Cron Jobs | Developer Guide

Overview
Creating a Cron Job
Cleaning Up After a Cron Job

Overview

A cron job builds on a regular job by allowing you to specifically schedule how the job should be run. Cron jobs are part of the Kubernetes API, which can be managed with oc commands like other object types.

A cron job creates a job object approximately once per execution time of its schedule, but there are circumstances in which it fails to create a job or two jobs might be created. Therefore, jobs should be idempotent and you should monitor and clean cron jobs frequently.

Creating a Cron Job

A cron job configuration consists of the following key parts:

A schedule specified in cron format.
A job template used when creating the next job.
An optional 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 is no deadline.
ConcurrencyPolicy: An optional concurrency policy, specifying how to treat concurrent jobs within a cron job. Only one of the following concurrent policies may be specified. If not specified, this defaults to allowing concurrent executions.
- Allow allows Cron Jobs to run concurrently.
- Forbid forbids concurrent runs, skipping the next run if the previous has not finished yet.
- Replace cancels the currently running job and replaces it with a new one.
An optional flag allowing the suspension of a cron job. If set to true, all subsequent executions will be suspended.

The following is an example of a CronJob resource:

apiVersion: batch/v2alpha1
kind: CronJob
metadata:
  name: pi
spec:
  schedule: "*/1 * * * *"  (1)
  jobTemplate:             (2)
    spec:
      template:
        metadata:
          labels:          (3)
            parent: "cronjobpi"
        spec:
          containers:
          - name: pi
            image: perl
            command: ["perl",  "-Mbignum=bpi", "-wle", "print bpi(2000)"]
          restartPolicy: OnFailure (4)

Schedule for the job. In this example, the job will run every minute.
Job template. This is similar to the job example.
Sets a label for jobs spawned by this cron job.
The restart policy of the pod. This does not apply to the job controller. See Known Issues and Limitations for details.

All cron job schedule times are based on the timezone of the master where the job is initiated.

You can also create and launch a cron job from a single command using oc run. The following command creates and launches the same cron job as specified in the previous example:

$ oc run pi --image=perl --schedule='*/1 * * * *' \
    --restart=OnFailure --labels parent="cronjobpi" \
    --command -- perl -Mbignum=bpi -wle 'print bpi(2000)'

With oc run, the --schedule option accepts schedules in cron format.

When creating a cron job, oc run only supports the Never or OnFailure restart policies (--restart).

Delete cron jobs that you no longer need:

$ oc delete cronjob/<cron_job_name>

Doing this prevents them from generating unnecessary artifacts.

Cleaning Up After a Cron Job

Cron jobs can leave behind artifact resources such as jobs or pods. Configuring history limits is important, so that old jobs and their pods are properly cleaned. There are two optional fields within cron job’s spec responsible for that:

apiVersion: batch/v2alpha1
kind: CronJob
metadata:
  name: pi
spec:
  successfulJobsHistoryLimit: 3 (1)
  failedJobsHistoryLimit: 1     (2)
  schedule: "*/1 * * * *"
  jobTemplate:
    spec:
  ...

1	The number of successful finished jobs to retain.
2	The number of failed finished jobs to retain.

At any point in time, you can manually verify all artifacts left over from a job execution by using the job name as their prefix. For example, given the cron job example:

$ oc get jobs
NAME            DESIRED   SUCCESSFUL   AGE
pi-1497848100   1         1            1m
pi-1497848160   1         1            49s

$ oc get pods
NAME                  READY     STATUS      RESTARTS   AGE
pi-1497848100-lxs4k   0/1       Completed   0          2m
pi-1497848160-6r0c8   0/1       Completed   0          59s

Delete each artifact if you no longer need them. To delete all jobs spawned by a cron job, specify the label set during cron job creation:

$ oc delete jobs -l <label>

For example, to delete only the jobs generated by the cron job example:

$ oc delete jobs -l parent=cronjobpi
job "pi-1497848100" deleted
job "pi-1497848160" deleted