Our previous article, we learned how DaemonSets can effectively collect data from Kubernetes nodes, allowing data to be structured and sent to appropriate tooling.
In this post, we'll delve into the topic of running a single job in Kubernetes through the use of Kubernetes Jobs.
Furthermore, we'll learn how Kubernetes enables the scheduling of jobs to be executed regularly, through the use of Cronjobs.
Job
Kubernetes Job objects incorporate a Job controller that creates a Pod from the spec provided, enabling it to execute an arbitrary command.
That said, the YAML file looks like the following:
kind: Job
apiVersion: batch/v1
metadata:
name: sleeper
spec:
template:
spec:
restartPolicy: Never
containers:
- name: sleeper
image: debian
command: ["sleep", "15"]
We can check that the job sleeper
was created:
$ kubectl get jobs
NAME COMPLETIONS DURATION AGE
sleeper 0/1 5s 5s
And a Pod was started from the job created above:
$ kubectl get pods
NAME READY STATUS RESTARTS AGE
sleeper-8mmtg 1/1 Running 0 8s
After the Pod is finished successfully, it goes to the Completed
status:
$ kubectl get pods
NAME READY STATUS RESTARTS AGE
sleeper-8mmtg 0/1 Completed 0 35s
And the Job completions is updated to 1/1
:
$ kubectl get jobs
NAME COMPLETIONS DURATION AGE
sleeper 1/1 35s 35s
Below is a visual representation of how Kubernetes handles Jobs, supported by a Pod that executes the command provided:
However, not rare we have to run a job regularly.
In UNIX-like systems, a program called crontab enables to run regularly, given a syntax for scheduling, an arbitrary command. Kubernetes allows Jobs to be scheduled in the same manner, using the the crontab syntax.
Meet Kubernetes CronJobs.
CronJob
In short, a Cronjob will regularly start a new Job, from which a new Pod will be started.
Here's an illustration of how CronJobs work in Kubernetes:
Below is the YAML representation of it:
kind: CronJob
apiVersion: batch/v1
metadata:
name: sleeper
spec:
schedule: "*/1 * * * *"
jobTemplate:
spec:
template:
spec:
restartPolicy: Never
containers:
- name: sleeper
image: debian
command: ["sleep", "15"]
- in the
schedule
key we provide the crontab syntax - the
jobTemplate
is exactly the Job specification that will run a Pod
Let's confirm that Kubernetes created a Cronjob object:
$ kubectl get cronjobs
NAME SCHEDULE SUSPEND ACTIVE LAST SCHEDULE AGE
sleeper */1 * * * * False 0 52s 9m29s
From the CronJob, a Job was created:
$ kubectl get jobs
NAME COMPLETIONS DURATION AGE
sleeper-28046804 0/1 22s 35s
And a Pod was started:
$ kubectl get pods
NAME READY STATUS RESTARTS AGE
sleeper-28046805-hqkwb 1/1 Running 0 40s
After a couple of minutes, we see that the CronJob created 3 Jobs which, by the way, is the default number of Jobs that Kubernetes will keep in the cluster (we can change that number at our need successfulJobsHistoryLimit
):
$ kubectl get jobs
NAME COMPLETIONS DURATION AGE
sleeper-28046806 1/1 21s 3m10s
sleeper-28046807 1/1 21s 2m10s
sleeper-28046808 1/1 21s 70s
Therefore, each Job will start a new Pod:
$ kubectl get pods
NAME READY STATUS RESTARTS AGE
sleeper-28046808-szdw7 0/1 Completed 0 2m26s
sleeper-28046809-fhxk8 0/1 Completed 0 86s
sleeper-28046810-2gvds 0/1 Completed 0 26s
Wrapping Up
This post demonstrated how to run arbitrary commands using the Kubernetes Job object.
Furthermore, we have learned that Kubernetes CronJob objects allow for the scheduling of Jobs to be executed regularly.
So far, we have covered the primary Kubernetes workload objects, namely ReplicaSet, Deployment, StatefulSet, DaemonSet, Job and CronJob.
In the following posts, we'll explore how these objects connect with each other within the cluster while also gaining an understanding of Kubernetes networking.
This post was written with the assistance of ChatGPT, which helped with some "eye candy" on grammar.
Top comments (2)
This series is so good. I'been working for years with Kubernetes APIs wrapped in some in-house tooling before reading this and better understanding what I actually do in a daily basis. Thank you very much for writing this. I will recommend this reading to everyone I can.
I’m happy hearing that from you!