Creating a new cron job is as easy as doing the following:

1. Creating a job definition under the <ModuleName>/etc/crontab.xml file
2. Creating a class with a public method that handles the job execution

Let's create a simple cron job. We will start off by creating the <MAGELICIOUS_DIR>/Core/etc/crontab.xml file with the following content:

<group id="default">
    <job name="the_job" instance="MageliciousCoreCronTheJob" method="execute">
        <schedule>*/15 * * * *</schedule>
    </job>
</group>

The instance and method values map to the class and method within that class, which will be executed when cron job is run. The schedule is a cron, like the expression for when the job is to be executed. Unless there are specific requirements telling us otherwise, we can safely use the default group.

We then create the <MAGELICIOUS_DIR>/Core/Cron/TheJob.php file with the following content:

class TheJob {
    public function execute() {
        // ...
    }
}

The Magento console command supports several console commands:

cron
  cron:install Generates and installs crontab for current user
  cron:remove Removes tasks from crontab
  cron:run Runs jobs by schedule

To get our cron job running, we need to make sure that crontab is installed, by running php bin/magento cron:install. This command generates and installs crontab for the current user. We can confirm that by following up with the crontab -e command, like so:

#~ MAGENTO START 6f7c468a10aea2972eab1da53c8d2fce
* * * * * /bin/php /magelicious/bin/magento cron:run 2>&1 | grep -v "Ran jobs by schedule" >> /magelicious/var/log/magento.cron.log
* * * * * /bin/php /magelicious/update/cron.php >> /magelicious/var/log/update.cron.log
* * * * * /bin/php /magelicious/bin/magento setup:cron:run >> /magelicious/var/log/setup.cron.log
#~ MAGENTO END 6f7c468a10aea2972eab1da53c8d2fce

Now, if we execute php bin/magento cron:run, the_job should find its way under the cron_schedule table.

Magento Open Source provides two cron groups: default and index. While the majority of times our cron jobs will be placed under the default group, there might be a need to create a completely new cron group. Luckily, this is quite easy.

To create a new cron group, all we need is a <MAGELICIOUS_DIR>/etc/cron_groups.xml file with the following content:

<config>
    <group id="magelicious">
        <schedule_generate_every>15</schedule_generate_every>
        <schedule_ahead_for>20</schedule_ahead_for>
        <schedule_lifetime>15</schedule_lifetime>
        <history_cleanup_every>10</history_cleanup_every>
        <history_success_lifetime>10080</history_success_lifetime>
        <history_failure_lifetime>10080</history_failure_lifetime>
        <use_separate_process>0</use_separate_process>
    </group>
</config>

While group information is not stored in the cron_schedule table, we can use it via the Magento CLI to run jobs that are specific to a certain group:

php bin/magento cron:run --group=default