MaintenanceTasks

A Rails engine for queuing and managing maintenance tasks.

Installation

To install the gem and run the install generator, execute:

$ bundle add maintenance_tasks
$ rails generate maintenance_tasks:install

The generator creates and runs a migration to add the necessary table to your database. It also mounts Maintenance Tasks in your config/routes.rb. By default the web UI can be accessed in the new /maintenance_tasks path.

In case you use an exception reporting service (e.g. Bugsnag) you might want to define an error handler. See Customizing the error handler for more information.

Active Job Dependency

The Maintenance Tasks framework relies on ActiveJob behind the scenes to run Tasks. The default queuing backend for ActiveJob is asynchronous. It is strongly recommended to change this to a persistent backend so that Task progress is not lost during code or infrastructure changes. For more information on configuring a queuing backend, take a look at the ActiveJob documentation.

Usage

Creating a Task

A generator is provided to create tasks. Generate a new task by running:

$ rails generate maintenance_tasks:task update_posts

This creates the task file app/tasks/maintenance/update_posts_task.rb.

The generated task is a subclass of MaintenanceTasks::Task that implements:

collection: return an Active Record Relation or an Array to be iterated over.
process: do the work of your maintenance task on a single record
count: return the number of rows that will be iterated over (optional, to be able to show progress)

Example:

# app/tasks/maintenance/update_posts_task.rb
module Maintenance
  class UpdatePostsTask < MaintenanceTasks::Task
    def collection
      Post.all
    end

    def count
      collection.count
    end

    def process(post)
      post.update!(content: 'New content!')
    end
  end
end

Creating a CSV Task

You can also write a Task that iterates on a CSV file. Note that writing CSV Tasks requires ActiveStorage to be configured. Ensure that the dependency is specified in your application's Gemfile, and that you've followed the setup instuctions.

Generate a CSV Task by running:

$ rails generate maintenance_tasks:task import_posts --csv

The generated task is a subclass of MaintenanceTasks::Task that implements:

process: do the work of your maintenance task on a CSV::Row

# app/tasks/maintenance/import_posts_task.rb
module Maintenance
  class ImportPostsTask < MaintenanceTasks::Task
    csv_collection

    def process(row)
      Post.create!(title: row["title"], content: row["content"])
    end
  end
end

# posts.csv
title,content
My Title,Hello World!

Considerations when writing Tasks

MaintenanceTasks relies on the queue adapter configured for your application to run the job which is processing your Task. The guidelines for writing Task may depend on the queue adapter but in general, you should follow these rules:

Duration of Task#process: processing a single element of the collection should take less than 25 seconds, or the duration set as a timeout for Sidekiq or the queue adapter configured in your application. It allows the Task to be safely interrupted and resumed.
Idempotency of Task#process: it should be safe to run process multiple times for the same element of the collection. Read more in this Sidekiq best practice. It's important if the Task errors and you run it again, because the same element that errored the Task may well be processed again. It especially matters in the situation described above, when the iteration duration exceeds the timeout: if the job is re-enqueued, multiple elements may be processed again.

Writing tests for a Task

The task generator will also create a test file for your task in the folder test/tasks/maintenance/. At a minimum, it's recommended that the #process method in your task be tested. You may also want to test the #collection and #count methods for your task if they are sufficiently complex.

Example:

# test/tasks/maintenance/update_posts_task_test.rb

require 'test_helper'

module Maintenance
  class UpdatePostsTaskTest < ActiveSupport::TestCase
    test "#process performs a task iteration" do
      post = Post.new

      Maintenance::UpdatePostsTask.process(post)

      assert_equal 'New content!', post.content
    end
  end
end

Writing tests for a CSV Task

You should write tests for your #process method in a CSV Task as well. It takes a CSV::Row as an argument. You can pass a row, or a hash with string keys to #process from your test.

# app/tasks/maintenance/import_posts_task_test.rb
module Maintenance
  class ImportPostsTaskTest < ActiveSupport::TestCase
    test "#process performs a task iteration" do
      assert_difference -> { Post.count } do
        Maintenance::UpdatePostsTask.process({
          'title' => 'My Title',
          'content' => 'Hello World!',
        })
      end

      post = Post.last
      assert_equal 'My Title', post.title
      assert_equal 'Hello World!', post.content
    end
  end
end

Running a Task

You can run your new Task by accessing the Web UI and clicking on "Run".

Alternatively, you can run your Task in the command line:

$ bundle exec maintenance_tasks perform Maintenance::UpdatePostsTask

To run a Task that processes CSVs from the command line, use the --csv option:

$ bundle exec maintenance_tasks perform Maintenance::ImportPostsTask --csv 'path/to/my_csv.csv'

You can also run a Task in Ruby by sending run with a Task name to Runner:

MaintenanceTasks::Runner.run(name: 'Maintenance::UpdatePostsTask')

To run a Task that processes CSVs using the Runner, provide a Hash containing an open IO object and a filename to run:

MaintenanceTasks::Runner.run(
  name: 'Maintenance::ImportPostsTask'
  csv_file: { io: File.open('path/to/my_csv.csv'), filename: 'my_csv.csv' }
)

Monitoring your Task's status

The web UI will provide updates on the status of your Task. Here are the states a Task can be in:

new: A Task that has not yet been run.
enqueued: A Task that is waiting to be performed after a user has instructed it to run.
running: A Task that is currently being performed by a job worker.
pausing: A Task that was paused by a user, but needs to finish work before stopping.
paused: A Task that was paused by a user and is not performing. It can be resumed.
interrupted: A Task that has been momentarily interrupted by the job infrastructure.
cancelling: A Task that was cancelled by a user, but needs to finish work before stopping.
cancelled: A Task that was cancelled by a user and is not performing. It cannot be resumed.
succeeded: A Task that finished successfully.
errored: A Task that encountered an unhandled exception while performing.

How Maintenance Tasks runs a Task

Maintenance tasks can be running for a long time, and the purpose of the gem is to make it easy to continue running tasks through deploys, Kubernetes Pod scheduling, Heroku dyno restarts or other infrastructure or code changes.

This means a Task can safely be interrupted, re-enqueued and resumed without any intervention at the end of an iteration, after the process method returns.

By default, a running Task will be interrupted after running for more 5 minutes. This is configured in the job-iteration gem and can be tweaked in an initializer if necessary.

Running tasks will also be interrupted and re-enqueued when needed. For example when Sidekiq workers shuts down for a deploy:

When Sidekiq receives a TSTP or TERM signal, it will consider itself to be stopping.
When Sidekiq is stopping, JobIteration stops iterating over the enumerator. The position in the iteration is saved, a new job is enqueued to resume work, and the Task is marked as interrupted.

When Sidekiq is stopping, it will give workers 25 seconds to finish before forcefully terminating them (this is the default but can be configured with the --timeout option). Before the worker threads are terminated, Sidekiq will try to re-enqueue the job so your Task will be resumed. However, the position in the collection won't be persisted so at least one iteration may run again.

Help! My Task is stuck

Finally, if the queue adapter configured for your application doesn't have this property, or if Sidekiq crashes, is forcefully terminated, or is unable to re-enqueue the jobs that were in progress, the Task may be in a seemingly stuck situation where it appears to be running but is not. In that situation, pausing or cancelling it will not result in the Task being paused or cancelled, as the Task will get stuck in a state of pausing or cancelling. As a work-around, if a Task is cancelling for more than 5 minutes, you will be able to cancel it for good, which will just mark it as cancelled, allowing you to run it again.

Configuring the gem

There are a few configurable options for the gem. Custom configurations should be placed in a maintenance_tasks.rb initializer.

Customizing the error handler

Exceptions raised while a Task is performing are rescued and information about the error is persisted and visible in the UI.

If you want to integrate with an exception monitoring service (e.g. Bugsnag), you can define an error handler:

# config/initializers/maintenance_tasks.rb
MaintenanceTasks.error_handler = ->(error, task_context, _errored_element) do
  Bugsnag.notify(error) do |notification|
    notification.add_tab(:task, task_context)
  end
end

The error handler should be a lambda that accepts three arguments:

error: The exception that was raised.
task_context: A hash with additional information about the Task and the error:
- task_name: The name of the Task that errored
- started_at: The time the Task started
- ended_at: The time the Task errored Note that task_context may be empty if the Task produced an error before any context could be gathered (for example, if deserializing the job to process your Task failed).
errored_element: The element, if any, that was being processed when the Task raised an exception. If you would like to pass this object to your exception monitoring service, make sure you sanitize the object to avoid leaking sensitive data and convert it to a format that is compatible with your bug tracker. For example, Bugsnag only sends the id and class name of ActiveRecord objects in order to protect sensitive data. CSV rows, on the other hand, are converted to strings and passed raw to Bugsnag, so make sure to filter any personal data from these objects before adding them to a report.

Customizing the maintenance tasks module

MaintenanceTasks.tasks_module can be configured to define the module in which tasks will be placed.

# config/initializers/maintenance_tasks.rb
MaintenanceTasks.tasks_module = 'TaskModule'

If no value is specified, it will default to Maintenance.

Customizing the underlying job class

MaintenanceTasks.job can be configured to define a Job class for your tasks to use. This is a global configuration, so this Job class will be used across all maintenance tasks in your application.

# config/initializers/maintenance_tasks.rb
MaintenanceTasks.job = 'CustomTaskJob'

# app/jobs/custom_task_job.rb
class CustomTaskJob < MaintenanceTasks::TaskJob
  queue_as :low_priority
end

The Job class must inherit from MaintenanceTasks::TaskJob.

Note that retry_on is not supported for custom Job classes, so failed jobs cannot be retried.

Customizing the rate at which task progress gets updated

MaintenanceTasks.ticker_delay can be configured to customize how frequently task progress gets persisted to the database. It can be a Numeric value or an ActiveSupport::Duration value.

# config/initializers/maintenance_tasks.rb
MaintenanceTasks.ticker_delay = 2.seconds

If no value is specified, it will default to 1 second.

Customizing which Active Storage service to use

The Active Storage framework in Rails 6.1 and up supports multiple storage services per environment. To specify which service to use, MaintenanceTasks.active_storage_service can be configured with the service's key, as specified in your application's config/storage.yml:

# config/storage.yml
user_data:
  service: GCS
  credentials: <%= Rails.root.join("path/to/user/data/keyfile.json") %>
  project: "my-project"
  bucket: "user-data-bucket"

internal:
  service: GCS
  credentials: <%= Rails.root.join("path/to/internal/keyfile.json") %>
  project: "my-project"
  bucket: "internal-bucket"

# config/initializers/maintenance_tasks.rb
MaintenanceTasks.active_storage_service = :internal

There is no need to configure this option if your application uses only one storage service per environment.

Upgrading

Use bundler to check for and upgrade to newer versions. After installing a new version, re-run the install command:

$ rails generate maintenance_tasks:install

This ensures that new migrations are installed and run as well.

Contributing

Would you like to report an issue or contribute with code? We accept issues and pull requests. You can find the contribution guidelines on CONTRIBUTING.md.

Dependabot updates

Whenever Dependabot creates a PR for a gem bump, check out the branch locally and run bin/update-gemfile <gem> to ensure all the gemfiles have the gem updated consistently.

Releasing new versions

Updates should be added to the latest draft release on GitHub as Pull Requests are merged.

Once a release is ready, follow these steps:

Update spec.version in maintenance_tasks.gemspec.
Run bin/gemfile-update install to bump the version in all the lockfiles.
Open a PR and merge on approval.
Deploy via Shipit and see the new version on https://rubygems.org/gems/maintenance_tasks.
Ensure the release has documented all changes and publish it.
Create a new draft release on GitHub with the title 'Upcoming Release'. The tag version can be left blank. This will be the starting point for documenting changes related to the next release.

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