Kubernetes Service：服务发现与负载均衡

Service概述

Service是Kubernetes中为一组Pod提供稳定访问入口的抽象。它通过标签选择器（Selector）关联Pod，并提供负载均衡能力。

Service类型

ClusterIP（默认）

集群内部访问的虚拟IP：

# clusterip-service.yaml
apiVersion: v1
kind: Service
metadata:
  name: backend-service
spec:
  type: ClusterIP
  selector:
    app: backend
  ports:
  - port: 80
    targetPort: 8080
    protocol: TCP

NodePort

在每个节点上开放固定端口：

# nodeport-service.yaml
apiVersion: v1
kind: Service
metadata:
  name: web-service
spec:
  type: NodePort
  selector:
    app: web
  ports:
  - port: 80
    targetPort: 80
    nodePort: 30080

LoadBalancer

使用云平台负载均衡器：

# loadbalancer-service.yaml
apiVersion: v1
kind: Service
metadata:
  name: api-service
  annotations:
    service.beta.kubernetes.io/aws-load-balancer-type: nlb
spec:
  type: LoadBalancer
  selector:
    app: api
  ports:
  - port: 443
    targetPort: 8443

ExternalName

映射到外部域名：

# externalname-service.yaml
apiVersion: v1
kind: Service
metadata:
  name: external-db
spec:
  type: ExternalName
  externalName: db.example.com

Headless Service

不分配ClusterIP，直接返回Pod IP：

# headless-service.yaml
apiVersion: v1
kind: Service
metadata:
  name: stateful-service
spec:
  clusterIP: None
  selector:
    app: stateful
  ports:
  - port: 80
    targetPort: 8080

# 查看Headless Service的Pod IP
kubectl get endpoints stateful-service

多端口Service

# multi-port-service.yaml
apiVersion: v1
kind: Service
metadata:
  name: multi-port-service
spec:
  selector:
    app: myapp
  ports:
  - name: http
    port: 80
    targetPort: 8080
  - name: https
    port: 443
    targetPort: 8443
  - name: metrics
    port: 9090
    targetPort: 9090

服务发现

环境变量

# Pod内会自动注入环境变量
# BACKEND_SERVICE_SERVICE_HOST=10.96.0.100
# BACKEND_SERVICE_SERVICE_PORT=80

DNS解析

# 集群内DNS解析
# 格式: <service-name>.<namespace>.svc.cluster.local

# 同一命名空间内
curl http://backend-service

# 跨命名空间
curl http://backend-service.production.svc.cluster.local

# Headless Service返回所有Pod IP
nslookup stateful-service.default.svc.cluster.local

Endpoints

Endpoints是Service关联的Pod IP列表：

# 查看Endpoints
kubectl get endpoints backend-service

# 手动创建Endpoints
kubectl create -f - <<EOF
apiVersion: v1
kind: Endpoints
metadata:
  name: external-service
subsets:
- addresses:
  - ip: 192.168.1.100
  - ip: 192.168.1.101
  ports:
  - port: 80
    protocol: TCP
EOF

实践：完整服务架构

# 完整的三层应用架构
---
# 前端Service
apiVersion: v1
kind: Service
metadata:
  name: frontend-service
spec:
  type: LoadBalancer
  selector:
    app: frontend
  ports:
  - port: 80
    targetPort: 80
---
# 前端Deployment
apiVersion: apps/v1
kind: Deployment
metadata:
  name: frontend
spec:
  replicas: 3
  selector:
    matchLabels:
      app: frontend
  template:
    metadata:
      labels:
        app: frontend
    spec:
      containers:
      - name: nginx
        image: nginx:alpine
        ports:
        - containerPort: 80
---
# 后端Service
apiVersion: v1
kind: Service
metadata:
  name: backend-service
spec:
  selector:
    app: backend
  ports:
  - port: 8080
    targetPort: 8080
---
# 后端Deployment
apiVersion: apps/v1
kind: Deployment
metadata:
  name: backend
spec:
  replicas: 3
  selector:
    matchLabels:
      app: backend
  template:
    metadata:
      labels:
        app: backend
    spec:
      containers:
      - name: api
        image: myapi:1.0
        ports:
        - containerPort: 8080
        env:
        - name: DB_HOST
          value: "mysql-service"
---
# 数据库Service（Headless）
apiVersion: v1
kind: Service
metadata:
  name: mysql-service
spec:
  clusterIP: None
  selector:
    app: mysql
  ports:
  - port: 3306
    targetPort: 3306

Service故障排查

# 查看Service详情
kubectl describe svc backend-service

# 查看Endpoints
kubectl get endpoints backend-service

# 测试Service连通性
kubectl run curl-test --rm -it --image=curlimages/curl -- \
    curl http://backend-service:8080/health

# 检查DNS解析
kubectl run dns-test --rm -it --image=busybox -- \
    nslookup backend-service

# 查看kube-proxy日志
kubectl logs -n kube-system -l k8s-app=kube-proxy

总结

Service是Kubernetes实现服务发现和负载均衡的核心机制。理解不同类型的Service及其适用场景，是构建可靠微服务架构的关键。