Patterns and Use Cases

About this guide

This guide explains typical messaging patterns and use cases. It only covers the most common scenarios. For comprehensive list of messaging patterns, consult books on this subject, for example, Enterprise Integration Patterns.

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

Covered versions

This guide covers Ruby amqp gem v0.8.0 and later.

Introduction

Messaging patterns are a lot like object-oriented design patterns: they are generalized reusable solutions to specific problems. They are not recipes, however, and their exact implementation may and will vary from application to application. Just like OO design patterns, they can classified:

Message construction patterns describe form, content and purpose of messages.
Message routing patterns outline how messages can be directed from producers to consumers.
Message transformation patterns change message content or metadata.

There are other, more specialized group of messaging patterns that are out of scope of this guide.

This guide demonstrates implementation of several common routing patterns plus explains how built-in AMQP 0.9.1 features can be used to implement message construction and message transformation patterns.

TBD

Request/Reply pattern

Description & Use cases

Request/Reply is a simple way of integration when one application issues a request and another application responds to it. This pattern is often referred to as"Remote Procedure Call", even when it is not entirely correct. Request/Reply pattern is a 1:1 communication pattern.

Some examples of Request/Reply pattern are:

The 1st application requests a document that the 2nd application generates or loads and returns.
End-user application issues a search request and another application returns results back.
One application requests a progress report from another application.

AMQP-based implementation

Implementation of Request/Reply pattern on top of AMQP 0.9.1 involves two messages: a request (Req) and a response (Res). Client app generates a request identifier and sets :message_id attribute on Req. Client also uses a server-named exclusive queue to receive replies and thus sets :reply_to Req attribute to the name of that queue.

Server app uses a well-known queue name to receive requests and sets :correlation_id to :message_id of the original request message (Req) to make it possible for the client to identify what request this reply is for.

Request message attributes

:message_id: Unique message identifier
:reply_to: Queue name server should send the response to

Response message attributes

:correlation_id: Identifier of the original request message (set to request’s :correlation_id)
:routing_key: Client’s replies queue name (set to request’s :reply_to)

Code example

Client code


require "amqp"

EventMachine.run do
  connection = AMQP.connect
  channel    = AMQP::Channel.new(connection)

  replies_queue = channel.queue("", :exclusive => true, :auto_delete => true)
  replies_queue.subscribe do |metadata, payload|
    puts "[response] Response for #{metadata.correlation_id}: #{payload.inspect}"
  end

  # request time from a peer every 3 seconds
  EventMachine.add_periodic_timer(3.0) do
    puts "[request] Sending a request..."
    channel.default_exchange.publish("get.time",
                                     :routing_key => "amqpgem.examples.services.time",
                                     :message_id  => Kernel.rand(10101010).to_s,
                                     :reply_to    => replies_queue.name,
                                     :immediate   => true)
  end


  Signal.trap("INT") { connection.close { EventMachine.stop } }
end

Server code


require "amqp"

EventMachine.run do
  connection = AMQP.connect
  channel    = AMQP::Channel.new(connection)

  requests_queue = channel.queue("amqpgem.examples.services.time", :exclusive => true, :auto_delete => true)
  requests_queue.subscribe(:ack => true) do |metadata, payload|
    puts "[requests] Got a request #{metadata.message_id}. Sending a reply..."
    channel.default_exchange.publish(Time.now.to_s,
                                     :routing_key    => metadata.reply_to,
                                     :correlation_id => metadata.message_id,
                                     :immediate      => true,
                                     :mandatory      => true)

    metadata.ack
  end


  Signal.trap("INT") { connection.close { EventMachine.stop } }
end

In the examples above messages are published with the :immediate attribute set. This is not necessary in all cases: sometimes it is OK for requests to sit in the queue without active consumers. Replies, on the other hand, assume an active consumer and existing replies queue, so if routing or immediate delivery do not succeed, server application will log returned messages. More on this in the Working With Exchanges guide.

Related patterns

Request/Reply
Event
Scatter/Gather
Smart Proxy

Command pattern

Description & Use cases

Command pattern is very similar to Request/Reply, except that there is no reply and messages are typed. For example, most modern Web applications have at least one “background task processor” that carries out a number of operations asynchronously, without sending any responses back. Command pattern usually assumes 1:1 communication.

Some specific examples of Command pattern are:

Account termination in a Web app triggers information archiving (or deletion) that is done by a separate app “in the background”.
After a document or profile update, a Web app sends out commands to a search indexer application.
Virtual machines control dashboard app sends virtual machine controller application a command to reboot.

AMQP-based implementation

Implementation of Command pattern on top of AMQP 0.9.1 involves well-known durable queues. Application that issues the command then can use default exchange to publish messages to well-known services directly. Request message :type attribute then indicates command type and message body (or body and headers) carry additional information consumer needs to carry it out.

Request message attributes

:type: Message type, as a string. For example: gems.install or commands.shutdown

Code example

Producer (Sender)


require "rubygems"
require "amqp"
require "yaml"

t = Thread.new { EventMachine.run }
sleep(0.5)


connection = AMQP.connect
channel    = AMQP::Channel.new(connection, :auto_recovery => true)

channel.prefetch(1)

# Acknowledgements are good for letting the server know
# that the task is finished. If the consumer doesn't send
# the acknowledgement, then the task is considered to be unfinished
# and will be requeued when consumer closes AMQP connection (because of a crash, for example).
channel.queue("amqpgem.examples.patterns.command", :durable => true, :auto_delete => false).subscribe(:ack => true) do |metadata, payload|
  case metadata.type
  when "gems.install"
    data = YAML.load(payload)
    puts "[gems.install] Received a 'gems.install' request with #{data.inspect}"

    # just to demonstrate a realistic example
    shellout = "gem install #{data[:gem]} --version '#{data[:version]}'"
    puts "[gems.install] Executing #{shellout}"; system(shellout)
    puts "[gems.install] Done"
    puts
  else
    puts "[commands] Unknown command: #{metadata.type}"
  end

  # message is processed, acknowledge it so that broker discards it
  metadata.ack
end

puts "[boot] Ready. Will be publishing commands every 10 seconds."
Signal.trap("INT") { connection.close { EventMachine.stop } }
t.join

Consumer (Recipient)


require "amqp"
require "yaml"

t = Thread.new { EventMachine.run }
sleep(0.5)

connection = AMQP.connect
channel    = AMQP::Channel.new(connection)

# publish new commands every 3 seconds
EventMachine.add_periodic_timer(10.0) do
  puts "Publishing a command (gems.install)"
  payload = { :gem => "rack", :version => "~> 1.3.0" }.to_yaml

  channel.default_exchange.publish(payload,
                                   :type        => "gems.install",
                                   :routing_key => "amqpgem.examples.patterns.command")
end

puts "[boot] Ready"
Signal.trap("INT") { connection.close { EventMachine.stop } }
t.join

Related patterns

Event
Request/Reply

Event pattern

Description & Use cases

Event pattern is a version of the Command pattern, but with 1 or more receivers (1:N communication). The world we live in is full of events, so applications of this pattern are endless.

Some specific use cases of Event pattern are

Event logging (one application asks event collector to record certain event and possibly take action)
Event propagation in MMO games
Live sport score updates
Various “push notifications” for mobile applications

AMQP-based implementation

Because Event pattern is a 1:N communication pattern, it typically uses a fanout exchange. Event listeners then use server-named exclusive queues and all bind to that exchange. Event messages use :type message attribute to indicate event type and message body (plus, possibly, message headers) to pass event context information.

More on fanout exchange type in the Working With Exchanges guide.

Code example

TBD

Related patterns

Command
Publish/Subscribe

Document Message pattern

Description & Use cases

TBD

AMQP-based implementation

TBD

Code example

TBD

Competing Consumers pattern

Authors

These guides were written by Michael Klishin and edited by Chris Duncan

Tell us what you think!

Please take a moment and tell us what you think about this guide on Twitter or Ruby AMQP mailing list: what was unclear? what wasn’t covered? maybe you don’t like guide style or grammar and spelling are incorrect? Readers feedback is key to making documentation better.

If mailing list communication is not an option for you for some reason, you can contact guides author directly