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d0378cd @tobi Lets try this again as textile instead of markdown
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1 h1. Delayed::Job
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2
2917b5f @chalkers Delayed_job not Delated_job
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3 Delayed_job (or DJ) encapsulates the common pattern of asynchronously executing longer tasks in the background.
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4
5 It is a direct extraction from Shopify where the job table is responsible for a multitude of core tasks. Amongst those tasks are:
6
7 * sending massive newsletters
8 * image resizing
9 * http downloads
10 * updating smart collections
11 * updating solr, our search server, after product changes
12 * batch imports
13 * spam checks
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14
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15 h2. Setup
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16
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17 The library evolves around a delayed_jobs table which can be created by using:
18 <pre><code>
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19 script/generate delayed_job
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20 </code></pre>
21
22 The created table looks as follows:
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23
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24 <pre><code>
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25 create_table :delayed_jobs, :force => true do |table|
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26 table.integer :priority, :default => 0 # Allows some jobs to jump to the front of the queue
27 table.integer :attempts, :default => 0 # Provides for retries, but still fail eventually.
28 table.text :handler # YAML-encoded string of the object that will do work
29 table.string :last_error # reason for last failure (See Note below)
30 table.datetime :run_at # When to run. Could be Time.now for immediately, or sometime in the future.
31 table.datetime :locked_at # Set when a client is working on this object
32 table.datetime :failed_at # Set when all retries have failed (actually, by default, the record is deleted instead)
33 table.string :locked_by # Who is working on this object (if locked)
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34 table.timestamps
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35 end
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36 </code></pre>
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37
38 On failure, the job is scheduled again in 5 seconds + N ** 4, where N is the number of retries.
39
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40 The default @MAX_ATTEMPTS@ is @25@. After this, the job either deleted (default), or left in the database with "failed_at" set.
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41 With the default of 25 attempts, the last retry will be 20 days later, with the last interval being almost 100 hours.
42
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43 The default @MAX_RUN_TIME@ is @4.hours@. If your job takes longer than that, another computer could pick it up. It's up to you to
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44 make sure your job doesn't exceed this time. You should set this to the longest time you think the job could take.
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46 By default, it will delete failed jobs (and it always deletes successful jobs). If you want to keep failed jobs, set
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47 @Delayed::Job.destroy_failed_jobs = false@. The failed jobs will be marked with non-null failed_at.
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48
49 Here is an example of changing job parameters in Rails:
50
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51 <pre><code>
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52 # config/initializers/delayed_job_config.rb
53 Delayed::Job.destroy_failed_jobs = false
54 silence_warnings do
55 Delayed::Job.const_set("MAX_ATTEMPTS", 3)
56 Delayed::Job.const_set("MAX_RUN_TIME", 5.minutes)
57 end
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58 </code></pre>
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59
60 Note: If your error messages are long, consider changing last_error field to a :text instead of a :string (255 character limit).
61
62
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63 h2. Usage
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65 Jobs are simple ruby objects with a method called perform. Any object which responds to perform can be stuffed into the jobs table.
66 Job objects are serialized to yaml so that they can later be resurrected by the job runner.
67
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68 <pre><code>
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69 class NewsletterJob < Struct.new(:text, :emails)
70 def perform
71 emails.each { |e| NewsletterMailer.deliver_text_to_email(text, e) }
72 end
73 end
74
75 Delayed::Job.enqueue NewsletterJob.new('lorem ipsum...', Customers.find(:all).collect(&:email))
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76 </code></pre>
77
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78 There is also a second way to get jobs in the queue: send_later.
79
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80 <pre><code>
81 BatchImporter.new(Shop.find(1)).send_later(:import_massive_csv, massive_csv)
82 </code></pre>
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83
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84 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
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85 which are stored as their text representation and loaded from the database fresh when the job is actually run later.
86
87
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88 h2. Running the jobs
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89
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90 You can invoke @rake jobs:work@ which will start working off jobs. You can cancel the rake task with @CTRL-C@.
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91
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92 You can also run by writing a simple @script/job_runner@, and invoking it externally:
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93
94 <pre><code>
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95 #!/usr/bin/env ruby
96 require File.dirname(__FILE__) + '/../config/environment'
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97
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98 Delayed::Worker.new.start
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99 </code></pre>
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100
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101 Workers can be running on any computer, as long as they have access to the database and their clock is in sync. You can even
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102 run multiple workers on per computer, but you must give each one a unique name:
103
104 <pre><code>
105 3.times do |n|
106 worker = Delayed::Worker.new
107 worker.name = 'worker-' + n.to_s
108 worker.start
109 end
110 </code></pre>
111
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112 Keep in mind that each worker will check the database at least every 5 seconds.
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114 Note: The rake task will exit if the database has any network connectivity problems.
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116 h3. Cleaning up
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118 You can invoke @rake jobs:clear@ to delete all jobs in the queue.
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120 h3. Changes
121
122 * 1.7.0: Added failed_at column which can optionally be set after a certain amount of failed job attempts. By default failed job attempts are destroyed after about a month.
123
124 * 1.6.0: Renamed locked_until to locked_at. We now store when we start a given job instead of how long it will be locked by the worker. This allows us to get a reading on how long a job took to execute.
125
126 * 1.5.0: 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 instead of relying on row level locks. This enables us to run as many worker processes as we need to speed up queue processing.
127
128 * 1.2.0: Added #send_later to Object for simpler job creation
129
130 * 1.0.0: Initial release
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