Hi Everyone!. In this post, I want to share with you a little guide that will show you how to use cron jobs in a Ruby on Rails application.
- Ruby (I use ruby 2.7.1p83)
- Ruby on Rails (I use Rails 188.8.131.52)
A cron job is a process that is scheduled to run at a certain time (every minute, day, week, or month). On Unix systems, we can run and manage these types of processes using the cron tool. The info of the process that will run the cron tool is stored in a file called crontab.
Examples of the use of cron jobs are the following:
- Generate reports. e.g, a report of daily payments from an e-commerce application.
- Send reminders to users. e.g, send offer reminders from an ecommerce application.
- Modify the status of the records. e.g, canceling payments that exceed a time limit to pay.
We can see our cron jobs using the following command
To delete user jobs we can use the following command
And to edit our crontab file, we can use the following command
The structure of a cron job is the following
.--------------- minute (0-59) | .------------ hour (0-23) | | .--------- day of month (1-31) | | | .------ month (1-12) | | | | .--- day of week (0-6) (sunday=0 or 7) | | | | | * * * * * command to execute
An example of how to define a cron job to execute every 5 minutes
*/5 * * * * /home/user/test.rb
Now that we know what's a cron job, we can start to write code. First, we are going to create our rails project.
rails new whenever_example
Whenever is a gem that helps us to define cron jobs in a readable way. So, we will add this gem to our
# Gemfile # Cron jobs gem 'whenever', require: false
Install our dependencies
We will use a rake task to run in our cron job. First, create a
# Create the rake task touch lib/tasks/whenever_test.rake
On our task, we will simply print a message.
# lib/tasks/whenever_test.rake desc 'Whenever rake task test' task whenever_call: :environment do Rails.logger.info "Whenever task" end
To see the rake tasks of our Rails application, we can use the following command.
# See all tasks bundle exec rake --tasks # See our task bundle exec rake --tasks | grep whenever_call # rake whenever_call # Whenever rake task test
We can execute our task to check that it works.
bundle exec rake whenever_call
In our logs file we should see the message we define.
tail log/development.log # Whenever task
First, we have to setup the whenever gem.
bundle exec wheneverize . # [add] writing `./config/schedule.rb' # [done] wheneverized!
This will create the
config/schedule.rb where we will define our cron jobs. We will create a simple task that will be executed every 2 minutes.
# config/schedule.rb # .... every 2.minute do rake 'whenever_call' end
Now, we have to update our crontab file.
If we list our cron jobs in the crontab file, we will see something like the following.
# crontab file 0,2,4,6,8,10,12,14,16,18,20,22,24,26,28,30,32,34,36,38,40,42,44,46,48,50,52,54,56,58 * * * * /bin/bash -l -c 'cd OUR_PROJECT_PATH && RAILS_ENV=production bundle exec rake whenever_call --silent'
If we see in our production logs, we can see the following.
# log/production.log I, [2020-11-15T20:50:02.590009 #53971] INFO -- : Whenever task I, [2020-11-15T20:52:02.914487 #54387] INFO -- : Whenever task
As, you can see the logs have 2 minutes of difference.
However, you can see that the task is logging into our
production.log. If you want to run the cron jobs in development environment, we can use the following.
whenever --update-crontab --set environment=development
Of course, we could specify more when the task runs. Here are some examples:
Scheduling a task every day at 3:00am
# config/schedule.rb # .... every 1.day, at: '3:00 am' do rake 'whenever_call' end
# crontab file 0 3 * * * /bin/bash -l -c 'cd OUR_PROJECT_PATH && RAILS_ENV=production bundle exec rake whenever_call --silent'
Scheduling a task every 2 days at 3:00am and 5:30pm
# config/schedule.rb # .... every 2.day, at: ['3:00 am', '5:30 pm'] do rake 'whenever_call' end
# crontab file 0 3 1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31 * * /bin/bash -l -c 'cd OUR_PROJECT_PATH && RAILS_ENV=production bundle exec rake whenever_call --silent' 30 17 1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31 * * /bin/bash -l -c 'cd OUR_PROJECT_PATH && RAILS_ENV=production bundle exec rake whenever_call --silent'
Scheduling a task on monday and sunday at 6:00pm
# config/schedule.rb # .... every [:monday, :sunday], at: '6:00 PM' do rake 'whenever_call' end
# crontab file 0 18 * * 1,0 /bin/bash -l -c 'cd OUR_PROJECT_PATH && RAILS_ENV=development bundle exec rake whenever_call --silent'
If you want to clear the contents of the crontab file, you can also use the following command.
# Clear crontab whenever -c
Thanks for reading this post and you can find more info about whenever in its official github repo.
If you are also interested in know how to create scheduled jobs in Elixir, I recommend you this amazing post 3 ways to schedule tasks in Elixir I have learned in 3 years working with it.