Day 12/40 Days of K8s: Exploring Kubernetes Daemonset, Job and Cronjob

🚀 Deployments vs DaemonSets

Deployments

In a Deployment, we specify the number of replicas, such as 3, to create and maintain that many pods across multiple nodes. The Deployment controller and scheduler are responsible for ensuring the specified number of pod replicas are always running. They manage the scheduling of pods based on node availability, resource constraints, taints, tolerations, and other factors.

DaemonSets

DaemonSet are used to run a single instance of a pod on each worker node, ensuring consistent deployment across the entire cluster. As soon as node deletion, DaemonSet gets deleted.

🌟 Use Cases for DaemonSets

Monitoring agents
Logging agents
Networking components (e.g., kube-proxy, CNI plugins)

💡 Note: DaemonSets ensure continuous operation on each node, ideal for node-specific tasks.

DaemonSet yaml:

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: nginx-ds 
  labels: 
    env: demo 
spec: 
  selector:
    matchLabels:
      env: demo 
  template:
    metadata:
      labels:
        env: demo 
      name: nginx 
    spec:
      containers:
      - name: nginx 
        image: nginx 
        ports:
        - containerPort: 80

🕒 CronJobs

A CronJob in Kubernetes is a specialized Job that runs on a schedule. It allows you to execute tasks at specified times, dates, or intervals. This is similar to the cron utility in Unix/Linux systems.

Uses cron-like syntax: * * * * * (minute, hour, day of month, month, day of week)

TASK: create a cronjob object in kubernetes that prints "40daysofkubernetes" after every 5 minutes and use busybox image

Cronjob yaml:

  # This will schedule cronjob to run every 5 minutes and spin up a container using image busybox and print out "40daysofkubernetes"  
  apiVersion: batch/v1
  kind: CronJob
  metadata:
    name: hello
  spec:
    schedule: "*/5 * * * *"
    jobTemplate:
      spec:
        template:
          spec:
            containers:
            - name: hello
              image: busybox:1.28
              imagePullPolicy: IfNotPresent
              command: ["sh", "-c", "echo '40daysofkubernetes'"]
            restartPolicy: OnFailure

So, every time when a cronjob is scheduled , in k8s a new job pod is created and once execution done, it marks the pod as completed.

Examples:

Every Saturday: * * * * 6
Every Saturday at 11:00 PM: 0 23 * * 6
Every 5 minutes: */5 * * * *

🌟 Jobs

Executes tasks only once and ensures the task completes successfully. Like part of installation script, performing a database backup, or any task that needs to run once and finish.

Job YAML:

  apiVersion: batch/v1
  kind: Job
  metadata:
    name: hello-world
  spec:
    template:
      spec:
        containers:
        - name: hello
          image: busybox
          command: ["echo", "Hello, World!"]
        restartPolicy: Never
    backoffLimit: 4

🔍 Verifying Workloads

Check kube-proxy DaemonSet on worker nodes:
```
  kubectl get ds kube-proxy -n kube-system
```
NOTE: Daemonset runs on each worker node but not on master because of taints on it.

💁 Key Points:

kube-proxy:
- A data plane component that runs in the kube-system namespace.
- kube-proxy runs on all nodes to ensure svc to pod traffic networking across the cluster.
nginx-ds DaemonSet: Runs on only worker nodes, not on master nodes to maintain workload separation and ensure efficient resource allocation.
Commands to View DaemonSets:
- To get the DaemonSet from all namespaces:
```
  kubectl get ds -A
```
- To view the DaemonSet specifically from the kube-system namespace:
```
  kubectl get ds -n kube-system
```

By understanding these Kubernetes resources like DaemonSet,Cronjob,Jobs, we can effectively manage various types of tasks and ensure our cluster operates smoothly across different scenarios.

#Kubernetes #DaemonSet #Cronjobs #Jobs #40DaysofKubernetes #CKASeries