There are certain things that we want to do regularly over fixed intervals, such as processing some items, downloading new data from some source, or something on those lines. The typical way to do that in Unix/Linux systems is called cron job. In a cron job, you indicate a program or a script and a schedule on which it is supposed to run. Just like a timer or an alarm clock, this will be regularly executed for you at the specified intervals.

This kind of feature is also available in Google App Engine and is called Scheduled tasks. However, the processing model is slightly different in the Unix/Linux cron jobs, including the format of how we define a schedule. These definitions are specified in a file called cron.yaml in the root directory of your application, and it gets automatically uploaded with each application deployment.

In cron.yaml, you can define multiple cron entries. Each entry must define at least the following things: