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Running Background Jobs in Ruby on Rails Revisited

A while back, we wrote an article on Running Background Jobs in Ruby on Rails.

The Ruby on Rails framework has a number of tools for running your code outside of the web-request, including the venerable script/runner for one-off tasks, but using them can be a little heavy on your server. If you want to run a task on the minute, or on demand, script/runner will load your entire Rails environment, which can be from 20-50 MB, depending on how many libraries and how much code you’re pulling in.

There are also a few other good guides, recipes, and libraries that we’ve mentioned before, including:

We’ve found that it’s not terribly hard to build your own job server that runs continuously in the background and can handle all kinds of jobs, including those that should run on a specified interval. Here’s how we did it.

We’re going to make use of the Daemons gem, so install it first:

    sudo gem install daemons 

Let’s go ahead and build in two types of jobs:

  • those that Run Once (immediately) and

  • those that Run on an interval (every x seconds or minutes or days)

We’ll use ActiveRecord’s Single Table Inheritance (STI) to handle both types of jobs and dictate their differing behaviors.

Create a PeriodicJob model:

     script/generate model PeriodicJob type:string   job:text interval:integer last_run_at:datetime 

And migrate up. Now, fill in the PeriodicJob#run! method:

     # app/models/periodic_job.rb class PeriodicJob < ActiveRecord::Base      # Runs a job and updates the +last_run_at+ field.   def run!     begin       eval(self.job)     rescue Exception       logger.error "'#{self.job}' could not run: #{$!.message}n#{$!.backtrace}"     end     self.last_run_at = Time.now.utc     self.save     end    end 

Note that we’re using Time.now.utc so as not to cause confusion – our AR is configured to use UTC by default.

Now, let’s create the subclass for Run Once jobs and let it inherit from our PeriodicJob model. We’ll add two more class methods to it, including a finder and a cleanup method:

     # app/models/run_once_periodic_job.rb class RunOncePeriodicJob < PeriodicJob      # RunOncePeriodicJobs run if they have no PeriodicJob#last_run_at time.   def self.find_all_need_to_run     self.find(:all, :conditions => ["last_run_at IS NULL"])   end      # Cleans up all jobs older than a day.   def self.cleanup     self.destroy_all ['last_run_at < ?', 1.day.ago]   end    end 

Now let’s define the Run on an Interval Job and add the interval specific finder:

     # app/models/run_interval_periodic_job.rb class RunIntervalPeriodicJob < PeriodicJob    # RunIntervalPeriodicJobs run if PeriodicJob#last_run_at time plus  # PeriodicJob#interval (in seconds) is past the current time (Time.now).   def self.find_all_need_to_run     self.find(:all).select {|job| job.last_run_at.nil? ||        (job.last_run_at + job.interval <= Time.now.utc)}   end    end 

Now, let’s write some tests, to make it clear how it should work:

     # test/unit/periodic_job_test.rb require File.dirname(__FILE__) + '/../test_helper'  class PeriodicJobTest < Test::Unit::TestCase   fixtures :periodic_jobs    def test_should_run_job     assert_nothing_thrown { periodic_jobs(:run_once_job).run! }   end    def test_should_find_run_once_job     assert RunOncePeriodicJob.find_all_need_to_run.include?(periodic_jobs(:run_once_job))   end      def test_should_not_find_run_job_already_run     assert !RunOncePeriodicJob.find_all_need_to_run.include?(periodic_jobs(:run_once_job_to_be_deleted))   end      def test_should_find_run_interval_job     assert RunIntervalPeriodicJob.find_all_need_to_run.include?(periodic_jobs(:run_interval_job_needs_run))           end      def test_should_not_find_run_interval_job_not_within_interval     assert !RunIntervalPeriodicJob.find_all_need_to_run.include?(periodic_jobs(:run_interval_job_does_not_need_run))   end      def test_should_cleanup_old_jobs     jobs_count = RunOncePeriodicJob.count          assert periodic_jobs(:run_once_job_to_be_deleted).last_run_at     RunOncePeriodicJob.cleanup          assert jobs_count - 1, RunOncePeriodicJob.count   end  end 

Here are our fixtures that setup the scenarios:

     # test/fixtures/periodic_jobs.yml run_once_job:   id: 1   type: RunOncePeriodicJob   job: 'what = "w00t once!"' run_interval_job_needs_run:   id: 2   type: RunIntervalPeriodicJob   interval: 60   job: 'what = "w00t on the minute dood!"'   last_run_at: <%= (Time.now.utc - 5.minutes).to_s(:db) %> run_interval_job_does_not_need_run:   id: 3   type: RunIntervalPeriodicJob   interval: 60   job: 'what = "w00t on the minute dood!"'   last_run_at: <%= (Time.now.utc - 5).to_s(:db) %> run_once_job_to_be_deleted:   id: 4   type: RunOncePeriodicJob   job: 'what = "w00t once!"'   last_run_at: <%= (Time.now.utc - 8.days).to_s(:db) %> run_interval_job_needs_run_never_run_before:   id: 5   type: RunIntervalPeriodicJob   interval: 60   job: 'what = "w00t on the minute dood!"' 

Now, we have a built in system for running Periodic Jobs. Note that all we have to do is create a new Periodic Job with the actual code we would normally toss to script/runner in the PeriodicJob#code field, and when we call the PeriodicJob#run! method, it will evaluate it.

We now need a way to always run a background task server to check these PeriodicJobs and run them.

Create a file called task_server.rb in your script directory.

     # script/task_server.rb #!/usr/bin/env ruby # # Background Task Server # # Relies on ActiveRecord PeriodicJob and STI table (periodic_jobs): # # type:         string    ("RunOncePeriodicJob", or "RunIntervalPeriodicJob") # interval:     integer   (in seconds) # job:          text      (actual ruby code to eval) # last_run_at:  datetime  (stored time of last run) # # Main algorithm is daemon process runs every XX seconds, wakes up and # looks for jobs. Jobs placed in the RunOncePeriodicJob queue are run  # immediately (if no last_run_at time) and stored until they are cleaned up  # (deleted). Jobs placed in the RunIntervalPeriodicJob queue are run if:  # their last_run_at time + their interval (in seconds) is past the current  # time (Time.now). #  options = {} ARGV.options do |opts|    opts.on( "-e", "--environment ENVIRONMENT", String,            "The Rails Environment to run under." ) do |environment|     options[:environment] = environment   end      opts.parse! end  RAILS_ENV = options[:environment] || 'development'    require File.dirname(__FILE__) + '/../config/environment.rb'  if RAILS_ENV == "development" or RAILS_ENV == "test"   SLEEP_TIME = 10 else   SLEEP_TIME = 60 end  
     loop do   # Find all Run Once jobs, and run them   RunOncePeriodicJob.find_all_need_to_run.each do |job|     job.run!   end    # Find all Run on Interval jobs, and run them     RunIntervalPeriodicJob.find_all_need_to_run.each do |job|     job.run!   end      # Cleans up periodic jobs, removes all RunOncePeriodicJobs over one   # day old.   RunOncePeriodicJob.cleanup      sleep(SLEEP_TIME) end 

That’s it. Now, we create a control script using the daemons gem.

     # script/task_server_control.rb #!/usr/bin/env ruby # # Background Task Server Control - A daemon for running jobs #  require 'rubygems' require 'daemons'  options = {}  default_pid_dir = "/var/run/task_server"  if File.exists?(default_pid_dir)   options[:dir_mode] = :normal   options[:dir] = default_pid_dir end  Daemons.run(File.dirname(__FILE__) + '/../script/task_server.rb', options) 

Create an optional /var/run/task_server dir if you’re running on a server (in production mode):

     mkdir -p /var/run/task_server chown deploy:deploy /var/run/task_server 

We can start it up in the normal server mode, as a daemon (using the start/stop commands, or we can start it up in interactive mode (so we can see the results) using the run command:

     ruby script/task_server_control.rb run 

In another window, add some jobs:

     ruby script/console >> RunOncePeriodicJob.create(:job => 'puts "This job will only run once."') => #... RunIntervalPeriodicJob.create(:job => 'puts "This job runs every 30 seconds, and it ran: #{Time.now.utc}"', :interval => 30) => #... 

You should see the task_server_control.rb file running these jobs as the task server wakes up.

And now, it wouldn’t be complete without some Capistrano support to enable restarting after we make code changes to the model, and to allow start/stop/restart:

     # config/deploy.rb   # In case you're running on multiple app servers, # we define the task_server to make sure that  # jobs only run on one server. role :task_server, "app_server1.example.com"  namespace :background_task_server do      task :setup, :roles => :task_server do     run "mkdir -p /var/run/task_server"     run "chown #{user}:#{group} /var/run/task_server"   end      # start background task server   task :start, :roles => :task_server do     run "#{current_path}/script/task_server_control.rb start -- -e production"   end      # stop background task server   task :stop, :roles => :task_server do     run "#{current_path}/script/task_server_control.rb stop -- -e production"   end    # start background task server   task :restart, :roles => :task_server do     # TODO: since restart won't cold_start, we could read call to status, if      # it returns:     #    task_server.rb: no instances running     # we could simply issue the start command     run "#{current_path}/script/task_server_control.rb restart -- -e production"   end    end  # optional: # after "deploy", "background_task_server:restart"  

Note the use of the task_server, so you can simply allow one app server to be your task server (if you’re running on multiple servers).

And now, because I’m feeling generous, let’s set monit up to monitor your task server, so that if it ever goes down for some strange reason, monit should boot it back up (this also ensures that restarts will boot your task server back up):

     # /etc/monit.d/task_server.conf  check process task-server with pidfile /var/run/task_server/task_server.rb.pid   group task-server   start program = "/usr/bin/ruby /var/www/apps/example/current/script/task_server_control.rb start -- --environment=production"   stop program  = "/usr/bin/ruby /var/www/apps/example/current/script/task_server_control.rb stop -- --environment=production" 

That’s it! If there’s any interest in forming a plugin around this with generators to create the migration and models, I’ll give it a go and stick it on github.

Feedback appreciated!

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