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 
 
== Changes == 
                   
1.5 Job runners can now be run in parallel. Two new database columns are needed: locked_until and locked_by. This allows us 
    to use pessimistic locking, which enables us to run as many worker processes as we need to speed up queue processing.
1.0 Initial release
            
== 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.datetime :locked_until   
    table.string   :locked_by       
    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 = 5
 
  trap('TERM') { puts 'Exiting...'; $exit = true }
  trap('INT')  { puts 'Exiting...'; $exit = true }
 
  puts "*** Staring job worker #{Delayed::Job.worker_name}"
 
  begin
 
    loop do  
      result = nil                                 
  
      realtime = Benchmark.realtime do  
        result = Delayed::Job.work_off      
      end                                                                          
  
      count = result.sum
    
      break if $exit
  
      if count.zero? 
        sleep(SLEEP)
        puts 'Waiting for more jobs...'
      else
        status = "#{count} jobs processed at %.4f j/s, %d failed ..." % [count / realtime, result.last]
        RAILS_DEFAULT_LOGGER.info status
        puts status
      end
    
      break if $exit
    end
 
  ensure 
    Delayed::Job.clear_locks!
  end