CronJobs

CronJobs is generally available (GA) in Google Kubernetes Engine (GKE) version 1.21 and later. This document explains how to run CronJobs in GKE. CronJobs are a built-in feature of Kubernetes. For more details, see the Kubernetes documentation about CronJobs.

Overview

CronJobs create Kubernetes Jobs on a repeating schedule. CronJobs allow you to automate regular tasks like making backups, creating reports, sending emails, or cleanup tasks.

CronJobs are created, managed, scaled, and deleted in the same way as Jobs. The exact number of Job objects created depends on several factors. For more information, see CronJob limitations.

For more information about Jobs, see Running a Job.

Before you begin

Before you start, make sure you have performed the following tasks:

  • Enable the Google Kubernetes Engine API.
  • Enable Google Kubernetes Engine API
  • If you want to use the Google Cloud CLI for this task, install and then initialize the gcloud CLI.

Creating a CronJob

You can create a CronJob using a manifest file. For example, the following YAML manifest prints the current time and a string once every minute, while retaining the default values for CronJob parameters:

# cronjob.yaml
apiVersion: batch/v1
kind: CronJob
metadata:
  name: hello
spec:
  schedule: "*/1 * * * *"
  concurrencyPolicy: Allow
  startingDeadlineSeconds: 100
  suspend: false
  successfulJobsHistoryLimit: 3
  failedJobsHistoryLimit: 1
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: hello
            image: busybox
            args:
            - /bin/sh
            - -c
            - date; echo "Hello, World!"
          restartPolicy: OnFailure

To create this CronJob, save the YAML manifest to a file and apply it to the cluster:

kubectl apply -f PATH_TO_FILE

Replace PATH_TO_FILE with the path to the YAML manifest.

Configuring a CronJob

You can specify the following parameters when you create a CronJob:

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 five 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 starting on Sunday)

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 example, 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. For more details on what to include in the spec.jobTemplate, see the Kubernetes CronJob documentation.

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/v1
kind: CronJob
metadata:
  name: hello
spec:
  schedule: "*/1 * * * *"
  startingDeadlineSeconds: 100
  jobTemplate:
    spec:
    ...

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.

For example, the following manifest instructs GKE to save a maximum of five successful CronJob executions and a maximum of 10 failed CronJob executions:

apiVersion: batch/v1
kind: CronJob
metadata:
  name: hello
spec:
  schedule: "*/1 * * * *"
  startingDeadlineSeconds: 100
  successfulJobsHistoryLimit: 5
  failedJobsHistoryLimit: 10
  jobTemplate:
    spec:
    ...

You can disable retention of successful or failed CronJob execution history by setting the respective value to 0. Disabling history retention might make debugging failures more difficult. For example, the following manifest instructs GKE to only save failed CronJob executions:

kind: CronJob
metadata:
  name: hello
spec:
  schedule: "*/1 * * * *"
  startingDeadlineSeconds: 100
  successfulJobsHistoryLimit: 0
  failedJobsHistoryLimit: 10
  jobTemplate:
    spec:
    ...

Inspecting a CronJob

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

kubectl describe cronjob CRONJOB_NAME

Replace CRONJOB_NAME with the name of the CronJob to inspect.

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 most recent failed Job. You can change or disable these defaults by changing the CronJob history limits.

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.

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 view the logs for a specific CronJob, run the following command:

kubectl logs POD_NAME

Replace POD_NAME with the name of the Pod you want to inspect.

The output is similar to the following:

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, run the following command:

kubectl delete cronjob CRONJOB_NAME

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

What's next