[Kubernetes] HPA(Horizontal Pod Autoscaler) 메트릭 기반 파드 확장

파드 → kubelet(cAdvisor) → metrics-server → Metrics API → HPA 컨트롤러

# metrics-server 설치 (로컬 클러스터에서는 TLS 검증 비활성화 필요할 수 있음)
kubectl apply -f https://github.com/kubernetes-sigs/metrics-server/releases/latest/download/components.yaml

# 로컬 Kind/Minikube 클러스터용: TLS 검증 비활성화 패치
kubectl patch deployment metrics-server -n kube-system \
  --type='json' \
  -p='[{"op":"add","path":"/spec/template/spec/containers/0/args/-","value":"--kubelet-insecure-tls"}]'

# 설치 확인 (30초~1분 소요)
kubectl rollout status deployment/metrics-server -n kube-system

# 메트릭 수집 확인
kubectl top nodes
# NAME          CPU(cores)   CPU%   MEMORY(bytes)   MEMORY%
# kind-worker   55m          1%     512Mi           26%

kubectl top pods -A
# NAMESPACE     NAME                     CPU(cores)   MEMORY(bytes)
# kube-system   coredns-xxx              5m           18Mi

# php-apache-deployment.yaml
# 부하 테스트에 자주 쓰이는 예시 이미지
apiVersion: apps/v1
kind: Deployment
metadata:
  name: php-apache
  namespace: hpa-demo
spec:
  replicas: 1
  selector:
    matchLabels:
      app: php-apache
  template:
    metadata:
      labels:
        app: php-apache
    spec:
      containers:
      - name: php-apache
        image: registry.k8s.io/hpa-example
        ports:
        - containerPort: 80
        resources:
          requests:
            cpu: "200m"    # HPA 계산 기준값 (필수!)
            memory: "128Mi"
          limits:
            cpu: "500m"
            memory: "256Mi"
---
apiVersion: v1
kind: Service
metadata:
  name: php-apache
  namespace: hpa-demo
spec:
  ports:
  - port: 80
  selector:
    app: php-apache

kubectl apply -f php-apache-deployment.yaml

# 방법 1: kubectl autoscale (빠른 생성)
kubectl autoscale deployment php-apache \
  -n hpa-demo \
  --cpu-percent=50 \
  --min=1 \
  --max=10

# 방법 2: YAML로 생성 (권장, GitOps 관리 가능)
cat <<EOF | kubectl apply -f -
apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
metadata:
  name: php-apache-hpa
  namespace: hpa-demo
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: php-apache
  minReplicas: 1
  maxReplicas: 10
  metrics:
  - type: Resource
    resource:
      name: cpu
      target:
        type: Utilization
        averageUtilization: 50   # CPU 사용률 50% 목표
EOF

# HPA 상태 확인
kubectl get hpa -n hpa-demo
# NAME              REFERENCE              TARGETS   MINPODS   MAXPODS   REPLICAS   AGE
# php-apache-hpa    Deployment/php-apache   3%/50%    1         10        1          30s

필요 파드 수 = ceil(현재파드수 × (현재사용률 / 목표사용률))

예: 파드 1개, 현재 CPU 150%, 목표 50%
→ ceil(1 × (150 / 50)) = ceil(3) = 3개

# 스케일링 정책 커스텀
apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
metadata:
  name: php-apache-hpa-custom
  namespace: hpa-demo
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: php-apache
  minReplicas: 2    # 최소 2개 유지 (HA 보장)
  maxReplicas: 20
  metrics:
  - type: Resource
    resource:
      name: cpu
      target:
        type: Utilization
        averageUtilization: 60
  behavior:
    scaleUp:
      stabilizationWindowSeconds: 0   # 즉시 스케일 업
      policies:
      - type: Percent
        value: 100                    # 한 번에 최대 100% 증가
        periodSeconds: 15
    scaleDown:
      stabilizationWindowSeconds: 300 # 5분 안정화 후 스케일 다운
      policies:
      - type: Pods
        value: 2                      # 한 번에 최대 2개씩 감소
        periodSeconds: 60