Getting Started with RabbitMQ on JRuby using March Hare

About this guide

This guide is a quick tutorial that helps you to get started with RabbitMQ and March Hare. It should take about 20 minutes to read and study the provided code examples. This guide covers:

• Installing RabbitMQ, a mature popular messaging broker server.
• Installing March Hare via Rubygems and Bundler.
• Running a "Hello, world" messaging example that is a simple demonstration of 1:1 communication.
• Creating a "Twitter-like" publish/subscribe example with one publisher and four subscribers that demonstrates 1:n communication.
• Creating a topic routing example with two publishers and eight subscribers showcasing n:m communication when subscribers only receive messages that they are interested in.

This work is licensed under a Creative Commons Attribution 3.0 Unported License (including images and stylesheets). The source is available on GitHub.

Which versions of March Hare does this guide cover?

This guide covers March Hare 3.0, including preview releases.

Installing RabbitMQ

The RabbitMQ site has a good installation guide that addresses many operating systems. On Mac OS X, the fastest way to install RabbitMQ is with Homebrew:

brew install rabbitmq

then run it:

rabbitmq-server

On Debian and Ubuntu, you can either download the RabbitMQ .deb package and install it with dpkg or make use of the apt repository that the RabbitMQ team provides.

For RPM-based distributions like RedHat or CentOS, the RabbitMQ team provides an RPM package.

Installing March Hare

Make sure that you have JRuby 9.x installed

This guide assumes that you have JRuby 9.0.0.0 installed.

You can use Rubygems to install March Hare

gem install march_hare

Adding March Hare as a dependency with Bundler

source "https://rubygems.org"

gem "march_hare", "~> 3.0"

Verifying your installation

Verify your installation with a quick irb session:

irb -rubygems
:001 > require "march_hare"
=> true
:002 > MarchHare::VERSION
=> "3.0.0"

"Hello, world" example

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

require "rubygems"
require "march_hare"

conn = MarchHare.connect

ch = conn.create_channel
q  = ch.queue("march_hare.examples.hello_world", :auto_delete => true)

c  = q.subscribe do |metadata, payload|
  puts "Received #{payload}"
end

q.publish("Hello!", :routing_key => q.name)

sleep 1.0

c.cancel
conn.close

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 "bunny.examples.hello_world". Let us go through the code step by step:

require "rubygems"
require "march_hare"

is the simplest way to load March Hare if you have installed it with RubyGems, but remember that you can omit the rubygems line if your environment does not need it. The following piece of code

conn = MarchHare.connect

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

The next line

ch = conn.create_channel

opens a new channel. AMQP 0.9.1 is a multi-channeled protocol that uses channels to multiplex a TCP connection.

Channels are opened on a connection. MarchHare::Session#create_channel will return only when March Hare receives a confirmation that the channel is open from RabbitMQ.

This line

q  = ch.queue("march_hare.examples.hello_world", :auto_delete => true)

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

x  = ch.default_exchange

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 use 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

c = q.subscribe do |delivery_info, metadata, payload|
  puts "Received #{payload}"
end

MarchHare::Queue#subscribe takes a block that will be called every time a message arrives. This will happen in a thread pool, so MarchHare::Queue#subscribe does not block the thread that invokes it. It returns a consumer, a message delivery subscription that can be cancelled.

Finally, we publish our message

x.publish("Hello!", :routing_key => q.name)

Routing key is one of the message properties. 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 "bunny.examples.hello_world" queue.

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

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

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. Three 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:

require "rubygems"
require "march_hare"

conn = MarchHare.connect

ch  = conn.create_channel
x   = ch.fanout("march_hare.nba.scores")

ch.queue("joe",   :auto_delete => true).bind(x).subscribe do |meta, payload|
  puts "#{payload} => joe"
end

ch.queue("aaron", :auto_delete => true).bind(x).subscribe do |meta, payload|
  puts "#{payload} => aaron"
end

ch.queue("bob",   :auto_delete => true).bind(x).subscribe do |meta, payload|
  puts "#{payload} => bob"
end

x.publish("BOS 101, NYK 89")
x.publish("ORL 85, ALT 88")
sleep 1.0

conn.close

Unlike the "Hello, world" example above, here we use a connection URI instead of the default arguments.

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

x   = ch.fanout("nba.scores")

The exchange that we declare above using MarchHare::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

ch.queue("joe",   :auto_delete => true).bind(x).subscribe do |meta, payload|
  puts "#{payload} => joe"
end

is similar to the subscription code that we used for message delivery previously, but what does that MarchHare::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.

x.publish("BOS 101, NYK 89")
x.publish("ORL 85, ALT 88")

publishes two messages using MarchHare::Exchange#publish. 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:

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 RabbitMQ fanout exchanges to do broadcasting.

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 RabbitMQ?" Well, next we are going to introduce you to topic exchanges and routing with patterns, one of the features that makes RabbitMQ 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:

require "rubygems"
require "march_hare"

connection = MarchHare.connect

ch  = connection.create_channel
# topic exchange name can be any string
x   = ch.topic("weathr", :auto_delete => true)

# Subscribers.
ch.queue("", :exclusive => true).bind(x, :routing_key => "americas.north.#").subscribe do |metadata, payload|
  puts "An update for North America: #{payload}, routing key is #{metadata.routing_key}"
end
ch.queue("americas.south").bind(x, :routing_key => "americas.south.#").subscribe do |metadata, payload|
  puts "An update for South America: #{payload}, routing key is #{metadata.routing_key}"
end
ch.queue("us.california").bind(x, :routing_key => "americas.north.us.ca.*").subscribe do |metadata, payload|
  puts "An update for US/California: #{payload}, routing key is #{metadata.routing_key}"
end
ch.queue("us.tx.austin").bind(x, :routing_key => "#.tx.austin").subscribe do |metadata, payload|
  puts "An update for Austin, TX: #{payload}, routing key is #{metadata.routing_key}"
end
ch.queue("it.rome").bind(x, :routing_key => "europe.italy.rome").subscribe do |metadata, payload|
  puts "An update for Rome, Italy: #{payload}, routing key is #{metadata.routing_key}"
end
ch.queue("asia.hk").bind(x, :routing_key => "asia.southeast.hk.#").subscribe do |metadata, payload|
  puts "An update for Hong Kong: #{payload}, routing key is #{metadata.routing_key}"
end

x.publish("San Diego update",     :routing_key => "americas.north.us.ca.sandiego")
x.publish("Berkeley update",      :routing_key => "americas.north.us.ca.berkeley")
x.publish("San Francisco update", :routing_key => "americas.north.us.ca.sanfrancisco")
x.publish("New York update",      :routing_key => "americas.north.us.ny.newyork")
x.publish("São Paolo update",     :routing_key => "americas.south.brazil.saopaolo")
x.publish("Hong Kong update",     :routing_key => "asia.southeast.hk.hongkong")
x.publish("Kyoto update",         :routing_key => "asia.southeast.japan.kyoto")
x.publish("Shanghai update",      :routing_key => "asia.southeast.prc.shanghai")
x.publish("Rome update",          :routing_key => "europe.italy.roma")
x.publish("Paris update",         :routing_key => "europe.france.paris")

sleep 1.0

connection.close

The first line that is different from the Blabbr example is

x = ch.topic("weathr", :auto_delete => true)

We use a topic exchange here. Topic exchanges are used for 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:

ruby ch.queue("americas.south").bind(exchange, :routing_key => "americas.south.#").subscribe do |metadata, payload| puts "An update for South America: #{payload}, routing key is #{metadata.routing_key}" end Here we bind a queue with the name of "americas.south" to the topic exchange declared earlier using the MarchHare::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 0.9.1 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:

As in the previous examples, the block that we pass to MarchHare::Queue#subscribe takes multiple arguments: delivery information, message metadata (properties) and message body (often called the payload). Long story short, the metadata 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:

ch.queue("us.tx.austin").bind(x, :routing_key => "#.tx.austin").subscribe do |metadata, payload|

  puts "An update for Austin, TX: #{payload}, routing key is #{metadata.routing_key}"
end

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:

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

The name of the server-named queue is generated by the broker and sent back to the client with a queue declaration confirmation.

Wrapping up

This is the end of the tutorial. Congratulations! You have learned quite a bit about both AMQP 0.9.1 and March Hare. This is only the tip of the iceberg. RabbitMQ has many more features to offer:

• 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
• High Availability features

and so on. Other guides explain these features in depth, as well as use cases for them. To stay up to date with March Hare development, follow @rubyamqp on Twitter and join our mailing list.

What to read next

Documentation is organized as a number of documentation guides, covering all kinds of topics including use cases for various exchange types, fault-tolerant message processing with acknowledgements and error handling.

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

• AMQP 0.9.1 Model Explained. A simple 2 page long introduction to the AMQP Model concepts and features. Understanding the AMQP 0.9.1 Model will make a lot of other documentation, both for March Hare and RabbitMQ itself, easier to follow. With this guide, you don't have to waste hours of time reading the whole specification.
• Connecting to the broker. This guide explains how to connect to an RabbitMQ and how to integrate March Hare into standalone and Web applications.
• Queues and Consumers. This guide focuses on features that consumer applications use heavily.
• Exchanges and Publishers. This guide focuses on features that producer applications use heavily.
• Error Handling and Recovery. This guide explains how to handle protocol errors, network failures and other things that may go wrong in real world projects.

Tell Us What You Think!

Please take a moment to tell us what you think about this guide on Twitter or the March Hare mailing list

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.