Meet Iteration, an extension for Sidekiq that makes your long-running jobs interruptible and resumable, saving all progress that the job has made (aka checkpoint for jobs).

You may consider pluck_in_batches gem to speedup iterating over large database tables.

Imagine the following job:

class SimpleJob
  include Sidekiq::Job

  def perform
    User.find_each do |user|
      user.notify_about_something
    end
  end
end

The job would run fairly quickly when you only have a hundred User records. But as the number of records grows, it will take longer for a job to iterate over all Users. Eventually, there will be millions of records to iterate and the job will end up taking hours or even days.

With frequent deploys and worker restarts, it would mean that a job will be either lost or restarted from the beginning. Some records (especially those in the beginning of the relation) will be processed more than once.

Cloud environments are also unpredictable, and there's no way to guarantee that a single job will have reserved hardware to run for hours and days. What if AWS diagnosed the instance as unhealthy and will restart it in 5 minutes? All job progress will be lost.

Software that is designed for high availability must be resilient to interruptions that come from the infrastructure. That's exactly what Iteration brings to Sidekiq.

• Ruby 2.7
• Sidekiq 6

Add this line to your application's Gemfile:

gem 'sidekiq-iteration'

And then execute:

$ bundle

In the job, include SidekiqIteration::Iteration module and start describing the job with two methods (build_enumerator and each_iteration) instead of perform:

class NotifyUsersJob
  include Sidekiq::Job
  include SidekiqIteration::Iteration

  def build_enumerator(cursor:)
    active_record_records_enumerator(User.all, cursor: cursor)
  end

  def each_iteration(user)
    user.notify_about_something
  end
end

each_iteration will be called for each User model in User.all relation. The relation will be ordered by primary key, exactly like find_each does. Iteration hooks into Sidekiq out of the box to support graceful interruption. No extra configuration is required.

class ArgumentsJob
  include Sidekiq::Job
  include SidekiqIteration::Iteration

  def build_enumerator(arg1, arg2, cursor:)
    active_record_records_enumerator(User.all, cursor: cursor)
  end

  def each_iteration(user, arg1, arg2)
    user.notify_about_something
  end
end

ArgumentsJob.perform_async(arg1, arg2)

class NotifyUsersJob
  include Sidekiq::Job
  include SidekiqIteration::Iteration

  def on_start
    # Will be called when the job starts iterating. Called only once, for the first time.
  end

  def around_iteration
    # Will be called around each iteration.
    # Can be useful for some metrics collection, performance tracking etc.
    yield
  end

  def on_resume
    # Called when the job resumes iterating.
  end

  def on_shutdown
    # Called each time the job is interrupted.
    # This can be due to throttling, `max_job_runtime` configuration, or sidekiq restarting.
  end

  def on_complete
    # Called when the job finished iterating.
  end

  # ...
end

class BatchesJob
  include Sidekiq::Job
  include SidekiqIteration::Iteration

  def build_enumerator(product_id, cursor:)
    active_record_batches_enumerator(
      Comment.where(product_id: product_id).select(:id),
      cursor: cursor,
      batch_size: 100,
    )
  end

  def each_iteration(batch_of_comments, product_id)
    comment_ids = batch_of_comments.map(&:id)
    CommentService.call(comment_ids: comment_ids)
  end
end

class RelationsJob
  include Sidekiq::Job
  include SidekiqIteration::Iteration

  def build_enumerator(product_id, cursor:)
    active_record_relations_enumerator(
      Product.find(product_id).comments,
      cursor: cursor,
      batch_size: 100,
    )
  end

  def each_iteration(comments_relation, product_id)
    # comments_relation will be a Comment::ActiveRecord_Relation
    comments_relation.update_all(deleted: true)
  end
end

class ArrayJob
  include Sidekiq::Job
  include SidekiqIteration::Iteration

  def build_enumerator(cursor:)
    array_enumerator(['build', 'enumerator', 'from', 'any', 'array'], cursor: cursor)
  end

  def each_iteration(array_element)
    # use array_element
  end
end

class CsvJob
  include Sidekiq::Job
  include SidekiqIteration::Iteration

  def build_enumerator(import_id, cursor:)
    import = Import.find(import_id)
    csv_enumerator(import.csv, cursor: cursor)
  end

  def each_iteration(csv_row, import_id)
    # insert csv_row to database
  end
end

class NestedIterationJob
  include Sidekiq::Job
  include SidekiqIteration::Iteration

  def build_enumerator(cursor:)
    nested_enumerator(
      [
        ->(cursor) { active_record_records_enumerator(Shop.all, cursor: cursor) },
        ->(shop, cursor) { active_record_records_enumerator(shop.products, cursor: cursor) },
        ->(_shop, product, cursor) { active_record_relations_enumerator(product.product_variants, cursor: cursor) }
      ],
      cursor: cursor
    )
  end

  def each_iteration(product_variants_relation)
    # do something
  end
end

• Iteration: how it works
• Job argument semantics
• Best practices
• Writing custom enumerator
• Throttling

For more detailed documentation, see rubydoc.

Iteration job must respond to build_enumerator and each_iteration methods. build_enumerator must return Enumerator object that respects the cursor value.

Advantages of this pattern over splitting a large job into many small jobs?

• Having one job is easier for redis in terms of memory, time and # of requests needed for enqueuing.
• It simplifies sidekiq monitoring, because you have a predictable number of jobs in the queues, instead of having thousands of them at one time and millions at another. Also easier to navigate its web UI.
• You can stop/pause/delete just one job, if something goes wrong. With many jobs it is harder and can take a long time, if it is critical to stop it right now.

Why can't I just iterate in #perform method and do whatever I want? You can, but then your job has to comply with a long list of requirements, such as the ones above. This creates leaky abstractions more easily, when instead we can expose a more powerful abstraction for developers without exposing the underlying infrastructure.

What happens when my job is interrupted? A checkpoint will be persisted to Redis after the current each_iteration, and the job will be re-enqueued. Once it's popped off the queue, the worker will work off from the next iteration.

What happens with retries? An interruption of a job does not count as a retry. The iteration of job that caused the job to fail will be retried and progress will continue from there on.

What happens if my iteration takes a long time? We recommend that a single each_iteration should take no longer than 30 seconds. In the future, this may raise an exception.

Why is it important that each_iteration takes less than 30 seconds? When the job worker is scheduled for restart or shutdown, it gets a notice to finish remaining unit of work. To guarantee that no progress is lost we need to make sure that each_iteration completes within a reasonable amount of time.

What do I do if each iteration takes a long time, because it's doing nested operations? If your each_iteration is complex, we recommend enqueuing another job, which will run your nested business logic. If each_iteration performs some other iterations, like iterating over child records, consider using nested iterations.

My job has a complex flow. How do I write my own Enumerator? See the guide on Custom Enumerators for details.

Thanks to job-iteration gem for the original implementation and inspiration.

After checking out the repo, run bundle install to install dependencies and start Redis. Run bundle exec rake to run the linter and tests. This project uses multiple Gemfiles to test against multiple versions of Sidekiq; you can run the tests against the specific version with BUNDLE_GEMFILE=gemfiles/sidekiq_6.gemfile bundle exec rake test.

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 tags, and push the .gem file to rubygems.org.

Bug reports and pull requests are welcome on GitHub at https://github.com/fatkodima/sidekiq-iteration.

The gem is available as open source under the terms of the MIT License.