Solid Queue

Solid Queue is a DB-based queuing backend for Active Job, designed with simplicity and performance in mind.

Solid Queue can be used with SQL databases such as MySQL, PostgreSQL or SQLite, and it leverages the FOR UPDATE SKIP LOCKED clause, if available, to avoid blocking and waiting on locks when polling jobs. It relies on Active Job for retries, discarding, error handling, serialization, or delays, and it's compatible with Ruby on Rails multi-threading.

Usage

To set Solid Queue as your Active Job's queue backend, you should add this to your environment config:

# config/environments/production.rb
config.active_job.queue_adapter = :solid_queue

Alternatively, you can set only specific jobs to use Solid Queue as their backend if you're migrating from another adapter and want to move jobs progressively:

# app/jobs/my_job.rb

class MyJob < ApplicationJob
  self.queue_adapter = :solid_queue
  # ...
end

Installation

Add this line to your application's Gemfile:

gem "solid_queue"

And then execute:

$ bundle

Or install it yourself as:

$ gem install solid_queue

Add the migration to your app and run it:

$ bin/rails solid_queue:install:migrations
$ bin/rails db:migrate

With this, you'll be ready to enqueue jobs using Solid Queue, but you need to start Solid Queue's supervisor to run them.

$ bundle exec rake solid_queue:start

This will start processing jobs in all queues using the default configuration. See below to learn more about configuring Solid Queue.

Requirements

Besides Rails 7, Solid Queue works best with MySQL 8+ or PostgreSQL 9.5+, as they support FOR UPDATE SKIP LOCKED. You can use it with older versions, but in that case, you might run into lock waits if you run multiple workers for the same queue.

Configuration

Workers and dispatchers

We have three types of processes in Solid Queue:

• Workers are in charge of picking jobs ready to run from queues and processing them. They work off the solid_queue_ready_executions table.
• Dispatchers are in charge of selecting jobs scheduled to run in the future that are due and dispatching them, which is simply moving them from the solid_queue_scheduled_jobs table over to the solid_queue_ready_executions table so that workers can pick them up. They also do some maintenance work related to concurrency controls.
• The supervisor forks workers and dispatchers according to the configuration, controls their heartbeats, and sends them signals to stop and start them when needed.

By default, Solid Queue will try to find your configuration under config/solid_queue.yml, but you can set a different path using the environment variable SOLID_QUEUE_CONFIG. This is what this configuration looks like:

production:
  dispatchers:
    - polling_interval: 1
      batch_size: 500
  workers:
    - queues: "*"
      threads: 3
      polling_interval: 2
    - queues: real_time,background
      threads: 5
      polling_interval: 0.1
      processes: 3

Everything is optional. If no configuration is provided, Solid Queue will run with one dispatcher and one worker with default settings.

• polling_interval: the time interval in seconds that workers and dispatchers will wait before checking for more jobs. This time defaults to 5 seconds for dispatchers and 1 second for workers.
• batch_size: the dispatcher will dispatch jobs in batches of this size.
• queues: the list of queues that workers will pick jobs from. You can use * to indicate all queues (which is also the default and the behaviour you'll get if you omit this). You can provide a comma-separated list of queues. Jobs will be polled from those queues in order, so for example, with real_time,background, no jobs will be taken from background unless there aren't any more jobs waiting in real_time. You can also provide a prefix with a wildcard to match queues starting with a prefix. For example:

staging:
  workers:
    - queues: staging*
      threads: 3
      polling_interval: 5

This will create a worker fetching jobs from all queues starting with staging. The wildcard * is only allowed on its own or at the end of a queue name; you can't specify queue names such as *_some_queue. These will be ignored.

Finally, you can combine prefixes with exact names, like staging*, background, and the behaviour with respect to order will be the same as with only exact names.

• threads: this is the max size of the thread pool that each worker will have to run jobs. Each worker will fetch this number of jobs from their queue(s), at most and will post them to the thread pool to be run. By default, this is 5. Only workers have this setting.
• processes: this is the number of worker processes that will be forked by the supervisor with the settings given. By default, this is 1, just a single process. This setting is useful if you want to dedicate more than one CPU core to a queue or queues with the same configuration. Only workers have this setting.

Queue order and priorities

As mentioned above, if you specify a list of queues for a worker, these will be polled in the order given, such as for the list real_time,background, no jobs will be taken from background unless there aren't any more jobs waiting in real_time.

Active Job also supports positive integer priorities when enqueuing jobs. In Solid Queue, the smaller the value, the higher the priority. The default is 0.

This is useful when you run jobs with different importance or urgency in the same queue. Within the same queue, jobs will be picked in order of priority, but in a list of queues, the queue order takes precedence, so in the previous example with real_time,background, jobs in the real_time queue will be picked before jobs in the background queue, even if those in the background queue have a higher priority (smaller value) set.

We recommend not mixing queue order with priorities but either choosing one or the other, as that will make job execution order more straightforward for you.

Threads, processes and signals

Workers in Solid Queue use a thread pool to run work in multiple threads, configurable via the threads parameter above. Besides this, parallelism can be achieved via multiple processes, configurable via different workers or the processes parameter above.

The supervisor is in charge of managing these processes, and it responds to the following signals:

• TERM, INT: starts graceful termination. The supervisor will send a TERM signal to its supervised processes, and it'll wait up to SolidQueue.shutdown_timeout time until they're done. If any supervised processes are still around by then, it'll send a QUIT signal to them to indicate they must exit.
• QUIT: starts immediate termination. The supervisor will send a QUIT signal to its supervised processes, causing them to exit immediately.

When receiving a QUIT signal, if workers still have jobs in-flight, these will be returned to the queue when the processes are deregistered.

If processes have no chance of cleaning up before exiting (e.g. if someone pulls a cable somewhere), in-flight jobs might remain claimed by the processes executing them. Processes send heartbeats, and the supervisor checks and prunes processes with expired heartbeats, which will release any claimed jobs back to their queues. You can configure both the frequency of heartbeats and the threshold to consider a process dead. See the section below for this.

Other configuration settings

There are several settings that control how Solid Queue works that you can set as well:

• logger: the logger you want Solid Queue to use. Defaults to the app logger.
• app_executor: the Rails executor used to wrap asynchronous operations, defaults to the app executor
• on_thread_error: custom lambda/Proc to call when there's an error within a thread that takes the exception raised as argument. Defaults to

-> (exception) { Rails.error.report(exception, handled: false) }

• connects_to: a custom database configuration that will be used in the abstract SolidQueue::Record Active Record model. This is required to use a different database than the main app. For example:

  # Use a separate DB for Solid Queue
  config.solid_queue.connects_to = { database: { writing: :solid_queue_primary, reading: :solid_queue_replica } }

• use_skip_locked: whether to use FOR UPDATE SKIP LOCKED when performing locking reads. This will be automatically detected in the future, and for now, you'd only need to set this to false if your database doesn't support it. For MySQL, that'd be versions < 8, and for PostgreSQL, versions < 9.5. If you use SQLite, this has no effect, as writes are sequential.
• process_heartbeat_interval: the heartbeat interval that all processes will follow—defaults to to 60 seconds.
• process_alive_threshold: how long to wait until a process is considered dead after its last heartbeat—defaults to to 5 minutes.
• shutdown_timeout: time the supervisor will wait since it sent the TERM signal to its supervised processes before sending a QUIT version to them requesting immediate termination—defaults to to 5 seconds.
• silence_polling: whether to silence Active Record logs emitted when polling for both workers and dispatchers—defaults to to false.
• supervisor_pidfile: path to a pidfile that the supervisor will create when booting to prevent running more than one supervisor in the same host, or in case you want to use it for a health check. It's nil by default.
• preserve_finished_jobs: whether to keep finished jobs in the solid_queue_jobs table—defaults to to true.
• clear_finished_jobs_after: period to keep finished jobs around, in case preserve_finished_jobs is true—defaults to to 1 day. Note: Right now, there's no automatic cleanup of finished jobs. You'd need to do this by periodically invoking SolidQueue::Job.clear_finished_in_batches, but this will happen automatically in the near future.
• default_concurrency_control_period: the value to be used as the default for the duration parameter in concurrency controls. It defaults to to 3 minutes.

Concurrency controls

Solid Queue extends Active Job with concurrency controls, that allows you to limit how many jobs of a certain type or with certain arguments can run at the same time. When limited in this way, jobs will be blocked from running, and they'll stay blocked until another job finishes and unblocks them, or after the set expiry time (concurrency limit's duration) elapses. Jobs are never discarded or lost, only blocked.

class MyJob < ApplicationJob
  limits_concurrency to: max_concurrent_executions, key: ->(arg1, arg2, **) { ... }, duration: max_interval_to_guarantee_concurrency_limit, group: concurrency_group

  # ...

• key is the only required parameter, and it can be a symbol, a string or a proc that receives the job arguments as parameters and will be used to identify the jobs that need to be limited together. If the proc returns an Active Record record, the key will be built from its class name and id.
• to is 1 by default, and duration is set to SolidQueue.default_concurrency_control_period by default, which itself defaults to 3 minutes, but that you can configure as well.
• group is used to control the concurrency of different job classes together. It defaults to the job class name.

When a job includes these controls, we'll ensure that, at most, the number of jobs (indicated as to) that yield the same key will be performed concurrently, and this guarantee will last for duration for each job enqueued. Note that there's no guarantee about the order of execution, only about jobs being performed at the same time (overlapping).

For example:

class DeliverAnnouncementToContactJob < ApplicationJob
  limits_concurrency to: 2, key: ->(contact) { contact.account }, duration: 5.minutes

  def perform(contact)
    # ...

Where contact and account are ActiveRecord records. In this case, we'll ensure that at most two jobs of the kind DeliverAnnouncementToContact for the same account will run concurrently. If, for any reason, one of those jobs takes longer than 5 minutes or doesn't release its concurrency lock within 5 minutes of acquiring it, a new job with the same key might gain the lock.

Let's see another example using group:

class Box::MovePostingsByContactToDesignatedBoxJob < ApplicationJob
  limits_concurrency key: ->(contact) { contact }, duration: 15.minutes, group: "ContactActions"

  def perform(contact)
    # ...

class Bundle::RebundlePostingsJob < ApplicationJob
  limits_concurrency key: ->(bundle) { bundle.contact }, duration: 15.minutes, group: "ContactActions"

  def perform(bundle)
    # ...

In this case, if we have a Box::MovePostingsByContactToDesignatedBoxJob job enqueued for a contact record with id 123 and another Bundle::RebundlePostingsJob job enqueued simultaneously for a bundle record that references contact 123, only one of them will be allowed to proceed. The other one will stay blocked until the first one finishes (or 15 minutes pass, whatever happens first).

Note that the duration setting depends indirectly on the value for concurrency_maintenance_interval that you set for your dispatcher(s), as that'd be the frequency with which blocked jobs are checked and unblocked. In general, you should set duration in a way that all your jobs would finish well under that duration and think of the concurrency maintenance task as a failsafe in case something goes wrong.

Finally, failed jobs that are automatically or manually retried work in the same way as new jobs that get enqueued: they get in the queue for gaining the lock, and whenever they get it, they'll be run. It doesn't matter if they had gained the lock already in the past.

Puma plugin

We provide a Puma plugin if you want to run the Solid Queue's supervisor together with Puma and have Puma monitor and manage it. You just need to add

plugin :solid_queue

to your puma.rb configuration.

Jobs and transactional integrity

⚠️ Having your jobs in the same ACID-compliant database as your application data enables a powerful yet sharp tool: taking advantage of transactional integrity to ensure some action in your app is not committed unless your job is also committed. This can be very powerful and useful, but it can also backfire if you base some of your logic on this behaviour, and in the future, you move to another active job backend, or if you simply move Solid Queue to its own database, and suddenly the behaviour changes under you.

If you prefer not to rely on this, or avoid relying on it unintentionally, you should make sure that:

• Your jobs relying on specific records are always enqueued on after_commit callbacks or otherwise from a place where you're certain that whatever data the job will use has been committed to the database before the job is enqueued.
• Or, to opt out completely from this behaviour, configure a database for Solid Queue, even if it's the same as your app, ensuring that a different connection on the thread handling requests or running jobs for your app will be used to enqueue jobs. For example:

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true

  connects_to database: { writing: :primary, reading: :replica }

  solid_queue.config.connects_to { database: { writing: :primary, reading: :replica } }

Inspiration

Solid Queue has been inspired by resque and GoodJob.

License

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