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:

1. 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.

2. nginx-ds DaemonSet: Runs on only worker nodes, not on master nodes to maintain workload separation and ensure efficient resource allocation.

3. 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