# Kubernetes Deployment Specification Reference

Comprehensive reference for Kubernetes Deployment resources, covering all key fields, best practices, and common patterns.

## Overview

A Deployment provides declarative updates for Pods and ReplicaSets. It manages the desired state of your application, handling rollouts, rollbacks, and scaling operations.

## Complete Deployment Specification

```yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
  namespace: production
  labels:
    app.kubernetes.io/name: my-app
    app.kubernetes.io/version: "1.0.0"
    app.kubernetes.io/component: backend
    app.kubernetes.io/part-of: my-system
  annotations:
    description: "Main application deployment"
    contact: "backend-team@example.com"
spec:
  # Replica management
  replicas: 3
  revisionHistoryLimit: 10

  # Pod selection
  selector:
    matchLabels:
      app: my-app
      version: v1

  # Update strategy
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxSurge: 1
      maxUnavailable: 0

  # Minimum time for pod to be ready
  minReadySeconds: 10

  # Deployment will fail if it doesn't progress in this time
  progressDeadlineSeconds: 600

  # Pod template
  template:
    metadata:
      labels:
        app: my-app
        version: v1
      annotations:
        prometheus.io/scrape: "true"
        prometheus.io/port: "9090"
    spec:
      # Service account for RBAC
      serviceAccountName: my-app

      # Security context for the pod
      securityContext:
        runAsNonRoot: true
        runAsUser: 1000
        fsGroup: 1000
        seccompProfile:
          type: RuntimeDefault

      # Init containers run before main containers
      initContainers:
        - name: init-db
          image: busybox:1.36
          command: ["sh", "-c", "until nc -z db-service 5432; do sleep 1; done"]
          securityContext:
            allowPrivilegeEscalation: false
            runAsNonRoot: true
            runAsUser: 1000

      # Main containers
      containers:
        - name: app
          image: myapp:1.0.0
          imagePullPolicy: IfNotPresent

          # Container ports
          ports:
            - name: http
              containerPort: 8080
              protocol: TCP
            - name: metrics
              containerPort: 9090
              protocol: TCP

          # Environment variables
          env:
            - name: POD_NAME
              valueFrom:
                fieldRef:
                  fieldPath: metadata.name
            - name: POD_NAMESPACE
              valueFrom:
                fieldRef:
                  fieldPath: metadata.namespace
            - name: DATABASE_URL
              valueFrom:
                secretKeyRef:
                  name: db-credentials
                  key: url

          # ConfigMap and Secret references
          envFrom:
            - configMapRef:
                name: app-config
            - secretRef:
                name: app-secrets

          # Resource requests and limits
          resources:
            requests:
              memory: "256Mi"
              cpu: "250m"
            limits:
              memory: "512Mi"
              cpu: "500m"

          # Liveness probe
          livenessProbe:
            httpGet:
              path: /health/live
              port: http
              httpHeaders:
                - name: Custom-Header
                  value: Awesome
            initialDelaySeconds: 30
            periodSeconds: 10
            timeoutSeconds: 5
            successThreshold: 1
            failureThreshold: 3

          # Readiness probe
          readinessProbe:
            httpGet:
              path: /health/ready
              port: http
            initialDelaySeconds: 5
            periodSeconds: 5
            timeoutSeconds: 3
            successThreshold: 1
            failureThreshold: 3

          # Startup probe (for slow-starting containers)
          startupProbe:
            httpGet:
              path: /health/startup
              port: http
            initialDelaySeconds: 0
            periodSeconds: 10
            timeoutSeconds: 3
            successThreshold: 1
            failureThreshold: 30

          # Volume mounts
          volumeMounts:
            - name: data
              mountPath: /var/lib/app
            - name: config
              mountPath: /etc/app
              readOnly: true
            - name: tmp
              mountPath: /tmp

          # Security context for container
          securityContext:
            allowPrivilegeEscalation: false
            readOnlyRootFilesystem: true
            runAsNonRoot: true
            runAsUser: 1000
            capabilities:
              drop:
                - ALL

          # Lifecycle hooks
          lifecycle:
            postStart:
              exec:
                command:
                  ["/bin/sh", "-c", "echo Container started > /tmp/started"]
            preStop:
              exec:
                command: ["/bin/sh", "-c", "sleep 15"]

      # Volumes
      volumes:
        - name: data
          persistentVolumeClaim:
            claimName: app-data
        - name: config
          configMap:
            name: app-config
        - name: tmp
          emptyDir: {}

      # DNS configuration
      dnsPolicy: ClusterFirst
      dnsConfig:
        options:
          - name: ndots
            value: "2"

      # Scheduling
      nodeSelector:
        disktype: ssd

      affinity:
        podAntiAffinity:
          preferredDuringSchedulingIgnoredDuringExecution:
            - weight: 100
              podAffinityTerm:
                labelSelector:
                  matchExpressions:
                    - key: app
                      operator: In
                      values:
                        - my-app
                topologyKey: kubernetes.io/hostname

      tolerations:
        - key: "app"
          operator: "Equal"
          value: "my-app"
          effect: "NoSchedule"

      # Termination
      terminationGracePeriodSeconds: 30

      # Image pull secrets
      imagePullSecrets:
        - name: regcred
```

## Field Reference

### Metadata Fields

#### Required Fields

- `apiVersion`: `apps/v1` (current stable version)
- `kind`: `Deployment`
- `metadata.name`: Unique name within namespace

#### Recommended Metadata

- `metadata.namespace`: Target namespace (defaults to `default`)
- `metadata.labels`: Key-value pairs for organization
- `metadata.annotations`: Non-identifying metadata

### Spec Fields

#### Replica Management

**`replicas`** (integer, default: 1)

- Number of desired pod instances
- Best practice: Use 3+ for production high availability
- Can be scaled manually or via HorizontalPodAutoscaler

**`revisionHistoryLimit`** (integer, default: 10)

- Number of old ReplicaSets to retain for rollback
- Set to 0 to disable rollback capability
- Reduces storage overhead for long-running deployments

#### Update Strategy

**`strategy.type`** (string)

- `RollingUpdate` (default): Gradual pod replacement
- `Recreate`: Delete all pods before creating new ones

**`strategy.rollingUpdate.maxSurge`** (int or percent, default: 25%)

- Maximum pods above desired replicas during update
- Example: With 3 replicas and maxSurge=1, up to 4 pods during update

**`strategy.rollingUpdate.maxUnavailable`** (int or percent, default: 25%)

- Maximum pods below desired replicas during update
- Set to 0 for zero-downtime deployments
- Cannot be 0 if maxSurge is 0

**Best practices:**

```yaml
# Zero-downtime deployment
strategy:
  type: RollingUpdate
  rollingUpdate:
    maxSurge: 1
    maxUnavailable: 0

# Fast deployment (can have brief downtime)
strategy:
  type: RollingUpdate
  rollingUpdate:
    maxSurge: 2
    maxUnavailable: 1

# Complete replacement
strategy:
  type: Recreate
```

#### Pod Template

**`template.metadata.labels`**

- Must include labels matching `spec.selector.matchLabels`
- Add version labels for blue/green deployments
- Include standard Kubernetes labels

**`template.spec.containers`** (required)

- Array of container specifications
- At least one container required
- Each container needs unique name

#### Container Configuration

**Image Management:**

```yaml
containers:
  - name: app
    image: registry.example.com/myapp:1.0.0
    imagePullPolicy: IfNotPresent # or Always, Never
```

Image pull policies:

- `IfNotPresent`: Pull if not cached (default for tagged images)
- `Always`: Always pull (default for :latest)
- `Never`: Never pull, fail if not cached

**Port Declarations:**

```yaml
ports:
  - name: http # Named for referencing in Service
    containerPort: 8080
    protocol: TCP # TCP (default), UDP, or SCTP
    hostPort: 8080 # Optional: Bind to host port (rarely used)
```

#### Resource Management

**Requests vs Limits:**

```yaml
resources:
  requests:
    memory: "256Mi" # Guaranteed resources
    cpu: "250m" # 0.25 CPU cores
  limits:
    memory: "512Mi" # Maximum allowed
    cpu: "500m" # 0.5 CPU cores
```

**QoS Classes (determined automatically):**

1. **Guaranteed**: requests = limits for all containers
   - Highest priority
   - Last to be evicted

2. **Burstable**: requests < limits or only requests set
   - Medium priority
   - Evicted before Guaranteed

3. **BestEffort**: No requests or limits set
   - Lowest priority
   - First to be evicted

**Best practices:**

- Always set requests in production
- Set limits to prevent resource monopolization
- Memory limits should be 1.5-2x requests
- CPU limits can be higher for bursty workloads

#### Health Checks

**Probe Types:**

1. **startupProbe** - For slow-starting applications

   ```yaml
   startupProbe:
     httpGet:
       path: /health/startup
       port: 8080
     initialDelaySeconds: 0
     periodSeconds: 10
     failureThreshold: 30 # 5 minutes to start (10s * 30)
   ```

2. **livenessProbe** - Restarts unhealthy containers

   ```yaml
   livenessProbe:
     httpGet:
       path: /health/live
       port: 8080
     initialDelaySeconds: 30
     periodSeconds: 10
     timeoutSeconds: 5
     failureThreshold: 3 # Restart after 3 failures
   ```

3. **readinessProbe** - Controls traffic routing
   ```yaml
   readinessProbe:
     httpGet:
       path: /health/ready
       port: 8080
     initialDelaySeconds: 5
     periodSeconds: 5
     failureThreshold: 3 # Remove from service after 3 failures
   ```

**Probe Mechanisms:**

```yaml
# HTTP GET
httpGet:
  path: /health
  port: 8080
  httpHeaders:
    - name: Authorization
      value: Bearer token

# TCP Socket
tcpSocket:
  port: 3306

# Command execution
exec:
  command:
    - cat
    - /tmp/healthy

# gRPC (Kubernetes 1.24+)
grpc:
  port: 9090
  service: my.service.health.v1.Health
```

**Probe Timing Parameters:**

- `initialDelaySeconds`: Wait before first probe
- `periodSeconds`: How often to probe
- `timeoutSeconds`: Probe timeout
- `successThreshold`: Successes needed to mark healthy (1 for liveness/startup)
- `failureThreshold`: Failures before taking action

#### Security Context

**Pod-level security context:**

```yaml
spec:
  securityContext:
    runAsNonRoot: true
    runAsUser: 1000
    runAsGroup: 1000
    fsGroup: 1000
    fsGroupChangePolicy: OnRootMismatch
    seccompProfile:
      type: RuntimeDefault
```

**Container-level security context:**

```yaml
containers:
  - name: app
    securityContext:
      allowPrivilegeEscalation: false
      readOnlyRootFilesystem: true
      runAsNonRoot: true
      runAsUser: 1000
      capabilities:
        drop:
          - ALL
        add:
          - NET_BIND_SERVICE # Only if needed
```

**Security best practices:**

- Always run as non-root (`runAsNonRoot: true`)
- Drop all capabilities and add only needed ones
- Use read-only root filesystem when possible
- Enable seccomp profile
- Disable privilege escalation

#### Volumes

**Volume Types:**

```yaml
volumes:
  # PersistentVolumeClaim
  - name: data
    persistentVolumeClaim:
      claimName: app-data

  # ConfigMap
  - name: config
    configMap:
      name: app-config
      items:
        - key: app.properties
          path: application.properties

  # Secret
  - name: secrets
    secret:
      secretName: app-secrets
      defaultMode: 0400

  # EmptyDir (ephemeral)
  - name: cache
    emptyDir:
      sizeLimit: 1Gi

  # HostPath (avoid in production)
  - name: host-data
    hostPath:
      path: /data
      type: DirectoryOrCreate
```

#### Scheduling

**Node Selection:**

```yaml
# Simple node selector
nodeSelector:
  disktype: ssd
  zone: us-west-1a

# Node affinity (more expressive)
affinity:
  nodeAffinity:
    requiredDuringSchedulingIgnoredDuringExecution:
      nodeSelectorTerms:
        - matchExpressions:
            - key: kubernetes.io/arch
              operator: In
              values:
                - amd64
                - arm64
```

**Pod Affinity/Anti-Affinity:**

```yaml
# Spread pods across nodes
affinity:
  podAntiAffinity:
    requiredDuringSchedulingIgnoredDuringExecution:
    - labelSelector:
        matchLabels:
          app: my-app
      topologyKey: kubernetes.io/hostname

# Co-locate with database
affinity:
  podAffinity:
    preferredDuringSchedulingIgnoredDuringExecution:
    - weight: 100
      podAffinityTerm:
        labelSelector:
          matchLabels:
            app: database
        topologyKey: kubernetes.io/hostname
```

**Tolerations:**

```yaml
tolerations:
  - key: "node.kubernetes.io/unreachable"
    operator: "Exists"
    effect: "NoExecute"
    tolerationSeconds: 30
  - key: "dedicated"
    operator: "Equal"
    value: "database"
    effect: "NoSchedule"
```

## Common Patterns

### High Availability Deployment

```yaml
spec:
  replicas: 3
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxSurge: 1
      maxUnavailable: 0
  template:
    spec:
      affinity:
        podAntiAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
            - labelSelector:
                matchLabels:
                  app: my-app
              topologyKey: kubernetes.io/hostname
      topologySpreadConstraints:
        - maxSkew: 1
          topologyKey: topology.kubernetes.io/zone
          whenUnsatisfiable: DoNotSchedule
          labelSelector:
            matchLabels:
              app: my-app
```

### Sidecar Container Pattern

```yaml
spec:
  template:
    spec:
      containers:
        - name: app
          image: myapp:1.0.0
          volumeMounts:
            - name: shared-logs
              mountPath: /var/log
        - name: log-forwarder
          image: fluent-bit:2.0
          volumeMounts:
            - name: shared-logs
              mountPath: /var/log
              readOnly: true
      volumes:
        - name: shared-logs
          emptyDir: {}
```

### Init Container for Dependencies

```yaml
spec:
  template:
    spec:
      initContainers:
        - name: wait-for-db
          image: busybox:1.36
          command:
            - sh
            - -c
            - |
              until nc -z database-service 5432; do
                echo "Waiting for database..."
                sleep 2
              done
        - name: run-migrations
          image: myapp:1.0.0
          command: ["./migrate", "up"]
          env:
            - name: DATABASE_URL
              valueFrom:
                secretKeyRef:
                  name: db-credentials
                  key: url
      containers:
        - name: app
          image: myapp:1.0.0
```

## Best Practices

### Production Checklist

- [ ] Set resource requests and limits
- [ ] Implement all three probe types (startup, liveness, readiness)
- [ ] Use specific image tags (not :latest)
- [ ] Configure security context (non-root, read-only filesystem)
- [ ] Set replica count >= 3 for HA
- [ ] Configure pod anti-affinity for spread
- [ ] Set appropriate update strategy (maxUnavailable: 0 for zero-downtime)
- [ ] Use ConfigMaps and Secrets for configuration
- [ ] Add standard labels and annotations
- [ ] Configure graceful shutdown (preStop hook, terminationGracePeriodSeconds)
- [ ] Set revisionHistoryLimit for rollback capability
- [ ] Use ServiceAccount with minimal RBAC permissions

### Performance Tuning

**Fast startup:**

```yaml
spec:
  minReadySeconds: 5
  strategy:
    rollingUpdate:
      maxSurge: 2
      maxUnavailable: 1
```

**Zero-downtime updates:**

```yaml
spec:
  minReadySeconds: 10
  strategy:
    rollingUpdate:
      maxSurge: 1
      maxUnavailable: 0
```

**Graceful shutdown:**

```yaml
spec:
  template:
    spec:
      terminationGracePeriodSeconds: 60
      containers:
        - name: app
          lifecycle:
            preStop:
              exec:
                command: ["/bin/sh", "-c", "sleep 15 && kill -SIGTERM 1"]
```

## Troubleshooting

### Common Issues

**Pods not starting:**

```bash
kubectl describe deployment <name>
kubectl get pods -l app=<app-name>
kubectl describe pod <pod-name>
kubectl logs <pod-name>
```

**ImagePullBackOff:**

- Check image name and tag
- Verify imagePullSecrets
- Check registry credentials

**CrashLoopBackOff:**

- Check container logs
- Verify liveness probe is not too aggressive
- Check resource limits
- Verify application dependencies

**Deployment stuck in progress:**

- Check progressDeadlineSeconds
- Verify readiness probes
- Check resource availability

## Related Resources

- [Kubernetes Deployment API Reference](https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.28/#deployment-v1-apps)
- [Pod Security Standards](https://kubernetes.io/docs/concepts/security/pod-security-standards/)
- [Resource Management](https://kubernetes.io/docs/concepts/configuration/manage-resources-containers/)
