Workhorse

Multi-threaded job backend with database queuing for ruby. Battle-tested and ready for production-use.

Introduction

How it works:

Jobs are instances of classes that support the perform method.
Jobs are persisted in the database using ActiveRecord.
Each job has a priority, the default being 0. Jobs with higher priorities (lower is higher, 0 the highest) get processed first.
Each job can be set to execute after a certain date / time.
You can start one or more worker processes.
Each worker is configurable as to which queue(s) it processes. Jobs in the same queue never run simultaneously. Jobs with no queue can always run in parallel.
Each worker polls the database and spawns a configurable number of threads to execute jobs of different queues simultaneously.

What it does not do:

It does not spawn new processes on the fly. Jobs are run in separate threads but not in separate processes (unless you manually start multiple worker processes).
It does not support timeouts.

Installation

Requirements

Ruby >= 2.0.0
Rails >= 3.2
A database and table handler that properly supports row-level locking (such as MySQL with InnoDB, PostgreSQL, or Oracle).
If you are planning on using the daemons handler:
- An operating system and file system that supports file locking.
- MRI ruby (aka "CRuby") as jRuby does not support fork. See the FAQ for possible workarounds.

Installing under Rails

Add workhorse to your Gemfile:
```
gem 'workhorse'
```
Install it using bundle install as usual.
Run the install generator:
```
bundle exec rails generate workhorse:install
```
This generates:
- A database migration for creating a table named jobs
- The initializer config/initializers/workhorse.rb for global configuration
- The daemon worker script bin/workhorse.rb
Please customize the initializer and worker script to your liking.

Queuing jobs

Basic jobs

Workhorse can handle any jobs that support the perform method and are serializable. To queue a basic job, use the static method Workhorse.enqueue. You can optionally pass a queue name and a priority.

class MyJob
  def initialize(name)
    @name = name
  end

  def perform
    puts "Hello #{@name}"
  end
end

Workhorse.enqueue MyJob.new('John'), queue: :test, priority: 2

RailsOps operations

Workhorse allows you to easily queue RailsOps operations using the static method Workhorse.enqueue_op:

Workhorse.enqueue_op Operations::Jobs::CleanUpDatabase, { queue: :maintenance, priority: 2 }, quiet: true

Params passed using the second argument will be used for operation instantiation at job execution.

If you do not want to pass any params to the operation, just omit the second hash:

Workhorse.enqueue_op Operations::Jobs::CleanUpDatabase, queue: :maintenance, priority: 2

Scheduling

Workhorse has no out-of-the-box functionality to support scheduling of regular jobs, such as maintenance or backup jobs. There are two primary ways of achieving regular execution:

Rescheduling by the same job after successful execution and setting perform_at

This is simple to set up and requires no additional dependencies. However, the time taken to execute a job and the time delay caused by the polling interval cannot easily be factored into the calculation of the interval, leading to a slight shift in effective execution date. (This can be mitigated by scheduling the job before knowing whether the current run will succeed. Proceed down this path at your own peril!)

Example: A job that takes 5 seconds to run and is set to reschedule itself after 10 minutes is started at 12:00 sharp. After one hour it will be set to execute at 13:00:30 at the earliest.

In its most basic form, the perform method of a job would look as follows:
```
class MyJob
  def perform
    # Do all the work

    # Perform again after 10 minutes (600 seconds)
    Workhorse.enqueue MyJob.new, perform_at: Time.now + 600
  end
end
```
Using an external scheduler

A more elaborate setup requires an external scheduler, but which can still be called from Ruby. One such scheduler is rufus-scheduler. A small example of an adapted bin/workhorse.rb to accommodate for the additional cog in the mechanism is given below:
```
#!/usr/bin/env ruby

require './config/environment'

Workhorse::Daemon::ShellHandler.run do |daemon|
  # Start scheduler process
  daemon.worker 'Scheduler' do
    scheduler = Rufus::Scheduler.new

    scheduler.cron '0/10 * * * *' do
      Workhorse.enqueue Workhorse::Jobs::CleanupSucceededJobs.new
    end

    Signal.trap 'TERM' do
      scheduler.shutdown
    end

    scheduler.join
  end

  # Start 5 worker processes with 3 threads each
  5.times do
    daemon.worker do
      Workhorse::Worker.start_and_wait(pool_size: 3, polling_interval: 10, logger: Rails.logger)
    end
  end
end
```
This allows starting and stopping the daemon with the usual interface. Note that the scheduler is handled like a Workhorse worker, the consequence of which is that only one 'worker' should be started by the ShellHandler. Otherwise there would be multiple jobs scheduled at the same time.

Please refer to the documentation on rufus-scheduler (or the scheduler of your choice) for further options concerning the timing of the jobs.

Configuring and starting workers

Workers poll the database for new jobs and execute them in one or more threads. Typically, one worker is started per process. While you can start workers manually, either in your main application process(es) or in a separate one, workhorse also provides you with a convenient way of starting one or multiple worker processes as daemons.

Start workers manually

Workers are created by instantiating, configuring, and starting a new Workhorse::Worker instance:

Workhorse::Worker.start_and_wait(
  pool_size: 5,                           # Processes 5 jobs concurrently
  quiet:     false,                       # Logs to STDOUT
  logger:    Rails.logger                 # Logs to Rails log. You can also
                                          # provide any custom logger.
)

See code documentation for more information on the arguments. All arguments passed to start_and_wait are passed to the initialize. All arguments passed to start_and_wait are in turn passed to the initializer of Workhorse::Worker.

Start workers using a daemon script

Using Workhorse::Daemon::ShellHandler, you can spawn one or multiple worker processes automatically. This is useful for cases where you want the workers to exist in separate processes as opposed to your main application process(es).

For this case, the workhorse install routine automatically creates the file bin/workhorse.rb, which can be used to start one or more worker processes.

The script can be called as follows:

RAILS_ENV=production bundle exec bin/workhorse.rb start|stop|kill|status|watch|restart|usage

Background and customization

Within the shell handler, you can instantiate, configure, and start a worker as described under Start workers manually:

Workhorse::Daemon::ShellHandler.run do |daemon|
  5.times do
    daemon.worker do
      # This will be run 5 times, each time in a separate process. Per process, it
      # will be able to process 3 jobs concurrently.
      Workhorse::Worker.start_and_wait(pool_size: 3, logger: Rails.logger)
    end
  end
end

Instant repolling

Per default, each worker only polls in the given interval. This means that if you schedule, for example, 50 jobs at once and have a polling interval of 1 minute with a queue size of 1, the poller would tackle the first job and then wait for a whole minute until the next poll. This would mean that these 50 jobs would take at least 50 minutes to be executed, even if they only take a few seconds each.

This is where instant repolling comes into play: Using the worker option instant_repolling, you can force the poller to automatically re-poll the database whenever a job has been performed. It then goes back to the usual polling interval.

This setting is recommended for all setups and may eventually be enabled by default.

Exception handling

Per default, exceptions occurring in a worker thread will only be visible in the respective log file, usually production.log. If you'd like to perform specific actions when an exception arises, set the global option on_exception to a callback of your linking, e.g.:

# config/initializers/workhorse.rb
Workhorse.setup do |config|
  config.on_exception = proc do |e|
    # Use gem 'exception_notification' for notifying about exceptions
    ExceptionNotifier.notify_exception(e)
  end
end

Handling database jobs

Jobs stored in the database can be accessed via the ActiveRecord model {Workhorse::DbJob}. This is the model representing a specific job database entry and is not to be confused with the actual job class you're enqueueing.

Obtaining database jobs

DbJobs are returned to you when enqueuing new jobs:

db_job = Workhorse.enqueue(MyJob.new)

You can also obtain a job via its ID that you either get from a returned job (see example above) or else by manually querying the database table:

db_job = Workhorse::DbJob.find(42)

Note that database job objects reflect the job at the point in time when the database job object has been instantiated. To make sure you're looking at the latest job info, use the in-place reload method:

db_job.reload

You can also retrieve a list of jobs in a specific state using one of the following methods:

DbJob.waiting
DbJob.locked
DbJob.started
DbJob.succeeded
DbJob.failed

Resetting jobs

Jobs in a state other than waiting are either being processed or else already in a final state such as succeeded and won't be performed again. Workhorse provides an API method for resetting jobs in the following cases:

A job has succeeded or failed (states succeeded and failed) and needs to re-run. In these cases, perform a non-forced reset:
```
db_job.reset!
```
This is always safe to do, even with workers running.
A job is stuck in state locked or started and the corresponding worker (check the database field locked_by) is not running anymore, i.e. due to a database connection loss or an unexpected worker crash. In these cases, the job will never be processed, and, if the job is in a queue, the entire queue is considered to be locked and no further jobs will be processed in this queue.

In these cases, make sure the worker is stopped and perform a forced reset:
```
db_job.reset!(true)
```

Performing a reset will reset the job state to waiting and it will be processed again. All meta fields will be reset as well. See inline documentation of Workhorse::DbJob#reset! for more details.

Using workhorse with Rails / ActiveJob

While workhorse can be used though its custom interface as documented above, it is also fully integrated into Rails using ActiveJob. See documentation of ActiveJob for more information on how to use it.

To use workhorse as your ActiveJob backend, set the queue_adapter to workhorse, either using config.active_job.queue_adapter in your application configuration or else using self.queue_adapter in a job class inheriting from ActiveJob. See ActiveJob documentation for more details.

Cleaning up jobs

Per default, jobs remain in the database, no matter in which state. This can eventually lead to a very large jobs database. You are advised to clean your jobs database on a regular interval. Workhorse provides the job Workhose::Jobs::CleanupSucceededJobs for this purpose that cleans up all succeeded jobs. You can run this using your scheduler in a specific interval.

Frequently asked questions

Please consult the FAQ.

tcberner/workhorse