scheduled jobs with kubernetes cronjobs¶

scheduled jobs with kubernetes cronjobs

what are scheduled jobs?¶

Scheduled Jobs are automated pieces of work that can be performed at a specific time or on a recurring schedule.

Example use cases:

Sending performance metrics on weekly/monthly basis
Analyse and send user behaviour metrics to admins on a daily basis

what are cronjobs?¶

A cron job is a Linux command used for scheduling tasks to be executed sometime in the future.
This is normally used to schedule a job that is executed periodically
Syntax:

# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of the month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday;
# │ │ │ │ │                                   7 is also Sunday on some systems)
# │ │ │ │ │
# │ │ │ │ │
# * * * * *

- https://crontab.guru/ will help us with configuration

Prerequisites¶

How Kubernetes works and schedules containers as pods
How Kubernetes manages objects and config in a declarative way
The differences between a service, deployment, and Horizontal Pod Autoscaler
Aware of usual Kubernetes terms like Node, Kubelet, and the likes.

kubernetes and cronjobs¶

Kubernetes comes with objects called cronjobs which works similar to the linux cronjobs
The timezone is taken from the k8s/kubernetes cluster
Cronjobs are a wrapper around kubernetes jobs

simple configuration yaml file for kubernetes cronjob¶

apiVersion: batch/v1beta1
kind: CronJob
metadata:
  name: simple-job-name
spec:
  schedule: "5 * * * *"
  concurrencyPolicy: Replace
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: hello
            image: busybox
            imagePullPolicy: IfNotPresent
            command:
            - /bin/sh
            - -c
            - date; echo "Hello from the Kubernetes cluster"
          restartPolicy: OnFailure

Above example is simple job which runs every 5 minutes and executes the command
We can have multiple containers
concurrencyPolicy can have values Allow, Forbid, Replace
- Allow (default): The cron job allows concurrently running jobs
- Forbid: The cron job does not allow concurrent runs; if it is time for a new job run and the previous job run hasn't finished yet, the cron job skips the new job run
- Replace: If it is time for a new job run and the previous job run hasn't finished yet, the cron job replaces the currently running job run with a new job run
restartPolicy can have values OnFailure, Never

Notes to remember before scheduling the cronjob¶

when restartPolicy is set to OnFailure then it will be restarted if any of the containers failed.
It won't follow the sheduled time when it runs for failure.
Make sure your command didn't create any issues if it runs at irregular intervals.

Commands to work with kubernets¶

Create the kubernetes cronjob

kubectl apply -f cronjob.yaml -n namespace

Get all scheduled cronjobs
```
kubectl get cronjobs -n namespace
```

suspend kubernetes cronjob

kubectl patch cronjobs <job-name> -p '{"spec" : {"suspend" : true }}' -n namespace

delete kubernetes cronjob

kubectl delete cronjobs <job-name> -n namespace

describe a cronjob

kubectl describe cronjobs <job-name> -n namespace

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