# Scheduled Tasks With Cron for Java - Google App Engine - Google Code

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# Scheduled Tasks With Cron for Java

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 10 minutes, but is subject to the same limits as other HTTP requests.

An application can have up to 20 scheduled tasks.

- About cron.xml
- The Schedule Format
- Securing URLs for Cron
- Cron and App Versions
- Uploading Cron Jobs
- Cron Support in the Admin Console
- Cron Support in the Development Server

## About cron.xml

A `cron.xml` file in the `WEB-INF` directory of your application (alongside `appengine-web.xml`) controls cron for your Java application. The following is an example `cron.xml` file:

    <?xml version="1.0" encoding="UTF-8"?>
    <cronentries>
      <cron>
        <url>/recache</url>
        <description>Repopulate the cache every 2 minutes</description>
        <schedule>every 2 minutes</schedule>
      </cron>
      <cron>
        <url>/weeklyreport</url>
        <description>Mail out a weekly report</description>
        <schedule>every monday 08:30</schedule>
        <timezone>America/New_York</timezone>
      </cron>
      <cron>
        <url>/weeklyreport</url>
        <description>Mail out a weekly report</description>
        <schedule>every monday 08:30</schedule>
        <timezone>America/New_York</timezone>
        <target>version-2</target>
      </cron>
    </cronentries>

For an XSD describing the format, check the file `docs/cron.xsd` in the SDK.

A `cron.xml` 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>`, and a `<target>`. The description is visible in the admin console.

The `url` url field is just a URL in your application. The format of the `schedule` field is covered more in The Schedule Format.

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

The `target` specifies the version of your application on which to invoke the URL. See About appengine-web.xml for more information about application versions.

## 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
    2nd,third mon,wed,thu of march 17:00
    every monday 09:00
    1st monday of sep,oct,nov 17:00
    every day 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.

## Securing URLs for Cron

You can prevent users from accessing URLs used by scheduled tasks by restricting access to administrator accounts. Scheduled tasks can access admin-only URLs. You can read about restricting URLs at Security and Authentication. An example you would use in `web.xml` to restrict everything starting with `/cron/` to admin-only is:

        <security-constraint>
            <web-resource-collection>
                <url-pattern>/cron/*</url-pattern>
            </web-resource-collection>
            <auth-constraint>
                <role-name>admin</role-name>
            </auth-constraint>
        </security-constraint>

Note: While cron jobs can use URL paths restricted with `<role-name>admin</role-name>`, they cannot use URL paths restricted with `<role-name>*</role-name>`.

For more on the format of `web.xml`, see the documentation on the deployment descriptor.

To test a cron job, sign in as an administrator and visit the URL of the handler in your browser.

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

    X-AppEngine-Cron: true

If you wish to ensure that only cron requests can trigger your handler, you should check for that header.

## 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

You can use `AppCfg` to upload cron jobs. When you upload your application to App Engine using `AppCfg update`, the Cron Service is updated with the contents of `cron.xml`. You can update just the cron configuration without uploading the rest of the application using `AppCfg update_cron`.

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

    <?xml version="1.0" encoding="UTF-8"?>
    <cronentries/>

## Cron Support in the Admin Console

The Admin Console allows you to view the state of your cron jobs. Select the "Cron Jobs" link from the side menu to view the state of the jobs, including the last time the job was run and the result of the job.

You can also see when cron jobs are added or removed by selecting the "Admin Logs" page from the Admin Console menu.

## Cron Support in the Development Server

The dev appserver doesn't automatically run your cron jobs. You can use your local desktop's cron or scheduled tasks interface to hit the URLs of your jobs with curl or a similar tool.