Delayed_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
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delayed_job 3.0.0 only supports Rails 3.0+. See the 2.0 branch for Rails 2.
delayed_job supports multiple backends for storing the job queue. See the wiki for other backends.
If you plan to use delayed_job with Active Record, add delayed_job_active_record to your Gemfile.
gem 'delayed_job_active_record'
If you plan to use delayed_job with Mongoid, add delayed_job_mongoid to your Gemfile.
gem 'delayed_job_mongoid'
Run bundle install to install the backend and delayed_job gems.
The Active Record backend requires a jobs table. You can create that table by running the following command:
$ rails generate delayed_job:active_record $ rake db:migrate
Delayed Job 3.0.0 introduces a new column to the delayed_jobs table.
If you’re upgrading from Delayed Job 2.x, run the upgrade generator to create a migration to add the column.
$ rails generate delayed_job:upgrade $ rake db:migrate
Call .delay.method(params) on any object and it will be processed in the background.
# without delayed_job @user.activate!(@device) # with delayed_job @user.delay.activate!(@device)
If a method should always be run in the background, you can call #handle_asynchronously after the method declaration:
class Device
def deliver
# long running method
end
handle_asynchronously :deliver
end
device = Device.new
device.deliver
handle_asynchronously can take as options anything you can pass to delay. In addition, the values can be Proc objects allowing call time evaluation of the value. For some examples:
class LongTasks
def send_mailer
# Some other code
end
handle_asynchronously :send_mailer, :priority => 20
def in_the_future
# Some other code
end
# 5.minutes.from_now will be evaluated when in_the_future is called
handle_asynchronously :in_the_future, :run_at => Proc.new { 5.minutes.from_now }
def self.when_to_run
2.hours.from_now
end
def call_a_class_method
# Some other code
end
handle_asynchronously :call_a_class_method, :run_at => Proc.new { when_to_run }
attr_reader :how_important
def call_an_instance_method
# Some other code
end
handle_asynchronously :call_an_instance_method, :priority => Proc.new {|i| i.how_important }
end
Due to how mailers are implemented in Rails 3, we had to do a little work around to get delayed_job to work.
# without delayed_job Notifier.signup(@user).deliver # with delayed_job Notifier.delay.signup(@user)
Remove the .deliver method to make it work. It’s not ideal, but it’s the best we could do for now.
DJ 3 introduces Resque-style named queues while still retaining DJ-style priority. The goal is to provide a system for grouping tasks to be worked by separate pools of workers, which may be scaled and controlled individually.
Jobs can be assigned to a queue by setting the queue option:
object.delay(:queue => 'tracking').method Delayed::Job.enqueue job, :queue => 'tracking' handle_asynchronously :tweet_later, :queue => 'tweets'
script/delayed_job can be used to manage a background process which will start working off jobs.
To do so, add gem "daemons" to your Gemfile and make sure you’ve run `rails generate delayed_job`.
You can then do the following:
$ RAILS_ENV=production script/delayed_job start $ RAILS_ENV=production script/delayed_job stop # Runs two workers in separate processes. $ RAILS_ENV=production script/delayed_job -n 2 start $ RAILS_ENV=production script/delayed_job stop # Set the --queue or --queues option to work from a particular queue. $ RAILS_ENV=production script/delayed_job --queue=tracking start $ RAILS_ENV=production script/delayed_job --queues=mailers,tasks start
Workers can be running on any computer, as long as they have access to the database and their clock is in sync. Keep in mind that each worker will check the database at least every 5 seconds.
You can also invoke rake jobs:work which will start working off jobs. You can cancel the rake task with CTRL-C.
Work off queues by setting the QUEUE or QUEUES environment variable.
QUEUE=tracking rake jobs:work QUEUES=mailers,tasks rake jobs:work
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))
You can define hooks on your job that will be called at different stages in the process:
class ParanoidNewsletterJob < NewsletterJob
def enqueue(job)
record_stat 'newsletter_job/enqueue'
end
def perform
emails.each { |e| NewsletterMailer.deliver_text_to_email(text, e) }
end
def before(job)
record_stat 'newsletter_job/start'
end
def after(job)
record_stat 'newsletter_job/after'
end
def success(job)
record_stat 'newsletter_job/success'
end
def error(job, exception)
Airbrake.notify(exception)
end
def failure
page_sysadmin_in_the_middle_of_the_night
end
end
The library revolves around a delayed_jobs table which looks as follows:
create_table :delayed_jobs, :force => true do |table| table.integer :priority, :default => 0 # Allows some jobs to jump to the front of the queue table.integer :attempts, :default => 0 # Provides for retries, but still fail eventually. table.text :handler # YAML-encoded string of the object that will do work table.text :last_error # reason for last failure (See Note below) table.datetime :run_at # When to run. Could be Time.zone.now for immediately, or sometime in the future. table.datetime :locked_at # Set when a client is working on this object table.datetime :failed_at # Set when all retries have failed (actually, by default, the record is deleted instead) table.string :locked_by # Who is working on this object (if locked) table.string :queue # The name of the queue this job is in table.timestamps end
On failure, the job is scheduled again in 5 seconds + N ** 4, where N is the number of retries.
The default Worker.max_attempts is 25. After this, the job either deleted (default), or left in the database with “failed_at” set.
With the default of 25 attempts, the last retry will be 20 days later, with the last interval being almost 100 hours.
The default Worker.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
make sure your job doesn’t exceed this time. You should set this to the longest time you think the job could take.
By default, it will delete failed jobs (and it always deletes successful jobs). If you want to keep failed jobs, set
Delayed::Worker.destroy_failed_jobs = false. The failed jobs will be marked with non-null failed_at.
By default all jobs are scheduled with priority = 0, which is top priority. You can change this by setting Delayed::Worker.default_priority to something else. Lower numbers have higher priority.
The default behavior is to read 5 jobs from the queue when finding an available job. You can configure this by setting Delayed::Worker.read_ahead.
It is possible to disable delayed jobs for testing purposes. Set Delayed::Worker.delay_jobs = false to execute all jobs realtime.
Here is an example of changing job parameters in Rails:
# config/initializers/delayed_job_config.rb Delayed::Worker.destroy_failed_jobs = false Delayed::Worker.sleep_delay = 60 Delayed::Worker.max_attempts = 3 Delayed::Worker.max_run_time = 5.minutes Delayed::Worker.read_ahead = 10 Delayed::Worker.delay_jobs = !Rails.env.test?
You can invoke rake jobs:clear to delete all jobs in the queue.
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