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concept	https://kubernetes.io

Kubernetes

Overview

Kubernetes is an open-source system for automating deployment, scaling, and management of containerized applications. It groups containers into logical units called Pods, which run on Nodes in a cluster. A simple solution to get up and running is k3s.

You can manage k8s clusters via kubectl. Most things are defined via yaml manifest files decleratively. You can throw these into your cluster with kubectl apply -f FILE.

Resources

Namespace

Logical separation of resources within a cluster.

apiVersion: v1
kind: Namespace
metadata:
  name: example-namespace

Pod

The smallest deployable unit in Kubernetes.

apiVersion: v1
kind: Pod
metadata:
  name: full-example-pod
  namespace: example-namespace
  labels:
    app: web
    tier: frontend
  annotations:
    description: "A full-featured pod example for demonstration purposes"
spec:
  restartPolicy: Always

  # Init container (runs before main containers)
  initContainers:
    - name: init-permissions
      image: busybox
      command: ["sh", "-c", "chmod 777 /mnt/data"]
      volumeMounts:
        - name: data-volume
          mountPath: /mnt/data

  containers:
    - name: main-app
      image: nginx:1.25
      imagePullPolicy: IfNotPresent
      ports:
        - containerPort: 80
          name: http
      env:
        # Environment
        - name: ENVIRONMENT
          value: production
        
        # Env from ConfigMap
        - name: CONFIG_TIMEOUT
          valueFrom:
            configMapKeyRef:
              name: example-config
              key: TIMEOUT
        
        # Env from Secret
        - name: SECRET_PASSWORD
          valueFrom:
            secretKeyRef:
              name: example-secret
              key: password
      volumeMounts:
        - name: data-volume
          mountPath: /usr/share/nginx/html
        - name: config-volume
          mountPath: /etc/config
          readOnly: true
      resources:
        limits:
          cpu: "500m"
          memory: "256Mi"
        requests:
          cpu: "250m"
          memory: "128Mi"
      livenessProbe:
        httpGet:
          path: /
          port: 80
        initialDelaySeconds: 10
        periodSeconds: 10
      readinessProbe:
        httpGet:
          path: /
          port: 80
        initialDelaySeconds: 5
        periodSeconds: 5
      lifecycle:
        preStop:
          exec:
            command: ["sh", "-c", "echo stopping..."]

    - name: sidecar-logger
      image: busybox
      args: ["sh", "-c", "tail -f /var/log/app.log"]
      volumeMounts:
        - name: log-volume
          mountPath: /var/log

  # Volumes
  volumes:
    # ConfigMap - inject config files
    - name: config-volume
      configMap:
        name: example-config
        items:
          - key: config.json
            path: config.json

    # Secret - inject sensitive data
    - name: secret-volume
      secret:
        secretName: example-secret
        items:
          - key: password
            path: password.txt

    # EmptyDir - ephemeral shared storage between containers
    - name: log-volume
      emptyDir:
        medium: ""
        sizeLimit: 500Mi

    # HostPath - access host node's filesystem (example: logs)
    - name: host-logs
      hostPath:
        path: /var/log
        type: Directory

Deployment

Ensures a specified number of identical Pods are running and up-to-date. Supports rolling updates and rollbacks.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: example-deployment
  namespace: example-namespace
spec:
  replicas: 3
  selector:
    matchLabels:
      app: example
  template:
    metadata:
      labels:
        app: example
    spec:
      containers:
        - name: web
          image: nginx:alpine
          ports:
            - containerPort: 80

StatefulSet

Like a Deployment, but for workloads requiring stable network IDs, persistent storage, and ordered startup/shutdown.

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: example-statefulset
  namespace: example-namespace
spec:
  serviceName: "example"
  replicas: 2
  selector:
    matchLabels:
      app: stateful-app
  template:
    metadata:
      labels:
        app: stateful-app
    spec:
      containers:
        - name: web
          image: nginx:alpine
          volumeMounts:
            - name: data
              mountPath: /usr/share/nginx/html

DaemonSet

Ensures a copy of a Pod runs on all (or some) Nodes in the cluster. Ideal for log collectors or system-level agents.

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: example-daemonset
  namespace: example-namespace
spec:
  selector:
    matchLabels:
      name: ds-app
  template:
    metadata:
      labels:
        name: ds-app
    spec:
      containers:
        - name: node-monitor
          image: busybox
          args: ["sh", "-c", "while true; do echo hello; sleep 10; done"]

Job

Runs a Pod (or multiple) to completion. Used for batch processing or one-off tasks.

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

CronJob

Schedules Jobs to run periodically, similar to UNIX cron.

apiVersion: batch/v1
kind: CronJob
metadata:
  name: example-cronjob
  namespace: example-namespace
spec:
  schedule: "*/5 * * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
            - name: hello
              image: busybox
              args: ["echo", "Hello from the CronJob"]
          restartPolicy: OnFailure

Note: You can quickly run CronJobs as a job with: kubectl create job --from=cronjob.batch/my_cron_job new_job

Service

Defines a stable network endpoint to access a set of Pods. Supports different types like ClusterIP, NodePort, and LoadBalancer.

apiVersion: v1
kind: Service
metadata:
  name: example-service
  namespace: example-namespace
spec:
  selector:
    app: example
  ports:
    - protocol: TCP
      port: 80
      targetPort: 80
  type: ClusterIP

ConfigMap

Injects configuration data (as key-value pairs) into Pods, keeping config decoupled from code.

apiVersion: v1
kind: ConfigMap
metadata:
  name: example-config
  namespace: example-namespace
data:
  APP_ENV: production
  TIMEOUT: "30"

Usage in a Pod:

envFrom:
  - configMapRef:
      name: example-config

Secret

Similar to ConfigMap, but for sensitive data like passwords, tokens, or keys.
If you want encryption on rest for your manifests, look at sops.

apiVersion: v1
kind: Secret
metadata:
  name: example-secret
  namespace: example-namespace
type: Opaque
data:
  username: YWRtaW4=   # base64 of 'admin'
  password: cGFzc3dvcmQ=   # base64 of 'password'