docs/diagrams/003_weathr_example_routing.png!


One more thing that is different from previous examples is that the block we pass to {AMQP::Queue#subscribe} now takes two arguments:
a _header_ and a _body_ (often called the _payload_). Long story short, the header parameter lets you access metadata associated with the message. Some
examples of message metadata attributes are:

 * message content type
 * message content encoding
 * message priority
 * message expiration time
 * message identifier
 * reply to (specifies which message this is a reply to)
 * application id (identifier of the application that produced the message)

and so on.

As the following binding demonstrates, "#" and "*" can also appear at the beginning of routing patterns:

<pre>
<code>
channel.queue("us.tx.austin").bind(exchange, :routing_key => "#.tx.austin").subscribe do |headers, payload|
  puts "An update for Austin, TX: #{payload}, routing key is #{headers.routing_key}"
end
</code>
</pre>

For this example the publishing of messages is no different from that of previous examples. If we were to run the program,
a message published with a routing key of "americas.north.us.ca.berkeley" would be routed to 2 queues: "us.california" and the
_server-named queue_ that we declared by passing a blank string as the name:

<pre>
<code>
channel.queue("", :exclusive => true) do |queue|
  queue.bind(exchange, :routing_key => "americas.north.#").subscribe do |headers, payload|
    puts "An update for North America: #{payload}, routing key is #{headers.routing_key}"
  end
end
</code>
</pre>

The name of the server-named queue is generated by the broker and sent back to the client with a queue declaration confirmation.
Because the queue name is not known before the reply arrives, we pass {AMQP::Channel#queue} a callback and it yields us back
a queue object once confirmation has arrived.


h3. Avoid race conditions

A word of warning: you may find examples on the Web of {AMQP::Channel#queue} usage that do not use
callbacks. We *recommend that you use a callback for server-named queues*, otherwise your code may be subject
to "race conditions":http://en.wikipedia.org/wiki/Race_condition. Even though the amqp gem tries to be reasonably smart and protect you from most common problems
(for example, binding operations will be delayed until after queue name is received from the broker), there
is no way it can do so for every case. The primary reason for supporting {AMQP::Channel#queue} usage without a callback for server-named queues is
backwards compatibility with earlier versions.



h2. Integration with objects

Since Ruby is a genuine object-oriented language, it is important to demonstrate how the Ruby amqp gem can be integrated
into rich object-oriented code.

The {AMQP::Queue#subscribe} callback does not have to be a block. It can be any Ruby object that responds to `call`.
A common technique is to combine {http://rubydoc.info/stdlib/core/1.8.7/Object:method Object#method} and {http://rubydoc.info/stdlib/core/1.8.7/Method:to_proc Method#to_proc}
and use object methods as message handlers.

An example to demonstrate this technique:

<pre>
<code>
class Consumer

  #
  # API
  #

  def handle_message(metadata, payload)
    puts "Received a message: #{payload}, content_type = #{metadata.content_type}"
  end # handle_message(metadata, payload)
end


class Worker

  #
  # API
  #


  def initialize(channel, queue_name = AMQ::Protocol::EMPTY_STRING, consumer = Consumer.new)
    @queue_name = queue_name

    @channel    = channel
    @channel.on_error(&method(:handle_channel_exception))

    @consumer   = consumer
  end # initialize

  def start
    @queue = @channel.queue(@queue_name, :exclusive => true)
    @queue.subscribe(&@consumer.method(:handle_message))
  end # start



  #
  # Implementation
  #

  def handle_channel_exception(channel, channel_close)
    puts "Oops... a channel-level exception: code = #{channel_close.reply_code}, message = #{channel_close.reply_text}"
  end # handle_channel_exception(channel, channel_close)
end
</code>
</pre>

The "Hello, world" example can be ported to use this technique:
<script src="https://gist.github.com/1009447.js"> </script>

The most important line in this example is

<pre>
<code>
@queue.subscribe(&@consumer.method(:handle_message))
</code>
</pre>

Ampersand (&) preceding an object is equivalent to calling the #to_proc method on it. We obtain a Consumer#handle_message method reference
with

<pre>
<code>
@consumer.method(:handle_message)
</code>
</pre>

and then the ampersand calls #to_proc on it. {AMQP::Queue#subscribe} then will be using this Proc instance to handle incoming messages.


Note that the `Consumer` class above can be easily tested in isolation, without spinning up any AMQP connections.
Here is one example using "RSpec":http://relishapp.com/rspec

<pre>
<code>
require "ostruct"
require "json"

# RSpec example
describe Consumer do
  describe "when a new message arrives" do
    subject { described_class.new }

    let(:metadata) do
      o = OpenStruct.new

      o.content_type = "application/json"
      o
    end
    let(:payload)  { JSON.encode({ :command => "reload_config" }) }

    it "does some useful work" do
      # check preconditions here if necessary

      subject.handle_message(metadata, payload)

      # add your code expectations here
    end
  end
end
</code>
</pre>


h2. Wrapping up

This is the end of the tutorial. Congratulations! You have learned quite a bit about both AMQP v0.9.1 and the amqp gem. This is only
the tip of the iceberg. AMQP has many more features built into the protocol:

 * Reliable delivery of messages
 * Message confirmations (a way to tell broker that a message was or was not processed successfully)
 * Message redelivery when consumer applications fail or crash
 * Load balancing of messages between multiple consumers
 * Message metadata attributes

and so on. Other guides explain these features in depth, as well as use cases for them. To stay up to date with amqp gem
development, "follow @rubyamqp on Twitter":http://twitter.com/rubyamqp and "join our mailing list":http://groups.google.com/group/ruby-amqp.




h2. What to read next

Documentation is organized as a number of {file:docs/DocumentationGuidesIndex.textile documentation guides}, covering all kinds of
topics from {file:docs/Exchanges.textile use cases for various exchange types} to {file:docs/ErrorHandling.textile error handling} and
{file:docs/VendorSpecificExchanges.textile Broker-specific AMQP 0.9.1 extensions}.

We recommend that you read the following guides next, if possible, in this order:

 * {file:docs/AMQP091ModelExplained.textile AMQP 0.9.1 Model Explained}. A simple 2 page long introduction to the AMQP Model concepts and features. Understanding the AMQP Model
   will make a lot of other documentation, both for the Ruby amqp gem and RabbitMQ itself, easier to follow. With this guide, you don't have to waste hours of time reading the whole specification.
 * {file:docs/ConnectingToTheBroker.textile Connection to the broker}. This guide explains how to connect to an AMQP broker and how to integrate the amqp gem into standalone and Web applications.
 * {file:docs/Queues.textile Working With Queues}. This guide focuses on features that consumer applications use heavily.
 * {file:docs/Exchanges.textile Working With Exchanges}. This guide focuses on features that producer applications use heavily.
 * {file:docs/PatternsAndUseCases.textile Patterns & Use Cases}. This guide focuses implementation of "common messaging patterns":http://www.eaipatterns.com/ using AMQP Model features as building blocks.
 * {file:docs/ErrorHandling.textile Error Handling & Recovery}. This guide explains how to handle protocol errors, network failures and other things that may go wrong in real world projects.

If you are migrating your application from earlier versions of the amqp gem (0.6.x and 0.7.x), to 0.8.x and later, there is the
{file:docs/08Migration.textile amqp gem 0.8 migration guide}.


h2. Authors

This guide was written by "Michael Klishin":http://twitter.com/michaelklishin and edited by "Chris Duncan":https://twitter.com/celldee.



h2. Tell us what you think!

Please take a moment to tell us what you think about this guide "on Twitter":http://twitter.com/rubyamqp or the "Ruby AMQP mailing list":http://groups.google.com/group/ruby-amqp.
 Let us know what was unclear or what has not been covered. Maybe you do not like the guide style or grammar or discover spelling mistakes. Reader feedback is
key to making the documentation better.

If, for some reason, you cannot use the communication channels mentioned above, you can "contact the author of the guides directly":mailto:michael@novemberain.com?subject=amqp%20gem%20documentation


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