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Active Job Basics
This guide provides you with all you need to get started in creating, enqueuing and executing background jobs.
After reading this guide, you will know:
- How to create jobs.
- How to enqueue jobs.
- How to run jobs in the background.
- How to send emails from your application asynchronously.
Introduction
Active Job is a framework for declaring jobs and making them run on a variety of queuing backends. These jobs can be everything from regularly scheduled clean-ups, to billing charges, to mailings. Anything that can be chopped up into small units of work and run in parallel, really.
The Purpose of Active Job
The main point is to ensure that all Rails apps will have a job infrastructure in place. We can then have framework features and other gems build on top of that, without having to worry about API differences between various job runners such as Delayed Job and Resque. Picking your queuing backend becomes more of an operational concern, then. And you'll be able to switch between them without having to rewrite your jobs.
NOTE: Rails by default comes with an asynchronous queuing implementation that runs jobs with an in-process thread pool. Jobs will run asynchronously, but any jobs in the queue will be dropped upon restart.
Creating a Job
This section will provide a step-by-step guide to creating a job and enqueuing it.
Create the Job
Active Job provides a Rails generator to create jobs. The following will create a
job in app/jobs (with an attached test case under test/jobs):
$ bin/rails generate job guests_cleanup
invoke test_unit
create test/jobs/guests_cleanup_job_test.rb
create app/jobs/guests_cleanup_job.rb
You can also create a job that will run on a specific queue:
$ bin/rails generate job guests_cleanup --queue urgent
If you don't want to use a generator, you could create your own file inside of
app/jobs, just make sure that it inherits from ApplicationJob.
Here's what a job looks like:
class GuestsCleanupJob < ApplicationJob
queue_as :default
def perform(*guests)
# Do something later
end
end
Note that you can define perform with as many arguments as you want.
Enqueue the Job
Enqueue a job like so:
# Enqueue a job to be performed as soon as the queuing system is
# free.
GuestsCleanupJob.perform_later guest
# Enqueue a job to be performed tomorrow at noon.
GuestsCleanupJob.set(wait_until: Date.tomorrow.noon).perform_later(guest)
# Enqueue a job to be performed 1 week from now.
GuestsCleanupJob.set(wait: 1.week).perform_later(guest)
# `perform_now` and `perform_later` will call `perform` under the hood so
# you can pass as many arguments as defined in the latter.
GuestsCleanupJob.perform_later(guest1, guest2, filter: 'some_filter')
That's it!
Job Execution
For enqueuing and executing jobs in production you need to set up a queuing backend, that is to say you need to decide for a 3rd-party queuing library that Rails should use. Rails itself only provides an in-process queuing system, which only keeps the jobs in RAM. If the process crashes or the machine is reset, then all outstanding jobs are lost with the default async backend. This may be fine for smaller apps or non-critical jobs, but most production apps will need to pick a persistent backend.
Backends
Active Job has built-in adapters for multiple queuing backends (Sidekiq, Resque, Delayed Job and others). To get an up-to-date list of the adapters see the API Documentation for ActiveJob::QueueAdapters.
Setting the Backend
You can easily set your queuing backend:
# config/application.rb
module YourApp
class Application < Rails::Application
# Be sure to have the adapter's gem in your Gemfile
# and follow the adapter's specific installation
# and deployment instructions.
config.active_job.queue_adapter = :sidekiq
end
end
You can also configure your backend on a per job basis.
class GuestsCleanupJob < ApplicationJob
self.queue_adapter = :resque
#....
end
# Now your job will use `resque` as it's backend queue adapter overriding what
# was configured in `config.active_job.queue_adapter`.
Starting the Backend
Since jobs run in parallel to your Rails application, most queuing libraries require that you start a library-specific queuing service (in addition to starting your Rails app) for the job processing to work. Refer to library documentation for instructions on starting your queue backend.
Here is a noncomprehensive list of documentation:
Queues
Most of the adapters support multiple queues. With Active Job you can schedule the job to run on a specific queue:
class GuestsCleanupJob < ApplicationJob
queue_as :low_priority
#....
end
You can prefix the queue name for all your jobs using
config.active_job.queue_name_prefix in application.rb:
# config/application.rb
module YourApp
class Application < Rails::Application
config.active_job.queue_name_prefix = Rails.env
end
end
# app/jobs/guests_cleanup_job.rb
class GuestsCleanupJob < ApplicationJob
queue_as :low_priority
#....
end
# Now your job will run on queue production_low_priority on your
# production environment and on staging_low_priority
# on your staging environment
The default queue name prefix delimiter is '_'. This can be changed by setting
config.active_job.queue_name_delimiter in application.rb:
# config/application.rb
module YourApp
class Application < Rails::Application
config.active_job.queue_name_prefix = Rails.env
config.active_job.queue_name_delimiter = '.'
end
end
# app/jobs/guests_cleanup_job.rb
class GuestsCleanupJob < ApplicationJob
queue_as :low_priority
#....
end
# Now your job will run on queue production.low_priority on your
# production environment and on staging.low_priority
# on your staging environment
If you want more control on what queue a job will be run you can pass a :queue
option to #set:
MyJob.set(queue: :another_queue).perform_later(record)
To control the queue from the job level you can pass a block to #queue_as. The
block will be executed in the job context (so you can access self.arguments)
and you must return the queue name:
class ProcessVideoJob < ApplicationJob
queue_as do
video = self.arguments.first
if video.owner.premium?
:premium_videojobs
else
:videojobs
end
end
def perform(video)
# Do process video
end
end
ProcessVideoJob.perform_later(Video.last)
NOTE: Make sure your queuing backend "listens" on your queue name. For some backends you need to specify the queues to listen to.
Callbacks
Active Job provides hooks to trigger logic during the life cycle of a job. Like other callbacks in Rails, you can implement the callbacks as ordinary methods and use a macro-style class method to register them as callbacks:
class GuestsCleanupJob < ApplicationJob
queue_as :default
around_perform :around_cleanup
def perform
# Do something later
end
private
def around_cleanup(job)
# Do something before perform
yield
# Do something after perform
end
end
The macro-style class methods can also receive a block. Consider using this style if the code inside your block is so short that it fits in a single line. For example, you could send metrics for every job enqueued:
class ApplicationJob
before_enqueue { |job| $statsd.increment "#{job.name.underscore}.enqueue" }
end
Available callbacks
before_enqueuearound_enqueueafter_enqueuebefore_performaround_performafter_perform
Action Mailer
One of the most common jobs in a modern web application is sending emails outside of the request-response cycle, so the user doesn't have to wait on it. Active Job is integrated with Action Mailer so you can easily send emails asynchronously:
# If you want to send the email now use #deliver_now
UserMailer.welcome(@user).deliver_now
# If you want to send the email through Active Job use #deliver_later
UserMailer.welcome(@user).deliver_later
NOTE: Using the asynchronous queue from a Rake task (for example, to
send an email using .deliver_later) will generally not work because Rake will
likely end, causing the in-process thread pool to be deleted, before any/all
of the .deliver_later emails are processed. To avoid this problem, use
.deliver_now or run a persistent queue in development.
Internationalization
Each job uses the I18n.locale set when the job was created. Useful if you send
emails asynchronously:
I18n.locale = :eo
UserMailer.welcome(@user).deliver_later # Email will be localized to Esperanto.
GlobalID
Active Job supports GlobalID for parameters. This makes it possible to pass live Active Record objects to your job instead of class/id pairs, which you then have to manually deserialize. Before, jobs would look like this:
class TrashableCleanupJob < ApplicationJob
def perform(trashable_class, trashable_id, depth)
trashable = trashable_class.constantize.find(trashable_id)
trashable.cleanup(depth)
end
end
Now you can simply do:
class TrashableCleanupJob < ApplicationJob
def perform(trashable, depth)
trashable.cleanup(depth)
end
end
This works with any class that mixes in GlobalID::Identification, which
by default has been mixed into Active Record classes.
Exceptions
Active Job provides a way to catch exceptions raised during the execution of the job:
class GuestsCleanupJob < ApplicationJob
queue_as :default
rescue_from(ActiveRecord::RecordNotFound) do |exception|
# Do something with the exception
end
def perform
# Do something later
end
end
Retrying or Discarding failed jobs
It's also possible to retry or discard a job if an exception is raised during execution. For example:
class RemoteServiceJob < ApplicationJob
retry_on CustomAppException # defaults to 3s wait, 5 attempts
discard_on ActiveJob::DeserializationError
def perform(*args)
# Might raise CustomAppException or ActiveJob::DeserializationError
end
end
To get more details see the API Documentation for ActiveJob::Exceptions.
Deserialization
GlobalID allows serializing full Active Record objects passed to #perform.
If a passed record is deleted after the job is enqueued but before the #perform
method is called Active Job will raise an ActiveJob::DeserializationError
exception.
Job Testing
You can find detailed instructions on how to test your jobs in the testing guide.