09-cronjob

Running a background process: cronjob

Introduction

In this section you will learn how to run background tasks using crons & jobs.

CronJob

Let's start with crons. Crons are like the cron in linux a time-based job scheduler.

apiVersion: batch/v1beta1
kind: CronJob
metadata:
  name: simple-cronjob
spec:
  schedule: "*/1 * * * *"
  jobTemplate:
    spec:
      template:
        spec:
          restartPolicy: Never
          containers:
          - name: pi
            image: perl
            command: ["perl",  "-Mbignum=bpi", "-wle", "print bpi(100)"]

• spec:
  • schedule: when to start this cron, in the same format as the linux crons. */2 * * * * means every 2 minutes
  • jobTemplate: the template of the container(s) to start
    • command: the command to run, here compute the first 100 digits of π.

Let's apply it:

$ kubectl apply -f 09-cronjob/01-simple-cronjob.yml
cronjob.batch "simple-cronjob" created

Wait a bit and access the logs of the pod created by the cron.

Job

If you need to run a one time job, you can use the Job in Kubernetes. In fact the CronJob will start a Job for you at the scheduled interval.

apiVersion: batch/v1
kind: Job
metadata:
  name: simple-job
spec:
  template:
    spec:
      containers:
      - name: c
        image: busybox
        command: ["sh", "-c", "echo Processing item $ITEM && sleep 5"]
      restartPolicy: Never

This manifest is fairly close to a CronJob.

Apply it and see what is happening. Does it restart?

$ kubectl apply -f 09-cronjob/02-simple-job.yml
job.batch "simple-job" created

If you have a long running background process - like a consumer of a queue - you can use a deployment without a service.

Exercises

1. Transform the simple-job to a cron job running every 2 minutes
2. Transform the simple-cronjob in a deployment of 1 replica, and compute the 1_000 first digits of π. What is happening when the container finishes?

Clean up

kubectl delete deployment,rs,service,cronjob,pod --all

Links

• https://kubernetes.io/docs/concepts/workloads/controllers/cron-jobs/