Scheduling Jobs with cron.yaml

The App Engine Cron Service allows you to configure regularly scheduled tasks that operate at defined times or regular intervals. These tasks are commonly known as cron jobs. These cron jobs are automatically triggered by the App Engine Cron Service. For instance, you might use this to send out a report email on a daily basis, to update some cached data every 10 minutes, or to update some summary information once an hour.

A cron job will invoke a URL, using an HTTP GET request, at a given time of day. An HTTP request invoked by cron can run for up to 60 minutes, but is subject to the same limits as other HTTP requests.

Free applications can have up to 20 scheduled tasks. Paid applications can have up to 250 scheduled tasks.

About cron.yaml

A cron.yaml file in the `WEB-INF` directory of your application (alongside app.yaml) configures scheduled tasks for your Java application. The following is an example cron.yaml file:

cron:
- description: "daily summary job"
  url: /tasks/summary
  schedule: every 24 hours
- description: "monday morning mailout"
  url: /mail/weekly
  schedule: every monday 09:00
  timezone: Australia/NSW
- description: "new daily summary job"
  url: /tasks/summary
  schedule: every 24 hours
  target: beta

The syntax of cron.yaml is the YAML format. For more information about this syntax, see the YAML website for more information.

A cron.yaml file consists of a number of job definitions. A job definition must have a url and a schedule. You can also optionally specify a description, timezone, target, and retry_parameters. The description is visible in the GCP Console and the development server's admin interface.

The url field specifies a URL in your application that will be invoked by the Cron Service. The format of the schedule field is covered in The Schedule Format.

The timezone should be the name of a standard zoneinfo time zone name. If you don't specify a timezone, the schedule will be in UTC (also known as GMT).

The target string is prepended to your app's hostname. It is usually the name of a service. The cron job will be routed to the default version of the named service. Note that if the default version of the service changes, the job will run in the new default version.

If there is no service with the name assigned to target, the name is assumed to be an app version, and App Engine will attempt to route the job to that version. For more information about routing, see How Requests are Routed.

If you use a dispatch file, your job might be re-routed. For example, given the following cron.yaml and dispatch.yaml files, the job will run in service2, even though its target is service1:

cron.yaml:

cron:
- description: "test dispatch vs target"
  url: /tasks/hello_service2
  schedule: every 1 mins
  target: service1

dispatch.yaml:

dispatch:
- url: '*/tasks/hello_service2'
  service: service2

The schedule format

Cron schedules are specified using a simple English-like format.

The following are examples of schedules:

every 12 hours
every 5 minutes from 10:00 to 14:00
every day 00:00
every monday 09:00
2nd,third mon,wed,thu of march 17:00
1st monday of sep,oct,nov 17:00
1 of jan,april,july,oct 00:00

If you don't need to run a recurring job at a specific time, but instead only need to run it at regular intervals, use the form:

every N (hours|mins|minutes) ["from" (time) "to" (time)]

The brackets are for illustration only, and quotes indicate a literal.

  • N specifies a number.
  • hours or minutes (you can also use mins) specifies the unit of time.
  • time specifies a time of day, as HH:MM in 24 hour time.

By default, an interval schedule starts the next interval after the last job has completed. If a from...to clause is specified, however, the jobs are scheduled at regular intervals independent of when the last job completed. For example:

every 2 hours from 10:00 to 14:00

This schedule runs the job three times per day at 10:00, 12:00, and 14:00, regardless of how long it takes to complete. You can use the literal "synchronized" as a synonym for from 00:00 to 23:59:

every 2 hours synchronized

If you want more specific timing, you can specify the schedule as:

("every"|ordinal) (days) ["of" (monthspec)] (time)

Where:

  • ordinal specifies a comma separated list of "1st", "first" and so forth (both forms are ok)
  • days specifies a comma separated list of days of the week (for example, "mon", "tuesday", with both short and long forms being accepted); "every day" is equivalent to "every mon,tue,wed,thu,fri,sat,sun"
  • monthspec specifies a comma separated list of month names (for example, "jan", "march", "sep"). If omitted, implies every month. You can also say "month" to mean every month, as in "1,8,15,22 of month 09:00".
  • time specifies the time of day, as HH:MM in 24 hour time.

Specifying retries

If a cron job's request handler returns a status code that is not in the range 200–299 (inclusive) App Engine considers the job to have failed. By default, failed jobs are not retried. You can cause failed jobs to be retried by including a retry_parameters block in your configuration file.

Here is a sample cron.yaml file that contains a single cron job configured to retry up to five times (the default) with a starting backoff of 2.5 seconds that doubles each time.

cron:
- description: "retry demo"
  url: /retry
  schedule: every 10 mins
  retry_parameters:
    min_backoff_seconds: 2.5
    max_doublings: 5

Cron retries syntax

The retry parameters are described in the table below.

Element Description
job_retry_limit The maximum number of retry attempts for a failed cron job not to exceed '5'. If specified with job_age_limit, App Engine retries the cron job until both limits are reached. When omitted from the parameters, the limit is set to '5' by default.
job_age_limit The time limit for retrying a failed cron job, measured from when the cron job was first run. The value is a number followed by a unit of time, where the unit is s for seconds, m for minutes, h for hours, or d for days. For example, the value 5d specifies a limit of five days after the cron job's first execution attempt. If specified with job_retry_limit, App Engine retries the cron job until both limits are reached.
min_backoff_seconds The minimum number of seconds to wait before retrying a cron job after it fails.
max_backoff_seconds The maximum number of seconds to wait before retrying a cron job after it fails.
max_doublings The maximum number of times that the interval between failed cron job retries will be doubled before the increase becomes constant. The constant is: 2**(max_doublings - 1) * min_backoff.

Validating cron requests

You might want to validate that requests to your cron URLs are coming from App Engine and not from another source. You can do so by validating an HTTP header and the source IP address for the request:

  • Requests from the Cron Service will also contain a HTTP header:

    X-Appengine-Cron: true
    

    The X-Appengine-Cron header is set internally by Google App Engine. If your request handler finds this header it can trust that the request is a cron request. The X- headers are stripped by App Engine when they originate from external sources so that you can trust this header.

  • Google App Engine issues cron requests from the IP address 10.0.0.1.

In Jetty or Tomcat, you might perform this validation in a filter.

Cron and app versions

If the target parameter has been set for a job, the request is sent to the specified version. Otherwise Cron requests are sent to the default version of the application.

Uploading cron jobs

To upload your cron jobs, you must specify the cron.yaml as a parameter to the following gcloud command:

gcloud app deploy cron.yaml

Deleting cron jobs

To delete all cron jobs, change the cron.yaml file to just contain:

cron:

Cron support in the Google Cloud Platform Console

The GCP Console Task queues page has a tab that shows the tasks that are running cron jobs.

You can also visit the Logs page see when cron jobs were added or removed.

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