A newbie’s information to Kubernetes Jobs and CronJobs

mcalizo

4 years ago

Kubernetes is the default orchestration engine for containers. Its choices for controlling and managing pods and containers embrace:

Deployments
StatefulSets
ReplicaSets

Each of those options has its personal objective, with the frequent operate to make sure that pods run constantly. In failure eventualities, these controllers both restart or reschedule pods to make sure the companies within the pods proceed operating.

As the Kubernetes documentation explains, a Kubernetes Job creates a number of pods and ensures specified variety of the pods terminates when the duty (Job) completes.

Just like in a typical working system, the power to carry out automated, scheduled jobs with out consumer interplay is vital within the Kubernetes world. But Kubernetes Jobs do extra than simply run automated jobs, and there are a number of methods to make the most of them via:

Jobs
CronJobs
Work queues (that is past the scope of this text)

Sounds easy proper? Well, possibly. Anyone who works on containers and microservice purposes is aware of that some require companies to be transient in order that they’ll do particular duties for purposes or inside the Kubernetes clusters.

In this text, I’ll go into why Kubernetes Jobs are vital, methods to create Jobs and CronJobs, and when to make use of them for purposes operating on the Kubernetes cluster.

Differences between Kubernetes Jobs and CronJobs

Kubernetes Jobs are used to create transient pods that carry out particular duties they’re assigned to. CronJobs do the identical factor, however they run duties primarily based on an outlined schedule.

Jobs play an vital position in Kubernetes, particularly for operating batch processes or vital ad-hoc operations. Jobs differ from different Kubernetes controllers in that they run duties till completion, fairly than managing the specified state comparable to in Deployments, ReplicaSets, and StatefulSets.

How to create Kubernetes Jobs and CronJobs

With that background in hand, you can begin creating Jobs and CronJobs.

Prerequisites

To do that train, you have to have the next:

A working Kubernetes cluster; you possibly can set up it with both:
The kubectl Kubernetes command line

Here is the Minikube deployment I used for this demonstration:

$ minikube model
minikube model: v1.eight.1$ kubectl cluster-info
Kubernetes grasp is operating at https://172.17.0.59:8443
KubeDNS is operating at https://172.17.0.59:8443/api/v1/namespaces/kube-system/companies/kube-dns:dns/proxy
$ kubectl get nodes
NAME       STATUS   ROLES    AGE   VERSION
minikube   Ready    grasp   88s   v1.17.three

Kubernetes Jobs

Just like anything within the Kubernetes world, you possibly can create Kubernetes Jobs with a definition file. Create a file referred to as sample-jobs.yaml utilizing your favourite editor.

Here is a snippet of the file that you need to use to create an instance Kubernetes Job:

apiVersion: batch/v1          ## The model of the Kubernetes API
variety: Job                     ## The sort of object for jobs
metadata:
 identify: job-test
spec:                         ## What state you need for the item
 template:
   metadata:
     identify: job-test
   spec:
     containers:
     - identify: job
       picture: busybox                  ##  Image used
       command: ["echo", "job-test"]   ##  Command used to create logs for verification later
     restartPolicy: OnFailure          ##  Restart Policy in case container failed

Next, apply the Jobs within the cluster:

$ kubectl apply -f sample-jobs.yaml

Wait a couple of minutes for the pods to be created. You can view the pod creation’s standing:

$ kubectl get pod –watch

After a number of seconds, you must see your pod created efficiently:

$ kubectl get pods
  NAME                  READY   STATUS          RESTARTS         AGE
  job-test                      zero/1     Completed       zero            11s

Once the pods are created, confirm the Job’s logs:

$ kubectl logs job-test job-test

You have created your first Kubernetes Job, and you’ll discover particulars about it:

$ kubectl describe job job-test

Clean up the Jobs:

$ kubectl delete jobs job-test

Kubernetes CronJobs

You can use CronJobs for cluster duties that should be executed on a predefined schedule. As the documentation explains, they’re helpful for periodic and recurring duties, like operating backups, sending emails, or scheduling particular person duties for a selected time, comparable to when your cluster is more likely to be idle.

As with Jobs, you possibly can create CronJobs by way of a definition file. Following is a snippet of the CronJob file cron-test.yaml. Use this file to create an instance CronJob:

apiVersion: batch/v1beta1            ## The model of the Kubernetes API
variety: CronJob                        ## The sort of object for Cron jobs
metadata:
  identify: cron-test
spec:
  schedule: "*/1 * * * *"            ## Defined schedule utilizing the *nix fashion cron syntax
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - identify: cron-test
            picture: busybox            ## Image used
            args:
            - /bin/sh
            - -c
            - date; echo Hello that is Cron check
          restartPolicy: OnFailure    ##  Restart Policy in case container failed

Apply the CronJob to your cluster:

$ kubectl apply -f cron-test.yaml
 cronjob.batch/cron-test created

Verify that the CronJob was created with the schedule within the definition file:

$ kubectl get cronjob cron-test
 NAME        SCHEDULE      SUSPEND   ACTIVE   LAST SCHEDULE   AGE
 cron-test   */1 * * * *   False     zero        <none>          10s

After a number of seconds, you’ll find the pods that the final scheduled job created and look at the usual output of one of many pods:

$ kubectl logs cron-test-1604870760
  Sun Nov  eight 21:26:09 UTC 2020
  Hello from the Kubernetes cluster

You have created a Kubernetes CronJob that creates an object as soon as per execution primarily based on the schedule schedule: "*/1 * * * *". Sometimes the creation will be missed due to environmental points within the cluster. Therefore, they should be idempotent.

Other issues to know

Unlike deployments and companies in Kubernetes, you possibly can’t change the identical Job configuration file and reapply it directly. When you make modifications within the Job configuration file, you could delete the earlier Job from the cluster earlier than you apply it.

Generally, making a Job creates a single pod and performs the given process, as within the instance above. But through the use of completions and parallelism, you possibly can provoke a number of pods, one after the opposite.

Use your Jobs

You can use Kubernetes Jobs and CronJobs to handle your containerized purposes. Jobs are vital in Kubernetes software deployment patterns the place you want a communication mechanism together with interactions between pods and the platforms. This might embrace circumstances the place an software wants a “controller” or a “watcher” to finish duties or must be scheduled to run periodically.