06 - Deployment and Scaling of Modular Monolith: Smart Scaling

# CI/CD Pipeline for modular monolith
# A single artifact for all environments

# .github/workflows/deploy.yml
name: Build and Deploy
on:
  push:
    branches: [main]

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - name: Build application
        run: ./gradlew build

      - name: Build Docker image
        run: |
          docker build -t ecommerce-app:$GITHUB_SHA .

      - name: Push to registry
        run: |
          docker push registry.example.com/ecommerce-app:$GITHUB_SHA

  deploy-staging:
    needs: build
    runs-on: ubuntu-latest
    steps:
      - name: Deploy to staging
        run: |
          kubectl set image deployment/ecommerce \
            app=registry.example.com/ecommerce-app:$GITHUB_SHA \
            --namespace staging

  deploy-production:
    needs: deploy-staging
    runs-on: ubuntu-latest
    environment: production  # requires manual approval
    steps:
      - name: Deploy to production
        run: |
          kubectl set image deployment/ecommerce \
            app=registry.example.com/ecommerce-app:$GITHUB_SHA \
            --namespace production
    

# Kubernetes: Blue-Green deployment with Service switch
# Blue deployment (currently active)
apiVersion: apps/v1
kind: Deployment
metadata:
  name: ecommerce-blue
  labels:
    app: ecommerce
    version: blue
spec:
  replicas: 4
  selector:
    matchLabels:
      app: ecommerce
      version: blue
  template:
    metadata:
      labels:
        app: ecommerce
        version: blue
    spec:
      containers:
      - name: app
        image: registry.example.com/ecommerce-app:v1.2.0
        ports:
        - containerPort: 8080
        readinessProbe:
          httpGet:
            path: /actuator/health/readiness
            port: 8080
          initialDelaySeconds: 30
          periodSeconds: 10
---
# Service pointing to the active deployment
apiVersion: v1
kind: Service
metadata:
  name: ecommerce-service
spec:
  selector:
    app: ecommerce
    version: blue    # Change to "green" for the switch
  ports:
  - port: 80
    targetPort: 8080
    

// Feature Flags with Spring Boot and Unleash

@Service
class OrderServiceImpl implements OrderModuleApi {

    private final FeatureFlagService featureFlags;

    @Override
    public OrderDto createOrder(CreateOrderCommand cmd) {
        Order order = Order.create(cmd);

        // Feature flag: new pricing system
        if (featureFlags.isEnabled("new-pricing-engine")) {
            order.applyNewPricing(pricingEngine.calculate(cmd));
        } else {
            order.applyLegacyPricing(cmd.getItems());
        }

        // Feature flag: push notification (gradual)
        if (featureFlags.isEnabled("push-notifications",
                cmd.getUserId())) {
            notificationModule.sendPush(order);
        }

        orderRepository.save(order);
        return order.toDto();
    }
}

// Feature Flags configuration with Unleash
@Configuration
class FeatureFlagConfig {

    @Bean
    public Unleash unleash() {
        return new DefaultUnleash(
            UnleashConfig.builder()
                .appName("ecommerce-app")
                .instanceId("instance-1")
                .unleashAPI("http://unleash:4242/api")
                .build()
        );
    }
}
    

# HPA for the modular monolith
apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
metadata:
  name: ecommerce-hpa
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: ecommerce-app
  minReplicas: 3
  maxReplicas: 12
  metrics:
  # Scale based on CPU
  - type: Resource
    resource:
      name: cpu
      target:
        type: Utilization
        averageUtilization: 70
  # Scale based on custom metrics
  - type: Pods
    pods:
      metric:
        name: http_requests_per_second
      target:
        type: AverageValue
        averageValue: "1000"
  behavior:
    scaleUp:
      stabilizationWindowSeconds: 60
      policies:
      - type: Pods
        value: 2
        periodSeconds: 60
    scaleDown:
      stabilizationWindowSeconds: 300
      policies:
      - type: Pods
        value: 1
        periodSeconds: 120
    

# Complete deployment for modular monolith
apiVersion: apps/v1
kind: Deployment
metadata:
  name: ecommerce-app
  labels:
    app: ecommerce
spec:
  replicas: 4
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxSurge: 1
      maxUnavailable: 0
  selector:
    matchLabels:
      app: ecommerce
  template:
    metadata:
      labels:
        app: ecommerce
    spec:
      containers:
      - name: app
        image: registry.example.com/ecommerce-app:latest
        ports:
        - containerPort: 8080
        env:
        - name: SPRING_PROFILES_ACTIVE
          value: "production"
        - name: JAVA_OPTS
          value: "-Xms512m -Xmx2g -XX:+UseG1GC"
        resources:
          requests:
            memory: "1Gi"
            cpu: "500m"
          limits:
            memory: "2Gi"
            cpu: "1000m"
        readinessProbe:
          httpGet:
            path: /actuator/health/readiness
            port: 8080
          initialDelaySeconds: 30
          periodSeconds: 10
        livenessProbe:
          httpGet:
            path: /actuator/health/liveness
            port: 8080
          initialDelaySeconds: 60
          periodSeconds: 30
      topologySpreadConstraints:
      - maxSkew: 1
        topologyKey: topology.kubernetes.io/zone
        whenUnsatisfiable: DoNotSchedule
        labelSelector:
          matchLabels:
            app: ecommerce