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Using AMQP as a Queuing Backend for Web Apps Should Be Easy
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README.md

Qusion

Qusion makes AMQP work with your web server with no fuss. It offers three features:

  • It sets up the required callbacks and/or worker threads so that AMQP will work with Passenger, Thin, or Mongrel. WEBrick, SCGI, and Evented Mongrel are experimentally supported, but not heavily tested.
  • A Channel Pool. You can cause problems for yourself if you create new channels (with MQ.new) for every request. The pool sets up a few of these when your app starts and reuses them.
  • YAML configuration files. In Rails, create config/amqp.yml then fill in the details for development, test and production. Use Qusion.start() in your environment.rb file (or an initializer) and you're good to go.

This fork of Qusion

This is a fork of James Tucker's fork of Dan DeLeo's original version. Tucker did a fair amount of cleanup, mostly to the threading logic, so I went with his fork. Improvements I've made include:

  • Conversion from a Rails plugin to this gem.
  • Support for Rails 3.
  • Support for Ruby AMQP 0.8.x (for 0.7.x see the amqp-0.7.x-stable branch).
  • Removed support for Merb (it complicated the code and tests; just use Rails 3).

Before You Start

Qusion makes it easy to just install and start using AMQP in your application. But there are many ways to use background jobs within a Rails app, so it's worth taking some time to consider the tradeoffs of each approach.

  • If your background job needs are simple and you're using a relational database, Delayed::Job lets you schedule background tasks through the database. You won't need to run another application (the AMQP Broker) to keep your app running.
  • It may make more sense to run your AMQP subscriber(s) as a daemon instead of via Qusion. This way it's easy to monitor/restart it if it goes down. Some prefer to publish messages using Bunny, a synchronous gem. This is a fairly common approach.
  • Qusion runs EventMachine in a separate thread on Phusion Passenger, Mongrel, and other non-evented servers. There are some inefficiencies in Ruby 1.8's threading model that make running EM in a thread quite slow. Joe Damato and Aman Gupta have created a patch for the problem which is included in an experimental branch of REE. You can learn more about the patch from Phusion's Blog.

Getting Started

First you'll need the amqp library and a working RabbitMQ installation. This entails:

  • Install Erlang for your platform
  • Install RabbitMQ for your platform
    • On OSX, use Homebrew to install Erlang and RabbitMQ: brew install rabbitmq
  • Install bundler: http://gembundler.com/
  • Include the qusion gem in your Rails project's Gemfile: gem "qusion"

Create an initializer (e.g. config/initializers/qusion.rb) and add:

Qusion.start

EM.next_tick do
  # do some AMQP stuff
end

And that's it! This will set up AMQP for any ruby app server (tested on mongrel, thin, and passenger). Now, you can use all of AMQP's functionality as normal. In your controllers or models, you might have:

AMQP::Channel.new.queue("my-work-queue").publish("do work, son!")

and it should just work.

Channel Pools

It's considered bad practice to use MQ.new over and over, as it creates a new AMQP channel, and that creates a new Erlang process in RabbitMQ. Erlang processes are super light weight, but you'll be wasting them and causing the Erlang VM GC headaches if you create them wantonly. So don't do that. Instead, use the channel pool provided by Qusion. It's simple: wherever you'd normally put MQ.new, just replace it with Qusion.channel. Examples:

# Create a queue:
Qusion.channel.queue("my-worker-queue")
# Topics:
Qusion.channel.topic("my-topic-exchange")
# etc.   

This feature is a bit experimental, so the optimal pool size isn't known yet. The default is 5. You can change it by adding something like the following to your environment.rb:

Qusion.channel_pool_size(3)

Configuration

You can put your AMQP server details in config/amqp.yml and Qusion will load it when you call Qusion.start(). Example:

# Put this in config/amqp.yml
default: &default
  host: localhost
  port: 5672
  user: guest
  pass: guest
  vhost: /
  timeout: 3600 # seconds
  logging: false
  ssl: false

development:
  <<: *default

test:
  <<: *default

If you're too hardcore for Rails (maybe you're using Sinatra or Ramaze), you can still use a YAML config file, but there's no support for different environments. So do something like this:

# Tell Qusion where your config file is:
Qusion.start("/path/to/amqp.yml")

# Your configuration looks like this:
application:
  host: localhost
  port: 5672
  ... 

If you just want to get started without configuring anything, Qusion.start() will use the default options if it can't find a config file. And, finally, you can give options directly to Qusion.start() like this:

Qusion.start(:host => "my-amqp-broker.mydomain.com", :user => "me", :pass => "am_I_really_putting_this_in_VCS?")

Bugs? Hacking?

If you find any bugs, or feel the need to add a feature, fork away. Pull requests are very welcome. You can also report an issues via Github.

Shouts

Original author: dan@kallistec.com Forked by: chmurph2+git@gmail.com

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