Delayed::Job
============
 
Delated_job (or DJ) encapsulates the common pattern of asynchronously executing longer tasks in the background. 
 
It is a direct extraction from Shopify where the job table is responsible for a multitude of core tasks. Amongst those tasks are:
 
* sending massive newsletters
* image resizing
* http downloads
* updating smart collections
* updating solr, our search server, after product changes
* batch imports 
* spam checks 
            
== Setup == 
            
The library evolves around a delayed_jobs table which looks as follows: 
 
  create_table :delayed_jobs, :force => true do |table|
    table.integer :priority, :default => 0
    table.integer :attempts, :default => 0
    table.text :handler
    table.string :last_error
    table.datetime :run_at   
    table.timestamps    
  end        
  
== Usage == 
 
Jobs are simple ruby objects with a method called perform. Any object which responds to perform can be stuffed into the jobs table.
Job objects are serialized to yaml so that they can later be resurrected by the job runner. 
 
  class NewsletterJob < Struct.new(:text, :emails)
    def perform
      emails.each { |e| NewsletterMailer.deliver_text_to_email(text, e) }
    end    
  end  
  
  Delayed::Job.enqueue NewsletterJob.new('lorem ipsum...', Customers.find(:all).collect(&:email))
           
There is also a second way to get jobs in the queue: send_later. 
 
  
  BatchImporter.new(Shop.find(1)).send_later(:import_massive_csv, massive_csv)                                                    
  
 
This will simply create a Delayed::PerformableMethod job in the jobs table which serializes all the parameters you pass to it. There are some special smarts for active record objects
which are stored as their text representation and loaded from the database fresh when the job is actually run later.
                                                                                                                              
                                                                                                                    
== Running the tasks ==        
 
You can invoke rake jobs:work which will start working off jobs. You can cancel the rake task by CTRL-C. 
 
At Shopify we run the the tasks from a simple script/job_runner which is being invoked by runnit: 
                                           
  #!/usr/bin/env ruby
  require File.dirname(__FILE__) + '/../config/environment'
 
  SLEEP = 15
  RESTART_AFTER = 1000
 
  trap('TERM') { puts 'Exiting...'; $exit = true }
  trap('INT')  { puts 'Exiting...'; $exit = true }
                
  # this script dies after several runs to prevent memory leaks. 
  # runnit will immediately start it again. 
  count, runs_left = 0, RESTART_AFTER
 
  loop do
  
    count = 0
                      
    # this requires the locking plugin, also from jadedPixel
    ActiveRecord::base.aquire_lock("jobs table worker", 10) do
      puts 'got lock'
    
      realtime = Benchmark.realtime do 
        count = Delayed::Job.work_off      
      end
    end
  
    runs_left -= 1
  
    break if $exit
  
    if count.zero? 
      sleep(SLEEP)
    else
      status = "#{count} jobs completed at %.2f j/s ..." % [count / realtime]
      RAILS_DEFAULT_LOGGER.info status
      puts status
    end
    
    if $exit or runs_left <= 0
      break 
    end
  end     
  
== Todo == 
                                                                                                                           
Work out a locking mechanism which would allow several job runners to run at the same time, spreading the load between them.