CronJobs

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This page explains how to run CronJobs in Google Kubernetes Engine. CronJobs are a native feature of Kubernetes; for more details, see the Kubernetes documentation about CronJobs.

Overview

You can use CronJobs to run tasks at a specific time or interval. CronJobs are a good choice for automatic tasks, such as backups, reporting, sending emails, or cleanup tasks.

CronJobs use Job objects to complete their tasks. A CronJob creates a Job object each time it runs. CronJobs are created, managed, scaled, and deleted in the same way as Jobs. For more information about Jobs, see Running a Job.

Before you begin

To prepare for this task, perform the following steps:

• Ensure that you have enabled the Google Kubernetes Engine API.
Enable Google Kubernetes Engine API
• Ensure that you have installed the Cloud SDK.
• Set your default project ID:

  gcloud config set project [PROJECT_ID]

• If you are working with zonal clusters, set your default compute zone:

  gcloud config set compute/zone [COMPUTE_ZONE]

• If you are working with regional clusters, set your default compute region:

  gcloud config set compute/region [COMPUTE_REGION]

• Update gcloud to the latest version:

  gcloud components update

Note: You can override these default settings in gcloud commands using the --project, --zone, and --region operational flags.

Creating a CronJob

This CronJob prints the current time and a string once per minute:

# cronjob.yaml
apiVersion: batch/v1beta1
kind: CronJob
metadata:
  name: hello
spec:
  schedule: "*/1 * * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: hello
            image: busybox
            args:
            - /bin/sh
            - -c
            - date; echo "Hello, World!"
          restartPolicy: OnFailure

To create this CronJob, you can save the YAML manifest to a file and apply it to the cluster using kubectl apply -f [FILENAME].

The next sections provide more details about specifying when the CronJob runs and what actually runs.

Specifying when the CronJob runs

The spec.schedule field defines when, and how often, the CronJob runs, using Unix standard crontab format. All CronJob times are in UTC. There are 5 fields, separated by spaces. These fields represent the following:

1. Minutes (between 0 and 59)
2. Hours (between 0 and 23)
3. Day of the month (between 1 and 31)
4. Month (between 1 and 12)
5. Day of the week (between 0 and 6)

You can use the following special characters in any of the spec.schedule fields:

• ? is a wildcard value that matches a single character.
• * is a wildcard value that matches zero or more characters.
• /, allows you to specify an interval for a field. For instance, if the first field (the minutes field) has a value of */5, it means "every 5 minutes". If the fifth field (the day-of-week field) is set to 0/5, it means "every fifth Sunday".

Specifying what the CronJob runs

The spec.jobTemplate describes what the CronJob does, including its container images, the commands the containers execute, and the restart policy for the CronJob.

Specifying a deadline

The optional startingDeadlineSeconds field indicates the maximum number of seconds the CronJob can take to start if it misses its scheduled time for any reason. Missed CronJobs are considered failures.

To specify a deadline, add the startingDeadlineSeconds value to the CronJob's spec field in the manifest file. For example, the following manifest specifies that the CronJob has 100 seconds to begin:

apiVersion: batch/v1beta1
kind: CronJob
metadata:
  name: hello
spec:
  schedule: "*/1 * * * *"
  startingDeadlineSeconds: 100
  jobTemplate:
    spec:
    ...

If you do not specify a startingDeadlineSeconds value, the CronJob never times out. This could lead to the same CronJob running multiple times simultaneously. To prevent this type of problem, see Specifying a concurrency policy.

Specifying a concurrency policy

The optional spec.concurrencyPolicy field specifies how to treat concurrent executions of a Job created by the CronJob controller. If you do not set a value, multiple concurrent Jobs are allowed by default

concurrencyPolicy accepts the following values:

Value	Meaning
Allow	Concurrent Jobs are allowed. This is the default.
Forbid	Concurrent jobs are forbidden, and new Jobs can't start until previous ones have completed or timed out.
Replace	Concurrent jobs are forbidden, and old jobs are cancelled in favor of new ones.

Suspending subsequent executions

The optional spec.suspend field, when set to true, prevents new Jobs from being run, but allows current executions to finish.

Specifying history limits

A CronJob creates a Pod each time it runs. Viewing the termination status of a CronJob's recent executions, as well as the logs of an individual Pod, are covered in Viewing CronJob history.

You can configure the number of successful and failed CronJob executions that are saved, by specifying values for spec.successfulJobsHistoryLimit and spec.failedJobsHistoryLimit. By default, successfulJobsHistoryLimit is set to 3 and failedJobsHistoryLimit is set to 1.

To disable retention of data about successful or failed jobs, set the respective value to 0. However, debugging failures may be more difficult.

Inspecting a CronJob

To check a CronJob's configuration, use kubectl describe:

kubectl describe cronjob [CRON_JOB]

Viewing CronJob history

A CronJob runs within a Pod. By default, Kubernetes preserves the logs for terminated Pods representing the last three successful runs of a CronJob and the single last failed job. You can change or disable these defaults.

To view a CronJob's history, first list all Pods. Completed CronJobs are shown with a status of Completed, and failed jobs have a status of RunContainerError, CrashLoopBackOff, or another status indicating a failure.

Note: You may see more entries than you expect. CronJob history is subject to periodic garbage collection. For example, in the output below, four instances of the hello CronJob are shown.

kubectl get pods

NAME                                READY   STATUS              RESTARTS   AGE
hello-1556555640-9bc5r              0/1     Completed           0          3m6s
hello-1556555700-cm6wk              0/1     Completed           0          2m6s
hello-1556555760-62wf5              0/1     Completed           0          66s
hello-1556555820-rl8kl              0/1     Completed           0          5s
hello-failed-1556555820-wrvt2       0/1     RunContainerError   1          5s

To cause the above job to fail, modify the example in Creating a CronJob and modify the spec.containers.[NAME] to use a path or command that does not exist, and change the metadata.name value to one that indicates that you are testing for failures.

To view the logs for a specific CronJob, use kubectl get logs with the name of the Pod:

kubectl get logs hello-failed-1556555820-wrvt2

container_linux.go:247: starting container process caused
"exec: \"/in/sh\": stat /in/sh: no such file or directory"

Deleting a CronJob

To delete a CronJob, use kubectl delete:

kubectl delete cronjob [CRON_JOB]

When you delete a CronJob, the Kubernetes garbage collector deletes the associated Jobs and prevents new Jobs from starting.

What's next

• Read the Kubernetes documentation for CronJobs.
• Learn how to run a Job.