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k8s-liveness-readiness-startup-probes

readiness probe - This is simply a signal to inform Kubernetes when to put this pod behind the load balancer and when to put this service behind the proxy to serve traffic. If you put an application behind the load balancer before it’s ready, then a user can reach this pod but won’t get the expected response of a healthy server.

liveness probe - The liveness probe let’s Kubernetes know if the pod is in a healthy state. If it isn’t healthy, then Kubernetes should restart it.

startup probes - The kubelet uses startup probes to know when a container application has started. If such a probe is configured, liveness and readiness probes do not start until it succeeds, making sure those probes don’t interfere with the application startup. This can be used to adopt liveness checks on slow starting containers, avoiding them getting killed by the kubelet before they are up and running.

verify

  • create deployment

    apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  selector:
    matchLabels:
      app: nginx
  replicas: 2
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
        - name: init-container
          image: busybox
          command: 
            - 'sh'
            - '-c'
            - |
              echo "Hello World" > /usr/share/nginx/html/index.html
              echo "Hello liveness" > /usr/share/nginx/html/liveness.html
              echo "Hello readiness" > /usr/share/nginx/html/readiness.html
              echo "Hello startup" > /usr/share/nginx/html/startup.html
              echo "Hello ELB" > /usr/share/nginx/html/elbcheck.html
              while true ; do sleep 60; echo ===; done
          volumeMounts:
          - name: nginx-html
            mountPath: /usr/share/nginx/html
        - name: nginx
          image: nginx:latest
          ports:
            - containerPort: 80
          volumeMounts:
            - name: nginx-html
              mountPath: /usr/share/nginx/html
          startupProbe:
            httpGet:
              path: /startup.html
              port: 80
            initialDelaySeconds: 2
            periodSeconds: 2
            failureThreshold: 3
          readinessProbe:
            httpGet:
              path: /readiness.html
              port: 80
            initialDelaySeconds: 10
            periodSeconds: 10
            failureThreshold: 1
          livenessProbe:
            httpGet:
              path: /liveness.html
              port: 80
            initialDelaySeconds: 15
            periodSeconds: 15
            failureThreshold: 1
      volumes:
        - name: nginx-html
          emptyDir: {}

  • ELB / startup / liveness / readiness probe 分别检查不同的 html

startup failed

  • 启动 probe 在 pod 正常启动后失效，不再检查
  • 除非 liveness 检查失效，container 被重启后，才会再次 check

liveness failed

  • nginx container 重启
  • init container 未重启
  • ip address 不变

readiness failed

  • remove from service / ELB
  • nginx container 未重启
  • init container 未重启
  • ip address 不变

结论

  • startup probe 用于慢速容器启动检查
  • readiness probe 检查基本雷同与 ELB 自带的健康检查，区别在于 readiness 检查由 kubelet 发起，在节点本地完成；ELB 的健康检查通过网络完成。使用其一即可
  • liveness probe 是唯一检查失败后会重启容器，如果 pod 有多个容器，则只检查启用了该 probe 的 container。pod 地址保持不变。
  • 场景1：ELB + liveness
    • ELB 检查失效 小于 liveness 检查失效
      • 允许服务临时不可用，ELB 检查失败将不再将流量转到该 POD。POD 在 liveness 检查失效前如果能回复，则自动加回（满足 ELB 检查成功）
    • ELB 检查失效 大于 liveness 检查失效
      • 服务将被重启
  • pod readiness gate 功能在 1.29 集群中未表现为加快 pod 注册到 ELB
    • 在 namespace 层面添加标签后，还需要额外配置 objectSelector 才能正常抓取到 default namespace 中的 POD

prestop-hook-

attachments/k8s-liveness-readiness-startup-probes/IMG-k8s-liveness-readiness-startup-probes.png

refer