💎 Resque::Plugins::LonelyJob

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resque-lonely_job is the original Resque worker-time serialization plugin. Extend a job with Resque::Plugins::LonelyJob to take a Redis mutex before perform. A job that cannot acquire the lock is re-enqueued instead of running concurrently.

By default the mutex is per queue, so at most one job from a queue runs at a time. Override redis_key(*args) to serialize a selected resource or partition instead.

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Install the gem and add to the application's Gemfile by executing:

bundle add resque-lonely_job

If bundler is not being used to manage dependencies, install the gem by executing:

gem install resque-lonely_job

LonelyJob has no global configuration object. Configure behavior on the job class:

class RebuildAccount
  extend Resque::Plugins::LonelyJob

  @queue = :maintenance
  @requeue_interval = 1

  def self.redis_key(account_id, *)
    "lonely_job:rebuild_account:#{account_id}"
  end
end

@requeue_interval defaults to one second and controls how long a worker waits before re-enqueuing a contested job. Lock expiry defaults to five days, so custom keys and job duration must be designed together. All workers must use the same Redis namespace.

require 'resque-lonely_job'

class StrictlySerialJob
  extend Resque::Plugins::LonelyJob

  @queue = :serial_work

  def self.perform
    # only one at a time in this block, no parallelism allowed for this
    # particular queue
  end
end

Let's say you want the serial constraint to apply at a more granular level. Instead of applying at the queue level, you can overwrite the .redis_key method.

require 'resque-lonely_job'

class StrictlySerialJob
  extend Resque::Plugins::LonelyJob

  @queue = :serial_work

  # Returns a string that will be used as the redis key
  # NOTE: it is recommended to prefix your string with the 'lonely_job:' to
  # namespace your key!
  def self.redis_key(account_id, *args)
    "lonely_job:strictly_serial_job:#{account_id}"
  end

  # Overwrite reenqueue to lpush instead of default rpush.  This attempts to
  # preserve job ordering but job order is *NOT* guaranteed and also not
  # likely. See the comment on SHA: e9912fb2 for why.
  def self.reenqueue(*args)
    Resque.redis.lpush("queue:#{Resque.queue_from_class(self)}", Resque.encode(class: self, args: args))
  end

  def self.perform(account_id, *args)
    # only one at a time in this block, no parallelism allowed for this
    # particular redis_key
  end
end

NOTE: Without careful consideration of your problem domain, worker starvation and/or unfairness is possible for jobs in this example. Imagine a scenario where you have three jobs in the queue with two resque workers:

+---------------------------------------------------+
| :serial_work                                      |
|---------------------------------------------------|
|             |             |             |         |
| redis_key:  | redis_key:  | redis_key:  | ...     |
|    A        |    A        |    B        |         |
|             |             |             |         |
| job 1       | job 2       | job 3       |         |
+---------------------------------------------------+
                                  ^
                                  |
  Possible starvation +-----------+
  for this job and
  subsequent ones

When the first worker grabs job 1, it'll acquire the mutex for processing redis_key A. The second worker tries to grab the next job off the queue but is unable to acquire the mutex for redis_key A so it places job 2 back at the head of the :serial_work queue. Until worker 1 completes job 1 and releases the mutex for redis_key A, no work will be done in this queue.

This issue may be avoided by employing dynamic queues, http://blog.kabisa.nl/2010/03/16/dynamic-queue-assignment-for-resque-jobs/, where the queue is a one to one mapping to the redis_key.

The secret to preserving job order semantics is to remove critical data from the resque job and store data in a separate redis list. Part of a running job's responsibility will be to grab data off of the separate redis list needed for it to complete its job.

+---------------------------------------------------+
| :serial_work for jobs associated with key A       |
|---------------------------------------------------|
|   data x    |   data y    |   data z    | ...     |
+---------------------------------------------------+

+---------------------------------------------------+
| :serial_work for jobs associated with key B       |
|---------------------------------------------------|
|   data m    |   data n    |   data o    | ...     |
+---------------------------------------------------+

+---------------------------------------------------+
| :serial_work                                      |
|---------------------------------------------------|
|             |             |             |         |
| redis_key:  | redis_key:  | redis_key:  | ...     |
|    A        |    A        |    B        |         |
|             |             |             |         |
| job 1       | job 2       | job 3       |         |
+---------------------------------------------------+

It now doesn't matter whether job 1 and job 2 are re-ordered as whichever goes first will perform an atomic pop on the redis list that contains the data needed for its job (data x, data y, data z).

The behavior when multiple jobs exist in a queue protected by resque-lonely_job is for one job to be worked, while the other is continuously dequeued and requeued until the first job is finished. This can result in that worker process pegging a CPU/core on a worker server. To guard against this, the default behavior is to sleep for 1 second before the requeue, which will allow the cpu to perform other work.

This can be customized using a @requeue_interval class instance variable in your job like so:

require 'resque-lonely_job'

class StrictlySerialJob
  extend Resque::Plugins::LonelyJob

  @queue = :serial_work
  @requeue_interval = 5         # sleep for 5 seconds before requeueing

  def self.perform
    # some implementation
  end
end

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This library follows for its public API where practical. For most applications, prefer the Pessimistic Version Constraint with two digits of precision.

For example:

spec.add_dependency("resque-lonely_job", "~> 1.0")

See CHANGELOG.md for a list of releases.

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

See LICENSE.md for the official copyright notice.

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