Running Automated Tasks with a CronJob

Running Automated Tasks with a CronJob

You can use a CronJob to run Jobs on a time-based schedule. These automated jobs run like Cron tasks on a Linux or UNIX system.

Cron jobs are useful for creating periodic and recurring tasks, like running backups or sending emails. Cron jobs can also schedule individual tasks for a specific time, such as if you want to schedule a job for a low activity period.

Cron jobs have limitations and idiosyncrasies. For example, in certain circumstances, a single cron job can create multiple jobs. Therefore, jobs should be idempotent.

For more limitations, see CronJobs.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:
- Katacoda
- Play with Kubernetes

Creating a Cron Job

Cron jobs require a config file. This example cron job config .spec file prints the current time and a hello message every minute:

application/job/cronjob.yaml

apiVersion: batch/v1beta1
kind: CronJob
metadata:
  name: hello
spec:
  schedule: "*/1 * * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: hello
            image: busybox
            imagePullPolicy: IfNotPresent
            args:
            - /bin/sh
            - -c
            - date; echo Hello from the Kubernetes cluster
          restartPolicy: OnFailure

Run the example CronJob by using this command:

kubectl create -f https://k8s.io/examples/application/job/cronjob.yaml

The output is similar to this:

cronjob.batch/hello created

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

kubectl get cronjob hello

The output is similar to this:

NAME    SCHEDULE      SUSPEND   ACTIVE   LAST SCHEDULE   AGE
hello   */1 * * * *   False     0        <none>          10s

As you can see from the results of the command, the cron job has not scheduled or run any jobs yet. Watch for the job to be created in around one minute:

kubectl get jobs --watch

The output is similar to this:

NAME               COMPLETIONS   DURATION   AGE
hello-4111706356   0/1                      0s
hello-4111706356   0/1           0s         0s
hello-4111706356   1/1           5s         5s

Now you’ve seen one running job scheduled by the “hello” cron job. You can stop watching the job and view the cron job again to see that it scheduled the job:

kubectl get cronjob hello

The output is similar to this:

NAME    SCHEDULE      SUSPEND   ACTIVE   LAST SCHEDULE   AGE
hello   */1 * * * *   False     0        50s             75s

You should see that the cron job hello successfully scheduled a job at the time specified in LAST SCHEDULE. There are currently 0 active jobs, meaning that the job has completed or failed.

Now, find the pods that the last scheduled job created and view the standard output of one of the pods.

Note: The job name and pod name are 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})

Show pod log:

kubectl logs $pods

The output is similar to this:

Fri Feb 22 11:02:09 UTC 2019
Hello from the Kubernetes cluster

Deleting a Cron Job

When you don’t need a cron job any more, delete it with kubectl delete cronjob <cronjob name>:

kubectl delete cronjob hello

Deleting the cron job removes all the jobs and pods it created and stops it from creating additional jobs. You can read more about removing jobs in garbage collection.

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 config also needs a .spec section.

Note: All modifications to a cron job, especially its .spec, are applied only to the following runs.

Schedule

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

The format also includes extended vixie cron step values. As explained in the FreeBSD manual:

Step values can be used in conjunction with ranges. Following a range with /<number> specifies skips of the number’s value through the range. For example, 0-23/2 can be used in the hours field to specify command execution every other hour (the alternative in the V7 standard is 0,2,4,6,8,10,12,14,16,18,20,22). Steps are also permitted after an asterisk, so if you want to say “every two hours”, just use */2.

Note: A 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.

Job Template

The .spec.jobTemplate is the template for the job, and it is required. It has exactly the same schema as a Job, except that it is nested and does not have an apiVersion or kind. For information about writing a job .spec, see Writing a Job Spec.

Starting Deadline

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. After the deadline, the cron job does not start the job. Jobs that do not meet their deadline in this way count as failed jobs. If this field is not specified, the jobs have no deadline.

The CronJob controller counts how many missed schedules happen for a cron job. If there are more than 100 missed schedules, the cron job is no longer scheduled. When .spec.startingDeadlineSeconds is not set, the CronJob controller counts missed schedules from status.lastScheduleTime until now.

For example, one cron job is supposed to run every minute, the status.lastScheduleTime of the cronjob is 5:00am, but now it’s 7:00am. That means 120 schedules were missed, so the cron job is no longer scheduled.

If the .spec.startingDeadlineSeconds field is set (not null), the CronJob controller counts how many missed jobs occurred from the value of .spec.startingDeadlineSeconds until now.

For example, if it is set to 200, it counts how many missed schedules occurred in the last 200 seconds. In that case, if there were more than 100 missed schedules in the last 200 seconds, the cron job is no longer scheduled.

Concurrency Policy

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

Allow (default): The cron job allows concurrently running jobs
Forbid: The cron job does not allow concurrent runs; if it is time for a new job run and the previous job run hasn’t finished yet, the cron job skips the new job run
Replace: If it is time for a new job run and the previous job run hasn’t finished yet, the cron job replaces the currently running job run with a new job run

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 it is set to true, all subsequent executions are suspended. This setting does not apply to already started executions. Defaults to false.

Caution: Executions that are suspended during their scheduled time count as missed jobs. When .spec.suspend changes from true to false on an existing cron job without a starting deadline, the missed jobs are scheduled immediately.

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.