Kubernetes Patterns

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
  namespace: my-namespace
  labels:
    app: my-app
    version: "1.0.0"
spec:
  replicas: 3
  selector:
    matchLabels:
      app: my-app
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxSurge: 1          # Allow 1 extra pod during update
      maxUnavailable: 0    # Never reduce below desired count
  template:
    metadata:
      labels:
        app: my-app
        version: "1.0.0"
    spec:
      # Security context at pod level
      securityContext:
        runAsNonRoot: true
        runAsUser: 1001
        fsGroup: 1001

      # Graceful shutdown
      terminationGracePeriodSeconds: 30

      containers:
        - name: my-app
          image: ghcr.io/org/my-app:1.0.0   # Never use :latest
          imagePullPolicy: IfNotPresent

          ports:
            - containerPort: 8080
              protocol: TCP

          # Resource requests AND limits are both required
          resources:
            requests:
              cpu: "100m"
              memory: "128Mi"
            limits:
              cpu: "500m"
              memory: "256Mi"

          # Container security context
          securityContext:
            allowPrivilegeEscalation: false
            readOnlyRootFilesystem: true
            capabilities:
              drop:
                - ALL

          # Probes (see Probes section below)
          startupProbe:
            httpGet:
              path: /health
              port: 8080
            failureThreshold: 30
            periodSeconds: 5
          livenessProbe:
            httpGet:
              path: /health
              port: 8080
            initialDelaySeconds: 0
            periodSeconds: 30
            failureThreshold: 3
          readinessProbe:
            httpGet:
              path: /ready
              port: 8080
            initialDelaySeconds: 5
            periodSeconds: 10
            failureThreshold: 2

          # Environment from ConfigMap and Secret
          envFrom:
            - configMapRef:
                name: my-app-config
          env:
            - name: DB_PASSWORD
              valueFrom:
                secretKeyRef:
                  name: my-app-secrets
                  key: db-password

          # Writable tmp directory when readOnlyRootFilesystem: true
          volumeMounts:
            - name: tmp
              mountPath: /tmp

      volumes:
        - name: tmp
          emptyDir: {}

# Correct pattern: startupProbe covers slow startup,
# then liveness/readiness take over
startupProbe:
  httpGet:
    path: /health
    port: 8080
  failureThreshold: 30  # 30 * 5s = 150s max startup time
  periodSeconds: 5

livenessProbe:
  httpGet:
    path: /health
    port: 8080
  periodSeconds: 30
  failureThreshold: 3   # 3 * 30s = 90s before restart

readinessProbe:
  httpGet:
    path: /ready         # Separate endpoint: checks DB, cache, etc.
    port: 8080
  periodSeconds: 10
  failureThreshold: 2

# WRONG: initialDelaySeconds without startupProbe
# If the app takes 60s to start, set a startupProbe instead
livenessProbe:
  httpGet:
    path: /health
    port: 8080
  initialDelaySeconds: 60   # BAD: Arbitrary wait, race condition

# ClusterIP (default) — internal-only
apiVersion: v1
kind: Service
metadata:
  name: my-app
  namespace: my-namespace
spec:
  selector:
    app: my-app
  ports:
    - port: 80
      targetPort: 8080
      protocol: TCP
  type: ClusterIP

# LoadBalancer — external traffic (cloud providers)
spec:
  type: LoadBalancer
  ports:
    - port: 443
      targetPort: 8080

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: my-app
  namespace: my-namespace
  annotations:
    nginx.ingress.kubernetes.io/ssl-redirect: "true"
    cert-manager.io/cluster-issuer: "letsencrypt-prod"
spec:
  ingressClassName: nginx
  tls:
    - hosts:
        - myapp.example.com
      secretName: my-app-tls
  rules:
    - host: myapp.example.com
      http:
        paths:
          - path: /
            pathType: Prefix
            backend:
              service:
                name: my-app
                port:
                  number: 80

apiVersion: v1
kind: ConfigMap
metadata:
  name: my-app-config
  namespace: my-namespace
data:
  LOG_LEVEL: "info"
  APP_ENV: "production"
  MAX_CONNECTIONS: "100"
  # Mount as a file for complex config
  app.yaml: |
    server:
      port: 8080
      timeout: 30s

# Mount ConfigMap as a file
volumes:
  - name: config
    configMap:
      name: my-app-config
      items:
        - key: app.yaml
          path: app.yaml
volumeMounts:
  - name: config
    mountPath: /etc/app
    readOnly: true

# Create secret from literal (CLI, then store in Vault/SOPS)
kubectl create secret generic my-app-secrets \
  --from-literal=db-password='s3cr3t' \
  --namespace=my-namespace \
  --dry-run=client -o yaml | kubectl apply -f -

apiVersion: v1
kind: Secret
metadata:
  name: my-app-secrets
  namespace: my-namespace
type: Opaque
# Values are base64-encoded (NOT encrypted — use Sealed Secrets or ESO for real encryption)
data:
  db-password: czNjcjN0  # base64 of 's3cr3t'

resources:
  requests:       # Scheduler uses this to place the pod
    cpu: "100m"   # 100 millicores = 0.1 CPU
    memory: "128Mi"
  limits:         # Container is killed/throttled above this
    cpu: "500m"
    memory: "256Mi"

# WRONG: No requests or limits — unpredictable scheduling, OOM evictions
containers:
  - name: app
    image: myapp:latest
    # Missing resources: {} — this is dangerous in production

# WRONG: Limits without requests — requests default to limits, over-reserves capacity
resources:
  limits:
    cpu: "2"
    memory: "1Gi"
  # requests missing — will default to limits values

# ServiceAccount with token disabled — safest default
apiVersion: v1
kind: ServiceAccount
metadata:
  name: my-app-sa
  namespace: my-namespace
automountServiceAccountToken: false   # No K8s API token injected into pods

# Reference in Deployment — no token, no API access
spec:
  template:
    spec:
      serviceAccountName: my-app-sa
      automountServiceAccountToken: false   # Belt-and-suspenders: also set at pod level

# 1. ServiceAccount — enable token for this SA
apiVersion: v1
kind: ServiceAccount
metadata:
  name: my-app-sa
  namespace: my-namespace
automountServiceAccountToken: true    # Token required: app calls K8s API

# 2. Role — grant only what the app needs (namespace-scoped)
apiVersion: rbac.authorization.k8s.io/v1
kind: Role
metadata:
  name: my-app-role
  namespace: my-namespace
rules:
  - apiGroups: [""]
    resources: ["configmaps"]
    verbs: ["get", "list", "watch"]    # Read-only, specific resource
  - apiGroups: [""]
    resources: ["secrets"]
    resourceNames: ["my-app-secrets"]  # Restrict to specific secret by name
    verbs: ["get"]

# 3. Bind Role to ServiceAccount
apiVersion: rbac.authorization.k8s.io/v1
kind: RoleBinding
metadata:
  name: my-app-rolebinding
  namespace: my-namespace
subjects:
  - kind: ServiceAccount
    name: my-app-sa
    namespace: my-namespace
roleRef:
  kind: Role
  apiGroup: rbac.authorization.k8s.io
  name: my-app-role

# 4. Reference SA in Deployment
spec:
  template:
    spec:
      serviceAccountName: my-app-sa
      # automountServiceAccountToken defaults to true from SA — token is injected