= Minion: super simple job queue over amqp

Minion makes processing jobs over AMQP simple and easy.

== Setup

Minion pulls the AMQP credentials out the environment via AMQP_URL.

  $ export AMQP_URL="amqp://johndoe:abc123@localhost/my_vhost"

Alternativly you can explicitly set it programmatically like this:

  Minion.amqp_url = "amqp://johndoe:abc123@localhost/my_vhost"

If no URL is supplied, Minion defaults to "amqp://guest:guest@localhost/" which
is the default credentials for Rabbitmq running locally.

== Principles

Minion treats your jobs with respect.  The queues are durable and not
autodelete.  When popping jobs off the queue, they will not receive an ack
until the job is done.  You can rest assured that once queued, the job will not
be lost.

Sends are done synchronously and receives are done asynchronously.  This allows
you to Minion.enqueue() from the console, or in a mongrel and you don't need to
worry about eventmachine.  It also means that when enqueue returns, the AMQP
server has received your message. Daemons set to receive messages however use
eventmachine.

Message processing is done one at a time (prefetch 1).  If you want tasks done
in parallel, run two minions.

== Push a job onto the queue

Its easy to push a job onto the queue.

  Minion.enqueue("make.sandwich", { "for" => "me", "with" => "bread" })

Minion expects a queue name (and will create it if needed). The second argument
needs to be a hash.  

== Processing a job

  require 'minion'

  include Minion

  job "make.sandwich" do |args|
    Sandwich.make(args["for"],args["with"])
  end

== Chaining multiple steps

If you have a task that requires more than one step just pass an array of
queues when you enqueue.

  Minion.enqueue([ "make.sandwich", "eat.sandwich" ], "for" => "me")

  job "make.sandwich" do
    ## this return value is merged with for => me and sent to the next queue
    { "type" => "ham on rye" }  
  end

  job "eat.sandwich" do |args|
    puts "I have #{args["type"]} sandwich for #{args["for"]}"
  end

== Conditional Processing

If you want a minion worker to only subscribe to a queue under specific
conditions there is a :when parameter that takes a lambda as an argument.  For
example, if you had a queue that makes sandwiches but only if there is bread
on hand, it would be.

  job "make.sandwich", :when => lambda { not Bread.out? } do
    Sandwich.make
  end

== Error handling

When an error is thrown in a job handler, the job is requeued to be done later
and the minion process exits.  If you define an error handler, however, the
error handler is run and the job is removed from the queue.

  error do |e|
    puts "got an error! #{e}"
  end

== Logging

Minion logs to stdout via "puts".  You can specify a custom logger like this:

  logger do |msg|
    puts msg
  end

== Meta

Created by Orion Henry

Patches contributed by Adam Wiggins, Kyle Drake

Released under the MIT License: www.opensource.org/licenses/mit-license.php