Concepts

Detailed explanations of Kubernetes system concepts and abstractions.

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Cron Jobs

What is a cron job?

A Cron Job manages time based Jobs, namely:

One CronJob object is like one line of a crontab (cron table) file. It runs a job periodically on a given schedule, written in Cron format.

Note: The question mark (?) in the schedule has the same meaning as an asterisk *, that is, it stands for any of available value for a given field.

Note: CronJob resource in batch/v2alpha1 API group has been deprecated starting from cluster version 1.8. You should switch to using batch/v1beta1, instead, which is enabled by default in the API server. Further in this document, we will be using batch/v1beta1 in all the examples.

A typical use case is:

Prerequisites

You need a working Kubernetes cluster at version >= 1.8 (for CronJob). For previous versions of cluster (< 1.8) you need to explicitly enable batch/v2alpha1 API by passing --runtime-config=batch/v2alpha1=true to the API server (see Turn on or off an API version for your cluster for more).

Creating a Cron Job

Here is an example Cron Job. Every minute, it runs a simple job to print current time and then say hello.

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 from the Kubernetes cluster
          restartPolicy: OnFailure

Run the example cron job by downloading the example file and then running this command:

$ kubectl create -f ./cronjob.yaml
cronjob "hello" created

Alternatively, use kubectl run to create a cron job without writing full config:

$ kubectl run hello --schedule="*/1 * * * *" --restart=OnFailure --image=busybox -- /bin/sh -c "date; echo Hello from the Kubernetes cluster"
cronjob "hello" created

After creating the cron job, get its status using this command:

$ kubectl get cronjob hello
NAME      SCHEDULE      SUSPEND   ACTIVE    LAST-SCHEDULE
hello     */1 * * * *   False     0         <none>

As you can see above, there’s no active job yet, and no job has been scheduled, either.

Watch for the job to be created in around one minute:

$ kubectl get jobs --watch
NAME               DESIRED   SUCCESSFUL   AGE
hello-4111706356   1         1         2s

Now you’ve seen one running job scheduled by “hello”. We can stop watching it and get the cron job again:

$ kubectl get cronjob hello
NAME      SCHEDULE      SUSPEND   ACTIVE    LAST-SCHEDULE
hello     */1 * * * *   False     0         Mon, 29 Aug 2016 14:34:00 -0700

You should see that “hello” successfully scheduled a job at the time specified in LAST-SCHEDULE. There are currently 0 active jobs, meaning that the job that’s scheduled is completed or failed.

Now, find the pods created by the job last scheduled and view the standard output of one of the pods. Note that your job name and pod name would be different.

# Replace "hello-4111706356" with the job name in your system
$ pods=$(kubectl get pods -a --selector=job-name=hello-4111706356 --output=jsonpath={.items..metadata.name})

$ echo $pods
hello-4111706356-o9qcm

$ kubectl logs $pods
Mon Aug 29 21:34:09 UTC 2016
Hello from the Kubernetes cluster

Deleting a Cron Job

Once you don’t need a cron job anymore, simply delete it with kubectl:

$ kubectl delete cronjob hello
cronjob "hello" deleted

This stops new jobs from being created and removes all the jobs and pods created by this cronjob. You can read more about it in garbage collection section.

Cron Job Limitations

A cron job creates a job object about once per execution time of its schedule. We say “about” because there are certain circumstances where two jobs might be created, or no job might be created. We attempt to make these rare, but do not completely prevent them. Therefore, jobs should be idempotent.

The job is responsible for retrying pods, parallelism among pods it creates, and determining the success or failure of the set of pods. A cron job does not examine pods at all.

Writing a Cron Job Spec

As with all other Kubernetes configs, a cron job needs apiVersion, kind, and metadata fields. For general information about working with config files, see deploying applications, and using kubectl to manage resources documents.

A cron job also needs a .spec section.

Note: All modifications to a cron job, especially its .spec, will be applied only to the next run.

Schedule

The .spec.schedule is a required field of the .spec. It takes a Cron format string, e.g. 0 * * * * or @hourly, as schedule time of its jobs to be created and executed.

Job Template

The .spec.jobTemplate is another required field of the .spec. It is a job template. It has exactly the same schema as a Job, except it is nested and does not have an apiVersion or kind, see Writing a Job Spec.

Starting Deadline Seconds

The .spec.startingDeadlineSeconds field is optional. It stands for the deadline (in seconds) for starting the job if it misses its scheduled time for any reason. Missed jobs executions will be counted as failed ones. If not specified, there’s no deadline.

Concurrency Policy

The .spec.concurrencyPolicy field is also optional. It specifies how to treat concurrent executions of a job created by this cron job. Only one of the following concurrent policies may be specified:

Note that concurrency policy only applies to the jobs created by the same cron job. If there are multiple cron jobs, their respective jobs are always allowed to run concurrently.

Suspend

The .spec.suspend field is also optional. If set to true, all subsequent executions will be suspended. It does not apply to already started executions. Defaults to false.

Jobs History Limits

The .spec.successfulJobsHistoryLimit and .spec.failedJobsHistoryLimit fields are optional. These fields specify how many completed and failed jobs should be kept. By default, they are set to 3 and 1 respectively. Setting a limit to 0 corresponds to keeping none of the corresponding kind of jobs after they finish.

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