AcidicJob

Idempotent operations for Rails apps (for ActiveJob or Sidekiq)

At the conceptual heart of basically any software are "operations"—the discrete actions the software performs. Rails provides a powerful abstraction layer for building operations in the form of ActiveJob, or we Rubyists can use the tried and true power of pure Sidekiq. With either we can easily trigger from other Ruby code throughout our Rails application (controller actions, model methods, model callbacks, etc.); we can run operations both synchronously (blocking execution and then returning its response to the caller) and asychronously (non-blocking and the caller doesn't know its response); and we can also retry a specific operation if needed seamlessly.

However, in order to ensure that our operational jobs are robust, we need to ensure that they are properly idempotent and transactional. As stated in the GitLab Sidekiq Style Guide:

As a general rule, a worker can be considered idempotent if:

• It can safely run multiple times with the same arguments.
• Application side-effects are expected to happen only once (or side-effects of a second run do not have an effect).

This is, of course, far easier said than done. Thus, AcidicJob.

AcidicJob provides a framework to help you make your operational jobs atomic ⚛️, consistent 🤖, isolated 🕴🏼, and durable ⛰️. Its conceptual framework is directly inspired by a truly wonderful loosely collected series of articles written by Brandur Leach, which together lay out core techniques and principles required to make an HTTP API properly ACIDic:

1. https://brandur.org/acid
2. https://brandur.org/http-transactions
3. https://brandur.org/job-drain
4. https://brandur.org/idempotency-keys

AcidicJob brings these techniques and principles into the world of a standard Rails application.

Installation

Add this line to your application's Gemfile:

gem 'acidic_job'

And then execute:

$ bundle install

Or simply execute to install the gem yourself:

$ bundle add acidic_job

Then, use the following command to copy over the AcidicJob::Run migration file.

rails generate acidic_job:install

Usage

AcidicJob is a concern that you include into your base ApplicationJob.

class ApplicationJob < ActiveJob::Base
  include AcidicJob
end

This is useful because the module needs to be mixed into any and all jobs that you want to either make acidic or enqueue acidicly.

It provides a suite of functionality that empowers you to create complex, robust, and acidic jobs.

TL;DR

Key Features

• Transactional Steps
  break your job into a series of steps, each of which will be run within an acidic database transaction, allowing retries to jump back to the last "recovery point".
• Steps that Await Jobs
  have workflow steps await other jobs, which will be enqueued and processed independently, and only when they all have finished will the parent job be re-enqueued to continue the workflow
• Iterable Steps
  define steps that iterate over some collection fully until moving on to the next step
• Persisted Attributes
  when retrying jobs at later steps, we need to ensure that data created in previous steps is still available to later steps on retry.
• Transactionally Staged Jobs
  enqueue additional jobs within the acidic transaction safely
• Custom Idempotency Keys
  use something other than the job ID for the idempotency key of the job run
• Sidekiq Callbacks
  bring ActiveJob-like callbacks into your pure Sidekiq Workers
• Run Finished Callbacks
  set callbacks for when a job run finishes fully

Transactional Steps

The first and foundational feature acidic_job provides is the with_acidity method, which takes a block of transactional step methods (defined via the step) method:

class RideCreateJob < ActiveJob::Base
  include AcidicJob

  def perform(user_id, ride_params)
    @user = User.find(user_id)
    @params = ride_params

    with_acidity providing: { ride: nil } do
      step :create_ride_and_audit_record
      step :create_stripe_charge
      step :send_receipt
    end
  end

  def create_ride_and_audit_record
    # ...
  end

  def create_stripe_charge
    # ...
  end

  def send_receipt
    # ...
  end
end

with_acidity takes only the providing: named parameter and a block where you define the steps of this operation. step simply takes the name of a method available in the job. That's all!

Now, each execution of this job will find or create an AcidicJob::Run record, which we leverage to wrap every step in a database transaction. Moreover, this database record allows acidic_job to ensure that if your job fails on step 3, when it retries, it will simply jump right back to trying to execute the method defined for the 3rd step, and won't even execute the first two step methods. This means your step methods only need to be idempotent on failure, not on success, since they will never be run again if they succeed.

Steps that Await Jobs

By simply adding the awaits option to your step declarations, you can attach any number of additional, asynchronous jobs to your step. This is profoundly powerful, as it means that you can define a workflow where step 2 is started if and only if step 1 succeeds, but step 1 can have 3 different jobs enqueued on 3 different queues, each running in parallel. Once all 3 jobs succeed, acidic_job will re-enqueue the parent job and it will move on to step 2. That's right, you can have workers that are executed in parallel, on separate queues, and asynchronously, but are still blocking—as a group—the next step in your workflow! This unlocks incredible power and flexibility for defining and structuring complex workflows and operations.

class RideCreateJob < ActiveJob::Base
  include AcidicJob

  def perform(user_id, ride_params)
    @user = User.find(user_id)
    @params = ride_params

    with_acidity providing: { ride: nil } do
      step :create_ride_and_audit_record, awaits: [SomeJob, AnotherJob]
      step :create_stripe_charge
      step :send_receipt
    end
  end
end

If you need to await a job that takes arguments, you can prepare that job along with its arguments using the with class method that acidic_job will add to your jobs:

class RideCreateJob < ActiveJob::Base
  include AcidicJob

  def perform(user_id, ride_params)
    @user = User.find(user_id)
    @params = ride_params

    with_acidity providing: { ride: nil } do
      step :create_ride_and_audit_record, awaits: awaits: [SomeJob.with('argument_1', keyword: 'value'), AnotherJob.with(1, 2, 3, some: 'thing')]
      step :create_stripe_charge
      step :send_receipt
    end
  end
end

If your step awaits multiple jobs (e.g. awaits: [SomeJob, AnotherJob.with('argument_1', keyword: 'value')]), your top level workflow job will only continue to the next step once all of the jobs in your awaits array have finished.

In some cases, you may need to dynamically determine the collection of jobs that the step should wait for; in these cases, you can pass the name of a method to the awaits option:

class RideCreateJob < ActiveJob::Base
  include AcidicJob
  set_callback :finish, :after, :delete_run_record

  def perform(user_id, ride_params)
    @user = User.find(user_id)
    @params = ride_params

    with_acidity providing: { ride: nil } do
      step :create_ride_and_audit_record, awaits: :dynamic_awaits
      step :create_stripe_charge
      step :send_receipt
    end
  end
  
  def dynamic_awaits
    if @params["key"].present?
      [SomeJob.with('argument_1', keyword: 'value')]
    else
      [AnotherJob.with(1, 2, 3, some: 'thing')]
    end
  end
end

Iterable Steps

Sometimes our workflows have steps that need to iterate over a collection and perform an action for each item in the collection before moving on to the next step in the workflow. In these cases, we can use the for_each option when defining our step to specific the collection, and acidic_job will pass each item into your step method for processing, keeping the same transactional guarantees as for any step. This means that if your step encounters an error in processing any item in the collection, when your job is retried, the job will jump right back to that step and right back to that item in the collection to try again.

class ExampleJob < ActiveJob::Base
  include AcidicJob

  def perform(record:)
    with_acidity providing: { collection: [1, 2, 3, 4, 5] } do
      step :process_item, for_each: :collection
      step :next_step
    end
  end
  
  def process_item(item)
    # do whatever work needs to be done with this individual item
  end
end

Note: This feature relies on the "Persisted Attributes" feature detailed below. This means that you can only iterate over collections that ActiveRecord can serialize.

Persisted Attributes

Any objects passed to the providing option on the with_acidity method are made available across retries. This means that you can set an attribute in step 1, access it in step 2, have step 2 fail, have the job retry, jump directly back to step 2 on retry, and have that object still accessible. This is done by serializing all objects to a field on the AcidicJob::Run and manually providing getters and setters that sync with the database record.

class RideCreateJob < ActiveJob::Base
  include AcidicJob

  def perform(user_id, ride_params)
    @user = User.find(user_id)
    @params = ride_params

    with_acidity providing: { ride: nil } do
      step :create_ride_and_audit_record
      step :create_stripe_charge
      step :send_receipt
    end
  end

  def create_ride_and_audit_record
    self.ride = Ride.create!
  end

  def create_stripe_charge
    Stripe::Charge.create(amount: 20_00, customer: @ride.user)
  end

  # ...
end

Note: This does mean that you are restricted to objects that can be serialized by ActiveJob (for more info, see here). This means you can persist ActiveRecord models, and any simple Ruby data types, but you can't persist things like Procs or custom class instances, for example.

Note: You will note the use of self.ride = ... in the code sample above. In order to call the attribute setter method that will sync with the database record, you must use this style. @ride = ... and/or ride = ... will both fail to sync the value with the database record.

The default pattern you should follow when defining your perform method is to make any values that your step methods need access to, but are present at the start of the perform method simply instance variables. You only need to provide attributes that will be set during a step. This means, the initial value will almost always be nil.

Transactionally Staged Jobs

A standard problem when inside of database transactions is enqueuing other jobs. On the one hand, you could enqueue a job inside of a transaction that then rollbacks, which would leave that job to fail and retry and fail. On the other hand, you could enqueue a job that is picked up before the transaction commits, which would mean the records are not yet available to this job.

In order to mitigate against such issues without forcing you to use a database-backed job queue, acidic_job provides perform_acidicly and deliver_acidicly methods to "transactionally stage" enqueuing other jobs from within a step (whether another ActiveJob or a Sidekiq::Worker or an ActionMailer delivery). These methods will create a new AcidicJob::Run record, but inside of the database transaction of the step. Upon commit of that transaction, a model callback pushes the job to your actual job queue. Once the job has been successfully performed, the AcidicJob::Run record is deleted so that this table doesn't grow unbounded and unnecessarily.

class RideCreateJob < ActiveJob::Base
  include AcidicJob

  def perform(user_id, ride_params)
    @user = User.find(user_id)
    @params = ride_params
    
    with_acidity providing: { ride: nil } do
      step :create_ride_and_audit_record
      step :create_stripe_charge
      step :send_receipt
    end
  end

  # ...

  def send_receipt
    RideMailer.with(user: @user, ride: @ride).confirm_charge.delivery_acidicly
  end
end

Custom Idempotency Keys

By default, AcidicJob uses the job identifier provided by the queueing system (ActiveJob or Sidekiq) as the idempotency key for the job run. The idempotency key is what is used to guarantee that no two runs of the same job occur. However, sometimes we need particular jobs to be idempotent based on some other criteria. In these cases, AcidicJob provides a collection of tools to allow you to ensure the idempotency of your jobs.

Firstly, you can configure your job class to explicitly use either the job identifier or the job arguments as the foundation for the idempotency key. A job class that calls the acidic_by_job_id class method (which is the default behavior) will simply make the job run's idempotency key the job's identifier:

class ExampleJob < ActiveJob::Base
  include AcidicJob
  acidic_by_job_id

  def perform
  end
end

Conversely, a job class can use the acidic_by_job_args method to configure that job class to use the arguments passed to the job as the foundation for the job run's idempotency key:

class ExampleJob < ActiveJob::Base
  include AcidicJob
  acidic_by_job_args

  def perform(arg_1, arg_2)
    # the idempotency key will be based on whatever the values of `arg_1` and `arg_2` are
  end
end

These options cover the two common situations, but sometimes our systems need finer-grained control. For example, our job might take some record as the job argument, but we need to use a combination of the record identifier and record status as the foundation for the idempotency key. In these cases you can pass a Proc to an acidic_by class method:

class ExampleJob < ActiveJob::Base
  include AcidicJob
  acidic_by ->(record:) { [record.id, record.status] }

  def perform(record:)
    # the idempotency key will be based on whatever the values of `record.id` and `record.status` are
  end
end

Note: The signature of the acidic_by proc needs to match the signature of the job's perform method.

Sidekiq Callbacks

In order to ensure that AcidicJob::Staged records are only destroyed once the related job has been successfully performed, whether it is an ActiveJob or a Sidekiq Worker, acidic_job also extends Sidekiq to support the ActiveJob callback interface.

This allows acidic_job to use an after_perform callback to delete the AcidicJob::Staged record, whether you are using the gem with ActiveJob or pure Sidekiq Workers. Of course, this means that you can add your own callbacks to any jobs or workers that include the AcidicJob module as well.

Run Finished Callbacks

When working with workflow jobs that make use of the awaits feature for a step, it is important to remember that the after_perform callback will be called as soon as the first awaits step has enqueued job, and not when the entire job run has finished. acidic_job allows the perform method to finish so that the queue for the workflow job is cleared to pick up new work while the awaits jobs are running. acidic_job will automatically re-enqueue the workflow job and progress to the next step when all of the awaits jobs have successfully finished. However, this means that after_perform is not necessarily the same as after_finish. In order to provide the opportunity for you to execute callback logic if and only if a job run has finished, we provide callback hooks for the finish event.

For example, you could use this hook to immediately clean up the AcidicJob::Run database record whenever the workflow job finishes successfully like so:

class RideCreateJob < ActiveJob::Base
  include AcidicJob
  set_callback :finish, :after, :delete_run_record

  def perform(user_id, ride_params)
    @user = User.find(user_id)
    @params = ride_params

    with_acidity providing: { ride: nil } do
      step :create_ride_and_audit_record, awaits: [SomeJob.with('argument_1', keyword: 'value')]
      step :create_stripe_charge, args: [1, 2, 3], kwargs: { some: 'thing' }
      step :send_receipt
    end
  end
  
  def delete_run_record
    return unless acidic_job_run.succeeded?

    acidic_job_run.destroy!
  end
end

Testing

When testing acidic jobs, you are likely to run into ActiveRecord::TransactionIsolationErrors:

ActiveRecord::TransactionIsolationError: cannot set transaction isolation in a nested transaction

This error is thrown because by default RSpec and most MiniTest test suites use database transactions to keep the test database clean between tests. The database transaction that is wrapping all of the code executed in your test is run at the standard isolation level, but acidic jobs then try to create another transaction run at a more conservative isolation level. You cannot have a nested transaction that runs at a different isolation level, thus, this error.

In order to avoid this error, you need to ensure firstly that your tests that run your acidic jobs are not using a database transaction and secondly that they use some different strategy to keep your test database clean. The DatabaseCleaner gem is a commonly used tool to manage different strategies for keeping your test database clean. As for which strategy to use, truncation and deletion are both safe, but their speed varies based on our app's table structure (see https://github.com/DatabaseCleaner/database_cleaner#what-strategy-is-fastest). Either is fine; use whichever is faster for your app.

In order to make this test setup simpler, AcidicJob provides a TestCase class that your MiniTest jobs tests can inherit from. It is simple; it inherits from ActiveJob::TestCase, sets use_transactional_tests to false, and ensures DatabaseCleaner is run for each of your tests. Moreover, it ensures that the system's original DatabaseCleaner configuration is maintained, options included, except that any transaction strategies for any ORMs are replaced with a deletion strategy. It does so by storing whatever the system DatabaseCleaner configuration is at the start of before_setup phase in an instance variable and then restores that configuration at the end of after_teardown phase. In between, it runs the configuration thru a pipeline that selectively replaces any transaction strategies with a corresponding deletion strategy, leaving any other configured strategies untouched.

For those of you using RSpec, you can require the acidic_job/rspec_configuration file, which will configure RSpec in the exact same way I have used in my RSpec projects to allow me to test acidic jobs with either the deletion strategy but still have all of my other tests use the fast transaction strategy:

require "database_cleaner/active_record"

# see https://github.com/DatabaseCleaner/database_cleaner#how-to-use
RSpec.configure do |config|
  config.use_transactional_fixtures = false

  config.before(:suite) do
    DatabaseCleaner.clean_with :truncation

    # Here we are defaulting to :transaction but swapping to deletion for some specs;
    # if your spec or its code-under-test uses
    # nested transactions then specify :transactional e.g.:
    #   describe "SomeWorker", :transactional do
    #
    DatabaseCleaner.strategy = :transaction

    config.before(:context, transactional: true) { DatabaseCleaner.strategy = :deletion }
    config.after(:context, transactional: true) { DatabaseCleaner.strategy = :transaction }
    config.before(:context, type: :system) { DatabaseCleaner.strategy = :deletion }
    config.after(:context, type: :system) { DatabaseCleaner.strategy = :transaction }
  end

  config.around(:each) do |example|
    DatabaseCleaner.cleaning do
      example.run
    end
  end
end

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and the created tag, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/[USERNAME]/acidic_job.