Kubernetes Fundamentals

Core building blocks and concepts of Kubernetes.

Pod

The smallest deployable unit. Hosts one or more containers.

apiVersion: v1
kind: Pod
metadata:
  name: my-app
  labels:
    app: my-app
spec:
  containers:
    - name: app
      image: nginx:alpine
      ports:
        - containerPort: 80
      resources:
        requests:
          memory: "64Mi"
          cpu: "250m"
        limits:
          memory: "128Mi"
          cpu: "500m"

Deployment

Declarative management of Pods. Supports rolling updates and rollbacks.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
spec:
  replicas: 3
  selector:
    matchLabels:
      app: my-app
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxSurge: 1
      maxUnavailable: 0
  template:
    metadata:
      labels:
        app: my-app
    spec:
      containers:
        - name: app
          image: my-app:v1.0
          ports:
            - containerPort: 8080

kubectl apply -f deployment.yaml
kubectl rollout status deployment/my-app
kubectl rollout history deployment/my-app
kubectl rollout undo deployment/my-app
kubectl scale deployment my-app --replicas=5

ReplicaSet

Ensures a specified number of Pod replicas are running. Usually managed by a Deployment.

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: my-app-rs
spec:
  replicas: 3
  selector:
    matchLabels:
      app: my-app
  template:
    metadata:
      labels:
        app: my-app
    spec:
      containers:
        - name: app
          image: my-app:v1.0

StatefulSet

For stateful applications (databases, message queues). Provides stable network identity and persistent storage.

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: postgres
spec:
  serviceName: postgres
  replicas: 3
  selector:
    matchLabels:
      app: postgres
  template:
    metadata:
      labels:
        app: postgres
    spec:
      containers:
        - name: postgres
          image: postgres:16
          ports:
            - containerPort: 5432
          volumeMounts:
            - name: data
              mountPath: /var/lib/postgresql/data
  volumeClaimTemplates:
    - metadata:
        name: data
      spec:
        accessModes: ["ReadWriteOnce"]
        resources:
          requests:
            storage: 10Gi

StatefulSet vs Deployment:

• Pods get ordered names: postgres-0, postgres-1, postgres-2
• Ordered startup and shutdown
• Dedicated PersistentVolume per Pod
• Stable DNS: postgres-0.postgres.namespace.svc.cluster.local

DaemonSet

Runs a Pod on every node. Ideal for monitoring and logging agents.

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: fluentd
spec:
  selector:
    matchLabels:
      app: fluentd
  template:
    metadata:
      labels:
        app: fluentd
    spec:
      containers:
        - name: fluentd
          image: fluentd:latest
          volumeMounts:
            - name: varlog
              mountPath: /var/log
      volumes:
        - name: varlog
          hostPath:
            path: /var/log

Service

Provides stable network access to Pods.

# ClusterIP (default, cluster-internal access)
apiVersion: v1
kind: Service
metadata:
  name: my-app-svc
spec:
  type: ClusterIP
  selector:
    app: my-app
  ports:
    - port: 80
      targetPort: 8080
---
# NodePort (external access, opens port on every node)
apiVersion: v1
kind: Service
metadata:
  name: my-app-nodeport
spec:
  type: NodePort
  selector:
    app: my-app
  ports:
    - port: 80
      targetPort: 8080
      nodePort: 30080
---
# LoadBalancer (cloud provider load balancer)
apiVersion: v1
kind: Service
metadata:
  name: my-app-lb
spec:
  type: LoadBalancer
  selector:
    app: my-app
  ports:
    - port: 80
      targetPort: 8080

ConfigMap and Secret

apiVersion: v1
kind: ConfigMap
metadata:
  name: app-config
data:
  DATABASE_HOST: "postgres"
  LOG_LEVEL: "info"
---
apiVersion: v1
kind: Secret
metadata:
  name: app-secrets
type: Opaque
data:
  DB_PASSWORD: cGFzc3dvcmQ=

# Usage in Pod
spec:
  containers:
    - name: app
      envFrom:
        - configMapRef:
            name: app-config
        - secretRef:
            name: app-secrets

Namespace

kubectl create namespace production
kubectl get all -n production
kubectl config set-context --current --namespace=production

Ingress

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: my-app-ingress
  annotations:
    nginx.ingress.kubernetes.io/rewrite-target: /
spec:
  rules:
    - host: app.example.com
      http:
        paths:
          - path: /
            pathType: Prefix
            backend:
              service:
                name: my-app-svc
                port:
                  number: 80