kustomize-overlays

What this skill does

# Kustomize Overlays

Master environment-specific Kubernetes configuration management using Kustomize overlays, strategic merge patches, and JSON patches for development, staging, and production environments.

## Overview

Overlays enable environment-specific customization of Kubernetes resources without duplicating configuration. Each overlay references a base configuration and applies environment-specific patches, transformations, and resource adjustments.

## Basic Overlay Structure

```
myapp/
├── base/
│   ├── kustomization.yaml
│   ├── deployment.yaml
│   ├── service.yaml
│   ├── configmap.yaml
│   └── ingress.yaml
└── overlays/
    ├── development/
    │   ├── kustomization.yaml
    │   ├── replica-patch.yaml
    │   └── namespace.yaml
    ├── staging/
    │   ├── kustomization.yaml
    │   ├── replica-patch.yaml
    │   ├── resource-patch.yaml
    │   └── namespace.yaml
    └── production/
        ├── kustomization.yaml
        ├── replica-patch.yaml
        ├── resource-patch.yaml
        ├── hpa.yaml
        └── namespace.yaml
```

## Base Configuration

### Base Kustomization

```yaml
# base/kustomization.yaml
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

metadata:
  name: myapp-base

# Resources to include
resources:
  - deployment.yaml
  - service.yaml
  - configmap.yaml
  - ingress.yaml

# Common labels applied to all resources
commonLabels:
  app: myapp
  managed-by: kustomize

# Common annotations
commonAnnotations:
  version: "1.0.0"
  team: platform

# Name prefix for all resources
namePrefix: myapp-

# Default namespace (can be overridden in overlays)
namespace: default

# Image transformations
images:
  - name: myapp
    newName: registry.example.com/myapp
    newTag: latest
```

### Base Deployment

```yaml
# base/deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: deployment
spec:
  replicas: 1
  selector:
    matchLabels:
      app: myapp
  template:
    metadata:
      labels:
        app: myapp
    spec:
      containers:
      - name: myapp
        image: myapp:latest
        ports:
        - containerPort: 8080
          name: http
        env:
        - name: LOG_LEVEL
          valueFrom:
            configMapKeyRef:
              name: config
              key: log-level
        resources:
          requests:
            memory: "128Mi"
            cpu: "100m"
          limits:
            memory: "256Mi"
            cpu: "200m"
        livenessProbe:
          httpGet:
            path: /health
            port: http
          initialDelaySeconds: 30
          periodSeconds: 10
        readinessProbe:
          httpGet:
            path: /ready
            port: http
          initialDelaySeconds: 5
          periodSeconds: 5
```

### Base Service

```yaml
# base/service.yaml
apiVersion: v1
kind: Service
metadata:
  name: service
spec:
  type: ClusterIP
  ports:
  - port: 80
    targetPort: http
    protocol: TCP
    name: http
  selector:
    app: myapp
```

### Base ConfigMap

```yaml
# base/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: config
data:
  log-level: "info"
  cache-enabled: "true"
  timeout: "30"
```

### Base Ingress

```yaml
# base/ingress.yaml
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: ingress
  annotations:
    kubernetes.io/ingress.class: nginx
spec:
  rules:
  - host: myapp.example.com
    http:
      paths:
      - path: /
        pathType: Prefix
        backend:
          service:
            name: myapp-service
            port:
              number: 80
```

## Development Overlay

### Development Kustomization

```yaml
# overlays/development/kustomization.yaml
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

# Reference the base
resources:
  - ../../base
  - namespace.yaml

# Override namespace
namespace: development

# Development-specific labels
commonLabels:
  environment: development
  cost-center: engineering

# Development-specific annotations
commonAnnotations:
  deployed-by: ci-cd
  environment: dev

# Name suffix for development resources
nameSuffix: -dev

# Image overrides for development
images:
  - name: myapp
    newName: registry.example.com/myapp
    newTag: dev-latest

# ConfigMap overrides
configMapGenerator:
  - name: config
    behavior: merge
    literals:
      - log-level=debug
      - cache-enabled=false
      - debug-mode=true

# Replica overrides
replicas:
  - name: myapp-deployment
    count: 1

# Strategic merge patches
patches:
  - path: replica-patch.yaml
    target:
      kind: Deployment
      name: myapp-deployment

# Inline patches
patchesStrategicMerge:
  - |-
    apiVersion: apps/v1
    kind: Deployment
    metadata:
      name: deployment
    spec:
      template:
        spec:
          containers:
          - name: myapp
            env:
            - name: ENVIRONMENT
              value: development
            - name: DEBUG
              value: "true"
```

### Development Namespace

```yaml
# overlays/development/namespace.yaml
apiVersion: v1
kind: Namespace
metadata:
  name: development
  labels:
    environment: development
    team: platform
```

### Development Replica Patch

```yaml
# overlays/development/replica-patch.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: deployment
spec:
  replicas: 1
  template:
    spec:
      containers:
      - name: myapp
        resources:
          requests:
            memory: "64Mi"
            cpu: "50m"
          limits:
            memory: "128Mi"
            cpu: "100m"
```

## Staging Overlay

### Staging Kustomization

```yaml
# overlays/staging/kustomization.yaml
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

resources:
  - ../../base
  - namespace.yaml

namespace: staging

commonLabels:
  environment: staging
  cost-center: engineering

commonAnnotations:
  deployed-by: ci-cd
  environment: staging

nameSuffix: -staging

images:
  - name: myapp
    newName: registry.example.com/myapp
    newTag: staging-v1.2.3

configMapGenerator:
  - name: config
    behavior: merge
    literals:
      - log-level=info
      - cache-enabled=true
      - cache-ttl=300

replicas:
  - name: myapp-deployment
    count: 2

patches:
  - path: replica-patch.yaml
    target:
      kind: Deployment
      name: myapp-deployment
  - path: resource-patch.yaml
    target:
      kind: Deployment
      name: myapp-deployment

patchesStrategicMerge:
  - |-
    apiVersion: apps/v1
    kind: Deployment
    metadata:
      name: deployment
    spec:
      template:
        metadata:
          annotations:
            prometheus.io/scrape: "true"
            prometheus.io/port: "8080"
        spec:
          containers:
          - name: myapp
            env:
            - name: ENVIRONMENT
              value: staging
            - name: METRICS_ENABLED
              value: "true"
          affinity:
            podAntiAffinity:
              preferredDuringSchedulingIgnoredDuringExecution:
              - weight: 100
                podAffinityTerm:
                  labelSelector:
                    matchExpressions:
                    - key: app
                      operator: In
                      values:
                      - myapp
                  topologyKey: kubernetes.io/hostname
```

### Staging Replica Patch

```yaml
# overlays/staging/replica-patch.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: deployment
spec:
  replicas: 2
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxSurge: 1
      maxUnavailable: 0
```

### Staging Resource Patch

```yaml
# overlays/staging/resource-patch.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: deployment
spec:
  template:
    spec:
      containers:
      - name: myapp
        resources:
          requests:
            memory: "256Mi"
            cpu: "200m"
          limits:
            memory: "512Mi"
            cpu: "500m"
```

## Production Overlay

### Production Kustomization

```yaml
# overlays/production/kustomiza