How to create a rake cron job in Heroku

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A few weeks ago I had to deal with setting up a cron job that periodically updated some db tables of a Ruby on Rails application running on Heroku. Here’s a brief step-by-step tutorial to make your life easier in case you were approaching the same task.

Go to the “tasks” folder of your application and create a file called “scheduler.rake”. Below you can find an example of a rake task:


desc 'my job description'
task :my_scheduler_job, :from_date do |t, args|
  # setup the db connection
  require 'pg'
  db_parts = ENV['DATABASE_URL'].split(/\/|:|@/)
  username = db_parts[3]
  password = db_parts[4]
  host = db_parts[5]
  db = db_parts[7]
  conn = PGconn.open(:host =>  host, :dbname => db, :user=> username, :password=> password)
  # read input param
  from_date = args[:from_date]
  if from_date
    date_clause = ' AND date > ' + from_date
  else
    date_clause = ''
  end
  # function that retrieves updated data
  updated_data = get_updated_data()
  updated_data.each{ |row|
    my_query = 'UPDATE MY_TABLE SET MY_FIELD = #{ row.my_field } WHERE ID = #{ row.id } ' + date_clause
    begin
      conn.exec(sql)
      puts 'Updating id: #{ row.id }'
    rescue Exception => e
      puts e.message
      puts e.backtrace.inspect
      puts 'There was an insert error on #{ index.to_s } in #{ my_query }'
    end
  }
end

Once your task is ready, go to your Heroku application and add the Heroku Scheduler add-on. Now go to the add-on dashboard and add your task by typing

rake my_scheduler_job

and set the desired frequency.

For debugging purposes, it’s possible to run your task from the command line by executing

 
heroku run rake my_scheduler_job['2013/05/01']

To check that the updates in your database were correctly executed, Heroku provides you a postgres console that can be accessed with the command

heroku pg:psql DATABASE_URL

Hope this tutorial will help you saving some time when having to create rake tasks in Heroku. As always, don’t hesitate writing your feedbacks/questions in the comment section below.

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