[Kubernetes] DaemonSet과 상태 저장형 앱 배포를 위한 StatefulSet 완벽 분석

# fluentd-daemonset.yaml
apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: fluentd
  namespace: kube-system
  labels:
    app: fluentd
spec:
  selector:
    matchLabels:
      app: fluentd
  template:
    metadata:
      labels:
        app: fluentd
    spec:
      tolerations:
      # 마스터 노드(control-plane)에도 배포하려면 필요
      - key: node-role.kubernetes.io/control-plane
        effect: NoSchedule
        operator: Exists
      containers:
      - name: fluentd
        image: fluent/fluentd-kubernetes-daemonset:v1.16-debian-elasticsearch8-1
        env:
        - name: FLUENT_ELASTICSEARCH_HOST
          value: "elasticsearch.logging.svc.cluster.local"
        - name: FLUENT_ELASTICSEARCH_PORT
          value: "9200"
        resources:
          requests:
            cpu: "100m"
            memory: "200Mi"
          limits:
            cpu: "500m"
            memory: "500Mi"
        volumeMounts:
        - name: varlog
          mountPath: /var/log
        - name: varlibdockercontainers
          mountPath: /var/lib/docker/containers
          readOnly: true
      volumes:
      - name: varlog
        hostPath:
          path: /var/log       # 노드의 로그 디렉토리를 직접 마운트
      - name: varlibdockercontainers
        hostPath:
          path: /var/lib/docker/containers

kubectl apply -f fluentd-daemonset.yaml

# 각 노드에 1개씩 파드가 배포됐는지 확인
kubectl get pods -n kube-system -l app=fluentd -o wide
# NAME            READY   STATUS    NODE
# fluentd-7k2p9   1/1     Running   worker-1
# fluentd-9m3q7   1/1     Running   worker-2
# fluentd-xn8tv   1/1     Running   worker-3

# DaemonSet 상태 요약
kubectl get daemonset fluentd -n kube-system
# NAME      DESIRED   CURRENT   READY   UP-TO-DATE   AVAILABLE   NODE SELECTOR   AGE
# fluentd   3         3         3       3            3           <none>          2m

# 특정 노드에만 배포 (nodeSelector 사용)
# 예: ssd=true 레이블이 있는 노드에만
kubectl label node worker-1 ssd=true

# mysql-statefulset.yaml
apiVersion: v1
kind: Service
metadata:
  name: mysql
  namespace: ds-sts-demo
  labels:
    app: mysql
spec:
  ports:
  - port: 3306
  clusterIP: None    # ← Headless Service (클러스터 IP 없음)
  selector:
    app: mysql
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: mysql
  namespace: ds-sts-demo
spec:
  serviceName: "mysql"    # Headless Service 이름과 일치해야 함
  replicas: 3
  selector:
    matchLabels:
      app: mysql
  template:
    metadata:
      labels:
        app: mysql
    spec:
      containers:
      - name: mysql
        image: mysql:8.0
        env:
        - name: MYSQL_ROOT_PASSWORD
          valueFrom:
            secretKeyRef:
              name: mysql-secret
              key: password
        ports:
        - containerPort: 3306
        volumeMounts:
        - name: data
          mountPath: /var/lib/mysql
        resources:
          requests:
            cpu: "500m"
            memory: "1Gi"
          limits:
            cpu: "2"
            memory: "2Gi"
  volumeClaimTemplates:     # ← 파드별 독립 PVC 자동 생성
  - metadata:
      name: data
    spec:
      accessModes: ["ReadWriteOnce"]
      storageClassName: "standard"
      resources:
        requests:
          storage: 10Gi

kubectl apply -f mysql-statefulset.yaml

# 파드 생성 순서 관찰 (0 → 1 → 2 순서로)
kubectl get pods -n ds-sts-demo -w
# NAME      READY   STATUS    AGE
# mysql-0   0/1     Pending   0s
# mysql-0   0/1     Running   3s
# mysql-0   1/1     Running   10s   ← 0번이 Ready된 후에야
# mysql-1   0/1     Pending   11s   ← 1번 생성 시작
# mysql-1   1/1     Running   25s
# mysql-2   0/1     Pending   26s
# mysql-2   1/1     Running   40s

# 각 파드별 독립 PVC 생성 확인
kubectl get pvc -n ds-sts-demo
# NAME           STATUS   VOLUME     CAPACITY   ACCESS MODES   STORAGECLASS
# data-mysql-0   Bound    pvc-xxx    10Gi       RWO            standard
# data-mysql-1   Bound    pvc-yyy    10Gi       RWO            standard
# data-mysql-2   Bound    pvc-zzz    10Gi       RWO            standard

일반 Service DNS:
mysql.ds-sts-demo.svc.cluster.local → 단일 ClusterIP (로드밸런싱)

Headless Service DNS:
mysql.ds-sts-demo.svc.cluster.local → 전체 파드 IP 목록
mysql-0.mysql.ds-sts-demo.svc.cluster.local → mysql-0 파드 IP
mysql-1.mysql.ds-sts-demo.svc.cluster.local → mysql-1 파드 IP
mysql-2.mysql.ds-sts-demo.svc.cluster.local → mysql-2 파드 IP

# DNS 확인 (클러스터 내 busybox 파드에서)
kubectl run dns-test --image=busybox:1.36 -it --rm -n ds-sts-demo \
  -- nslookup mysql-0.mysql.ds-sts-demo.svc.cluster.local
# Server:    10.96.0.10
# Address 1: 10.96.0.10 kube-dns.kube-system.svc.cluster.local
# Name:      mysql-0.mysql.ds-sts-demo.svc.cluster.local
# Address 1: 10.244.1.5 mysql-0.mysql.ds-sts-demo.svc.cluster.local

# MySQL 레플리케이션 설정 예시 (mysql-1에서 실행)
kubectl exec mysql-1 -n ds-sts-demo -- mysql -uroot -p$MYSQL_ROOT_PASSWORD -e \
  "CHANGE MASTER TO MASTER_HOST='mysql-0.mysql.ds-sts-demo.svc.cluster.local', MASTER_PORT=3306;"