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Jobs Part 1
Jobs Part 2

ReplicaSets and Deployments are Kubernetes resources to describe long-running processes (LRPs). For web applications, for example, application instances, once started, are often running continuously and keeping them alive is a major goal.

However, there are also - compared to LRPs - short-lived workloads. These include one-off tasks as well as tasks requiring recurring execution both of varying complexity.

One-Off Tasks will be covered in this lesson while recurring tasks will be covered in the CRON Job Lesson.

When to use a Job?

Whenever it's not a long-running process but still needs to run code from a container image, it may be a Job.

A few examples include running tasks such as

  • database schema migrations
  • administrative one-off tasks in general
  • Analytic workflows
  • in general batch jobs which are run (partially) in sequence and/or (partially) in parallel.

Note that these tasks may be "long" running in dimension of hours or even days but compared to the average web application's lifecycle this is still comparatively short.

What is a Job?

A Job is a utility to execute short-lived workloads by starting Pods up until successful completion.

The following examples will help you understand the concept of Jobs in greater details.

Creating Jobs

Let's start creating Job by creating the simplest Job: a single container one-off Task.

Single Container One-Off Task

In this example a simple task is to be executed. The task is to run a simple shell command and then exit. In this case a single container will suffice. Neither sequential nor parallel processing of multiple containers or Pods is necessary.

Running a simple Job appears familiar to you as you have already done it in an earlier lesson:

kubectl run -i --tty busybox --image=busybox --restart=Never -- sh

If all you care about is the outcome of a shell command there is no need to attach an interactive terminal (-i --tty):

kubectl run simple-one-off-task --image=busybox --restart=Never -- echo "I represent a very important maintenance task."

Retrieve the corresponding job:

kubectl get jobs

As you can see there is no job because kubectl run doesn't create one. It's just a regular Pod. If that gets the (metaphorical) job done kubectl run is a way to go.

Now proceed with creating a real Job in file 20-simple-job.yaml:

apiVersion: batch/v1
kind: Job
name: simple-one-off-job-from-yaml
- name: simple-one-off-job-container
image: busybox
imagePullPolicy: Always
command: ['echo']
- 'I represent a very important maintenance task'
restartPolicy: OnFailure

Apply it:

kubectl apply -f 20-simple-job.yaml

And look again at:

kubectl get jobs

There should be your job.

Let's have a look at its description:

kubectl describe job simple-one-off-job-from-yaml

You should see a list of events containing a single event from job-controller.

As well as a field called Labels. It likely contains a key value pair such as job-name=simple-one-off-job-from-yaml.

Let's use this Label obtained from our Job metadata to query the corresponding Pod:

kubectl get pods -l job-name=simple-one-off-job-from-yaml

You should find a completed Pod with a name similar to simple-one-off-job-from-yaml-bbwkw. The suffix bbwkw is a random string attached by the JobController. Looking up the Pod of a Job in an environment with many Pods may become tricky so searching by label comes in handy.

Now we can also have a look at the Pods log:

kubectl logs simple-one-off-job-from-yaml-bbwkw

Which will tell us: "I represent a very important maintenance task".

So you can see that both the Job and corresponding Pod(s) exist past their execution. This is good for debugging but bad if many Jobs are executed and nobody does the garbage collection.

Deleting a Job

Deleting a Job works as expected:

kubectl delete job simple-one-off-job-from-yaml

Now look for the corresponding Pods:

kubectl get pods -l job-name=simple-one-off-job-from-yaml

They are gone too. So keep in mind: deleting a Job also deletes corresponding Pods.

  1. Kubernetes Documentation, Tasks, Jobs,
  2. Kubernetes Documentation, Concepts, Jobs - Run to Completion,