Sidekiq limited capacity worker

It is possible to limit the number of concurrent running jobs for a worker class by using the LimitedCapacity::Worker concern.

The worker must implement three methods:

  • perform_work: The concern implements the usual perform method and calls perform_work if there’s any available capacity.
  • remaining_work_count: Number of jobs that have work to perform.
  • max_running_jobs: Maximum number of jobs allowed to run concurrently. 
class MyDummyWorker
  include ApplicationWorker
  include LimitedCapacity::Worker

  def perform_work(*args)
  end

  def remaining_work_count(*args)
    5
  end

  def max_running_jobs
    25
  end
end

Additional to the regular worker, a cron worker must be defined as well to backfill the queue with jobs. the arguments passed to perform_with_capacity are passed to the perform_work method. 

class ScheduleMyDummyCronWorker
  include ApplicationWorker
  include CronjobQueue

  def perform(*args)
    MyDummyWorker.perform_with_capacity(*args)
  end
end

How many jobs are running?

It runs max_running_jobs at almost all times.

The cron worker checks the remaining capacity on each execution and it schedules at most max_running_jobs jobs. Those jobs on completion re-enqueue themselves immediately, but not on failure. The cron worker is in charge of replacing those failed jobs.

Handling errors and idempotence

This concern disables Sidekiq retries, logs the errors, and sends the job to the dead queue. This is done to have only one source that produces jobs and because the retry would occupy a slot with a job to perform in the distant future.

We let the cron worker enqueue new jobs, this could be seen as our retry and back off mechanism because the job might fail again if executed immediately. This means that for every failed job, we run at a lower capacity until the cron worker fills the capacity again. If it is important for the worker not to get a backlog, exceptions must be handled in #perform_work and the job should not raise.

The jobs are deduplicated using the :none strategy, but the worker is not marked as idempotent!.

Metrics

This concern exposes three Prometheus metrics of gauge type with the worker class name as label:

  • limited_capacity_worker_running_jobs
  • limited_capacity_worker_max_running_jobs
  • limited_capacity_worker_remaining_work_count