Running Cron Jobs on Linux Instances - AWS OpsWorks

Running Cron Jobs on Linux Instances

Important

The AWS OpsWorks Stacks service reached end of life on May 26, 2024 and has been disabled for both new and existing customers. We strongly recommend customers migrate their workloads to other solutions as soon as possible. If you have questions about migration, reach out to the AWS Support Team on AWS re:Post or through AWS Premium Support.

A Linux cron job directs the cron daemon to run one or more commands on a specified schedule. For example, suppose your stack supports a PHP e-commerce application. You can set up a cron job to have the server send you a sales report at a specified time every week. For more information about cron, see cron on Wikipedia. For more information about how to run a cron job directly on a Linux-based computer or instance, see What are cron and crontab, and how do I use them? on the Indiana University knowledge base website.

Although you can manually set up cron jobs on individual Linux-based instances by connecting to them with SSH, and editing their crontab entries, a key advantage of AWS OpsWorks Stacks is that you can direct it to run the task across an entire layer of instances. The following procedure describes how to set up a cron job on a PHP App Server layer's instances, but you can use the same approach with any layer.

To set up a cron job on a layer's instances

  1. Implement a cookbook with a recipe with a cron resource that sets up the job. The example assumes that the recipe is named cronjob.rb; the implementation details are described later. For more information on cookbooks and recipes, see Cookbooks and Recipes.

  2. Install the cookbook on your stack. For more information, see Installing Custom Cookbooks.

  3. Have AWS OpsWorks Stacks run the recipe automatically on the layer's instances by assigning it to the following lifecycle events. For more information, see Automatically Running Recipes.

    • Setup – Assigning cronjob.rb to this event directs AWS OpsWorks Stacks to run the recipe on all new instances.

    • Deploy – Assigning cronjob.rb to this event directs AWS OpsWorks Stacks to run the recipe on all online instances when you deploy or redeploy an app to the layer.

    You can also manually run the recipe on online instances by using the Execute Recipes stack command. For more information, see Run Stack Commands.

The following is the cronjob.rb example, which sets up a cron job to run a user-implemented PHP application once a week that collects the sales data from the server and mails a report. For more examples of how to use a cron resource, see cron. 


cron "job_name" do
  hour "1"
  minute "10"
  weekday "6"
  command "cd /srv/www/myapp/current && php .lib/mailing.php"
end

cron is a Chef resource that represents a cron job. When AWS OpsWorks Stacks runs the recipe on an instance, the associated provider handles the details of setting up the job.

  • job_name is a user-defined name for the cron job, such as weekly report.

  • hour/minute/weekday specify when the commands should run. This example runs the commands every Saturday at 1:10 AM.

  • command specifies the commands to be run.

    This example runs two commands. The first navigates to the /srv/www/myapp/current directory. The second runs the user-implemented mailing.php application, which collects the sales data and sends the report.

Note

The bundle command does not work with cron jobs by default. The reason is that AWS OpsWorks Stacks installs bundler in the /usr/local/bin directory. To use bundle with a cron job, you must explicitly add the path /usr/local/bin to the cron job. Also, because the $PATH environment variable may not expand in the cron job, a best practice is to explicitly add any necessary path information to the job without relying on expansion of the $PATH variable. The following examples show two ways to use bundle in a cron job.


cron "my first task" do
  path "/usr/local/bin"
  minute "*/10"
  command "cd /srv/www/myapp/current && bundle exec my_command"
end

cron_env = {"PATH" => "/usr/local/bin"}
cron "my second task" do
  environment cron_env
  minute "*/10"
  command "cd /srv/www/myapp/current && /usr/local/bin/bundle exec my_command"
end

If your stack has multiple application servers, assigning cronjob.rb to the PHP App Server layer's lifecycle events might not be an ideal approach. For example, the recipe runs on all of the layer's instances, so you will receive multiple reports. A better approach is to use a custom layer to ensure that only one server sends a report.

To run a recipe on just one of a layer's instances

  1. Create a custom layer called, for example, PHPAdmin and assign cronjob.rb to its Setup and Deploy events. Custom layers don't necessarily have to do very much. In this case, PHPAdmin just runs one custom recipe on its instances.

  2. Assign one of the PHP App Server instances to AdminLayer. If an instance belongs to more than one layer, AWS OpsWorks Stacks runs each layer's built-in and custom recipes.

Because only one instance belongs to the PHP App Server and PHPAdmin layers, cronjob.rb runs only on that instance and you receive just one report.