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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: ScheduledJob resource was introduced in Kubernetes version 1.4, but starting from version 1.5 its current name is CronJob.

A typical use case is:

Prerequisites

You need a working Kubernetes cluster at version >= 1.4 (for ScheduledJob), >= 1.5 (for CronJob), with batch/v2alpha1 API turned on by passing --runtime-config=batch/v2alpha1=true while bringing up 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/v2alpha1
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 --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. However, running jobs won’t be stopped, and no jobs or their pods will be deleted. To clean up those jobs and pods, you need to list all jobs created by the cron job, and delete them all:

$ kubectl get jobs
NAME               DESIRED   SUCCESSFUL   AGE
hello-1201907962   1         1            11m
hello-1202039034   1         1            8m
...

$ kubectl delete jobs hello-1201907962 hello-1202039034 ...
job "hello-1201907962" deleted
job "hello-1202039034" deleted
...

Once the jobs are deleted, the pods created by them are deleted as well. Note that all jobs created by cron job “hello” will be prefixed “hello-“. You can delete them at once with kubectl delete jobs --all, if you want to delete all jobs in the current namespace (not just the ones created by “hello”).

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, configuring containers, 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, there are no limits, and all successful and failed jobs are kept. However, jobs can pile up quickly when running a cron job, and setting these fields is recommended. Setting a limit to 0 corresponds to keeping none of the corresponding kind of jobs after they finish.

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