Scheduling Tasks With Cron for Java 8 (gcloud CLI-based)

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 a cron job to send out an email report on a daily basis, or to update some cached data every 10 minutes, or refresh summary information once an hour.

A cron job makes a scheduled HTTP GET request to the specified endpoint in the same app where the cron job is configured. The handler for that endpoint executes the logic when it is called.

The App Engine Cron Service cannot be used to call web endpoints outside the App Engine host app. It cannot be used to call App Engine endpoints from other apps besides the host app.

A cron job request is subject to the same limits as those for push task queues.

Before you begin

To deploy or update schedules, your account requires one of the following IAM roles:

  • Owner
  • Editor

You can set the permission on the IAM page in the Google Cloud console.

Creating a cron job

  1. Create the cron.yaml file in the root directory of your application (alongside app.yaml).

  2. Add one or more <cron> entries to your file and define the necessary elements for your job, including the required <url> and <schedule> elements. Review the cron.yaml syntax and options for more details about the elements of the cron.yaml file.

    The following example creates a basic cron job that runs daily:

    cron:
- description: "daily summary job"
  url: /tasks/summary
  target: beta
  schedule: every 24 hours

    The target specification is optional and is the name of a service/version. If present, the target is prepended to your app's hostname, causing the job to be routed to that service/version. If no target is specified, the job will run in the versions of the default service that are configured for traffic.

  3. Create a handler for the cron job URL. The handler should execute any tasks that you want scheduled. The handler should respond with an HTTP status code between 200 and 299 (inclusive) to indicate success. Other status codes can be returned and can be used to retry the cron job.

The handler can be as simple as a Servlet in the app. The Servlet URL mapping in web.xml should be the same as the cron job URL.

Testing cron jobs in the development server

The local development server doesn't automatically run your cron jobs. You can make requests directly to your cron job's URL to test your functionality. You can use your local cron or scheduled tasks interface to trigger the URLs of your jobs with curl or a similar tool.

Retrying cron jobs that fail

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 unless a 503 status code is returned, in which case it is retried every minute until it succeeds or returns a 200-299 status code.

To set failed jobs to be retried:

  1. Include a retry_parameters block in your cron.yaml file.

  2. Choose and set the retry parameters in the retry_parameters block.

    For example, this sample cron.yaml file contains a single cron job that is 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

Learn more about the cron retry options.

Deploying cron jobs

To deploy the cron jobs specified in your cron.yaml configuration file, run the following command:

gcloud

gcloud app deploy cron.yaml

Maven

mvn appengine:deployCron cron.yaml

Gradle

gradle appengineDeployCron cron.yaml

IDE

If you use IntelliJ or Eclipse, select the individual configuration files to be deployed using the deployment form.

Deleting all cron jobs

To delete all cron jobs:

  1. Edit the contents of the cron.yaml file to:

    cron:

  2. Deploy the cron.yaml file to App Engine.

Securing URLs for cron

A cron handler is just a normal handler defined in app.yaml. 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 restrict a URL by adding login: admin to the handler configuration in app.yaml.

An example might look like this in app.yaml:

application: hello-cron
version: 1
runtime: java
api_version: 1

handlers:
- url: /report/weekly
  servlet: mysite.server.CronServlet
  login: admin

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

The X-Appengine-Cron header is set internally by App Engine. If your request handler finds this header it can trust that the request is a cron request. If the header is present in an external user request to your app, it is stripped, except for requests from logged in administrators of the application, who are allowed to set the header for testing purposes.

App Engine issues Cron requests from the IP address 0.1.0.2. For Cron jobs created with older gcloud versions (earlier than 326.0.0), Cron requests will come from 0.1.0.1.

Calling Google Cloud Endpoints

You cannot specify a Google Cloud Endpoint in the url field of a cron job. If you want your cron job to call a Google Cloud Endpoint, issue a request to a target that is served by a handler in your app, and call the endpoint class and method from the handler code.

Viewing cron jobs in the Google Cloud console

You can view scheduled cron jobs in Cloud Scheduler's App Engine Cron Jobs tab.

You can also view logs to see when cron jobs were added or removed.

Learn more

See detailed information about defining cron jobs in the cron.yaml Reference.