40 Days Of Kubernetes (12/40)

Day 12/40

Kubernetes Daemonset Explained - Daemonsets, Job and Cronjob in Kubernetes

Video Link
@piyushsachdeva
Git Repository
My Git Repo

This section is about cronjob, job and daemonset.
As the definition, DaemonSets are Kubernetes API objects that allow you to run Pods as a daemon on each of your Nodes. New Nodes that join the cluster will automatically start running Pods that are part of a DaemonSet.

DaemonSets are often used to run long-lived background services such as Node monitoring systems and log collection agents. To ensure complete coverage, it’s important that these apps run a Pod on every Node in your cluster. source

---

1. Create a daemonset:

apiVersion: apps/v1

kind:  DaemonSet

metadata:
  name: nginx-ds
  labels:
    env: demo

spec:
  template:
    metadata:
      labels:
        env: demo
      name: nginx
    spec:
      containers:
      - image: nginx
        name: nginx
        ports:
        - containerPort: 80
  selector:
    matchLabels:
      env: demo

root@localhost:~# kubectl apply -f daemonset.yaml
daemonset.apps/nginx-ds created
root@localhost:~# kubectl get po
NAME             READY   STATUS    RESTARTS   AGE
nginx-ds-mf4gz   1/1     Running   0          11s
nginx-ds-rslrm   1/1     Running   0          11s
root@localhost:~# kubectl get po -o wide
NAME             READY   STATUS    RESTARTS   AGE   IP            NODE                 NOMINATED NODE   READINESS GATES
nginx-ds-mf4gz   1/1     Running   0          19s   10.244.2.9    lucky-luke-worker2   <none>           <none>
nginx-ds-rslrm   1/1     Running   0          19s   10.244.1.12   lucky-luke-worker    <none>           <none>

As you can see, our custom workload as daemonset is running on all worker nodes and because it is not a control-plane component and the control-plane node doesn't tolerate custom workload, it doesn't run on control-plane node. (but we can change it)

If one of these pod removed, it will run another one on that node

root@localhost:~# kubectl get po
NAME             READY   STATUS    RESTARTS   AGE
nginx-ds-mf4gz   1/1     Running   0          9m40s
nginx-ds-rslrm   1/1     Running   0          9m40s
root@localhost:~# kubectl delete pod nginx-ds-mf4gz
pod "nginx-ds-mf4gz" deleted
root@localhost:~# kubectl get po
NAME             READY   STATUS    RESTARTS   AGE
nginx-ds-946m4   1/1     Running   0          5s
nginx-ds-rslrm   1/1     Running   0          10m

• See all daemonset on our cluster:

root@localhost:~# kubectl get ds --all-namespaces
NAMESPACE     NAME         DESIRED   CURRENT   READY   UP-TO-DATE   AVAILABLE   NODE SELECTOR            AGE
default       nginx-ds     2         2         2       2            2           <none>                   10m
kube-system   kindnet      3         3         3       3            3           kubernetes.io/os=linux   3d
kube-system   kube-proxy   3         3         3       3            3           kubernetes.io/os=linux   3d

---

2. Cronjobs

See the Cronitor website for understand how it can be configured for tasks run in specific time.

(Photo from the video)

• Sample from Kubernetes website:

apiVersion: batch/v1
kind: CronJob
metadata:
  name: hello
spec:
  schedule: "* * * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: hello
            image: busybox:1.28
            imagePullPolicy: IfNotPresent
            command:
            - /bin/sh
            - -c
            - date; echo Hello from the Kubernetes cluster
          restartPolicy: OnFailure

3. Jobs

A Job creates one or more Pods and will continue to retry execution of the Pods until a specified number of them successfully terminate. As pods successfully complete, the Job tracks the successful completions. When a specified number of successful completions is reached, the task (ie, Job) is complete. Deleting a Job will clean up the Pods it created. Suspending a Job will delete its active Pods until the Job is resumed again. source

• Sample from Kubernetes website

apiVersion: batch/v1
kind: Job
metadata:
  name: pi
spec:
  template:
    spec:
      containers:
      - name: pi
        image: perl:5.34.0
        command: ["perl",  "-Mbignum=bpi", "-wle", "print bpi(2000)"]
      restartPolicy: Never
  backoffLimit: 4