Mastering Kubernetes Controllers: DaemonSet, Job, and CronJob Hands‑On Guide

Why Controllers Matter

Kubernetes pods created directly are vulnerable: if a pod is deleted it does not self‑recover. Controllers continuously monitor pod health and enforce the desired state, automatically restarting failed pods, maintaining replica counts, and handling node changes.

DaemonSet Overview

A DaemonSet ensures that a copy of a pod runs on every node in the cluster. When a new node joins, the DaemonSet creates a pod on it; when a node leaves, the associated pod is removed. Unlike Deployments, each node runs at most one DaemonSet pod.

DaemonSet Specification

Key fields include apiVersion: apps/v1, kind: DaemonSet, metadata, and spec with selector and template. The template defines pod metadata, tolerations, containers, resource limits, volume mounts, and termination grace period.

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: fluentd-elasticsearch
  namespace: kube-system
  labels:
    k8s-app: fluentd-logging
spec:
  selector:
    matchLabels:
      name: fluentd-elasticsearch
  template:
    metadata:
      labels:
        name: fluentd-elasticsearch
    spec:
      tolerations:
      - key: node-role.kubernetes.io/master
        effect: NoSchedule
      containers:
      - name: fluentd-elasticsearch
        image: xianchao/fluentd:v2.5.1
        resources:
          limits:
            memory: 200Mi
          requests:
            cpu: 100m
            memory: 200Mi
        volumeMounts:
        - name: varlog
          mountPath: /var/log
        - name: varlibdockercontainers
          mountPath: /var/lib/docker/containers
          readOnly: true
      terminationGracePeriodSeconds: 30
      volumes:
      - name: varlog
        hostPath:
          path: /var/log
      - name: varlibdockercontainers
        hostPath:
          path: /var/lib/docker/containers

Apply the manifest with kubectl apply -f daemonset.yaml and verify creation using kubectl get ds -n kube-system and kubectl get pods -n kube-system -o wide.

DaemonSet Rolling Update

The updateStrategy field controls how pods are updated. Supported types are RollingUpdate and OnDelete. For rolling updates, you can set maxUnavailable to control the number of pods removed before new ones are created. Example command to update the image:

kubectl set image daemonsets fluentd-elasticsearch=ikubernetes/filebeat:5.6.6-alpine -n kube-system

Job Controller

A Job runs a pod to completion for one‑time tasks such as database backups. Important spec fields include completions, parallelism, activeDeadlineSeconds, backoffLimit, and ttlSecondsAfterFinished. Restart policy must be Never or OnFailure.

apiVersion: batch/v1
kind: Job
metadata:
  name: my-busybox-job
spec:
  ttlSecondsAfterFinished: 20
  activeDeadlineSeconds: 50
  completions: 6
  parallelism: 3
  backoffLimit: 6
  template:
    spec:
      restartPolicy: Never
      containers:
      - name: my-container-job
        image: busybox
        imagePullPolicy: IfNotPresent
        command: ['sh', '-c']
        args: ['echo "Welcome to xianchao"; sleep 60; echo "Next to Meet you"']

Deploy with kubectl apply -f job.yaml, monitor with kubectl get job, and view logs using kubectl logs <pod-name>.

CronJob Controller

CronJob adds scheduling to Jobs, allowing periodic execution. The schedule field follows standard cron syntax. The jobTemplate contains the Job spec.

apiVersion: batch/v1beta1
kind: CronJob
metadata:
  name: hello
spec:
  schedule: "*/1 * * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: hello
            image: busybox
            imagePullPolicy: IfNotPresent
            command:
            - /bin/sh
            - -c
            - date; echo Hello from the Kubernetes cluster
          restartPolicy: OnFailure

Apply with kubectl apply -f cronjob.yaml, then watch scheduling with kubectl get cronjob and kubectl get jobs --watch. Logs of each run can be retrieved via kubectl logs <job-pod>.

Key Takeaways

Controllers automate pod lifecycle management, improving reliability.

DaemonSet is ideal for node‑level services such as log collectors or storage agents.

Jobs handle batch or one‑off tasks; CronJobs add time‑based automation.

Understanding spec fields and update strategies enables safe rolling updates and resource cleanup.