AsyncTask

Lightweight, asynchronous, and database-backed execution of singleton methods.

This gem provides generators and mixins to queue up tasks in database transactions to be performed later. Doing so prevents (1) tasks from being run twice if performed within a transaction and (2) tasks from synchronously blocking.

transaction do
  order = Order.create!(number: 7355608)

  # RPC Call
  OrderClient.fulfill(customer_token: customer_token, order_number: order.number)

  raise
end

Despite database transaction rolling back the creation of the Order record, the RPC call executes. This problem becomes more difficult in nested transactions. To avoid doing something regrettable, we create an AsyncTask::Attempt record inside the database. These records are then performed at a later time using a job:

transaction do
  order = Order.create!(number: 1)

  # To be performed by a job later
  AsyncTask::Attempt.create!(
    target: OrderClient,
    method_name: :fulfill,
    method_args: { customer_token: customer_token, order_number: order.number },
  )

  raise
end

The above pattern ensures we will not act when there is a rollback later in the transaction.

The gem provides the following:

• Models

  • Generators for the AsyncTask::Attempt migration, model, factory, and specs.
  • Choice between using async tasks with encrypted or unencrypted method arguments.
  • Tracking completion using completed_at.
  • Fields for target, method_name, and method_args.
  • AsyncTask::BaseAttempt mixin to provide model methods.
  • A num_attempts field gives you flexibility to handle retries and other failure scenarios.
  • status and completed_at are fields that track state.
  • idempotence_token field for rolling your own presence, uniqueness, or idempotence checks.

• Jobs

  • Generators for AsyncTask::AttemptJob, AsyncTask::AttemptBatchJob, and their specs.
  • AsyncTask::BaseAttemptJob and AsyncTask::BaseAttemptBatchJob mixins.

Getting Started

1. Add the gem to your application's Gemfile and execute bundle install to install it:

gem 'async_task'

2. Generate migrations, base models, jobs, and specs. Feel free to add any additional columns you need to the generated migration file:

$ rails g async_task:install

3. Rename the model and migrations as you see fit. Make sure your model contains include AsyncTask::BaseAttempt. Use self.table_name= if necessary.

class AsyncTask::Attempt < ApplicationRecord
  include AsyncTask::BaseAttempt
end

4. Implement the handle_perform_error in your AsyncTask::Attempt model. This methods is used by AsyncTask::BaseAttempt when exceptions are thrown performing the task.

5. This gem provides no encryptor by default. Implement an encryptor (see below) if you need encrypted params.

6. Create AsyncTask::Attempts to be sent later by a job (generated) that includes a AsyncTask::BaseAttemptJob:

class AsyncTask::AttemptJob < ActiveJob::Base
  include AsyncTask::BaseAttemptJob
end

AsyncTask::Attempt.create!(
  target: OrderClient,
  method_name: :fulfill,
  method_args: { customer_token: customer_token, order_number: order.number },
)

7. Make sure to schedule the AsyncTask::AttemptJob to run frequently using something like Clockwork.

Cautionary Situations When Using This Gem

Idempotence

The target, method_name, and method_args should be idempotent because the AsyncTask::AttemptBatchJob could schedule multiple AsyncTask::AttemptJobs if the job queue is backed up.

Nested Transactions

Task execution occurs inside of a with_lock block, which executes the body inside of a database transaction. Keep in mind that transactions inside the #{target}.#{method_name} will be nested. You may have to consider implementing transaction(require: new) or creating transactions in separate threads.

Cookbook

Custom Encryptors

Implement the interface present in AsyncTask::NullEncryptor to provide your own encryptor.

module AesEncryptor
  extend self

  def decrypt(content)
    AesClient.decrypt(content)
  end

  def encrypt(content)
    AesClient.encrypt(content)
  end
end

Delayed Execution

Setting the scheduled_at field allows delayed execution to be possible. A task that has an scheduled_at before Time.current will be executed by AsyncTask::BaseAttemptBatchJob.

Handling AsyncTask::BaseAttempt Errors

class AsyncTask::Attempt < ActiveRecord::Base
  include AsyncTask::BaseAttempt

  def handle_perform_error(error)
    Raven.capture_exception(error)
  end
end

Lastly, the num_attempts field in AsyncTask::Attempt allows you to track the number of attempts the task has undergone. Use this to implement retries and permanent failure thresholds for your tasks.

Proper Usage of expire! / fail!

expire! should be used for tasks that should no longer be run.

fail! should be used to mark permanent failure for a task.

Design Motivations

We're relying heavily on generators and mixins. Including the AsyncTask::BaseAttempt module allows us to generate a model that can inherit from both ActiveRecord::Base (Rails 4) and ApplicationRecord (Rails 5). The BaseAttempt module's methods can easily be overridden, giving callers flexibility to handle errors, extend functionality, and inherit (STI). Lastly, the generated migrations provide fields used by the BaseAttempt module, but the developer is free to add their own fields and extend the module's methods while calling super.

Development

• Install dependencies with bin/setup.
• Run tests/lints with rake
• For an interactive prompt that will allow you to experiment, run bin/console.

Acknowledgments

• RickCSong

License

Copyright 2017 Square, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.