k8s【PostStart钩子】

PostStart

容器生命周期钩子(Container Lifecycle Hooks)监听容器生命周期的特定事件,并在事件发生时

spec:
  containers:
  - name: lifecycle-demo-container
    image: nginx
    lifecycle:
      postStart:
        exec:
          command: ["/bin/sh", "-c", "echo Hello from the postStart handler > /usr/share/message"]
      preStop:
        exec:
          command: ["/usr/sbin/nginx","-s","quit"]

执行已注册的回调函数。支持两种钩子:

  • postStart: 容器启动后执行,注意由于是异步执行,它无法保证一定在ENTRYPOINT之后运行。如果失败,容器会被杀死,并根据RestartPolicy决定是否重启
  • preStop:容器停止前执行,常用于资源清理。如果失败,容器同样也会被杀死

而钩子的回调函数支持两种方式:

  • exec:在容器内执行命令
  • httpGet:向指定URL发起GET请求

关于postStart异步执行测试

apiVersion: v1
kind: Pod
metadata:
  name: test-post-start
spec:
  containers:
  - name: test-post-start-container
    image: busybox
    command: ["/bin/sh", "-c", "sleep 5 && echo $(date) 'written by entrypoint' >> log.log && sleep 600"]
    lifecycle:
      postStart:
        exec:
          command: ["/bin/sh", "-c", "sleep 10 && echo $(date) 'written by post start' >> log.log"]

创建上面的pod,通过进入pod,查看log.log打印日志,证明:

  • PostStart是否会挡住主进程的启动
  • PostStart是否是异步执行

如果 PostStart 会阻挡 ENTRYPOINT 的启动,则日志文件内容应该是:

(时间点 T)written by post start
(时间点 T + 约 10 秒)written by entrypoint

否则内容应该是:

(时间点 T)written by entrypoint
(时间点 T + 约 5 秒)written by post start
```log

实验结果:
```log
/ # cat log.log
Thu Jun 4 06:14:50 UTC 2020 written by entrypoint
Thu Jun 4 06:14:55 UTC 2020 written by post start

修改YML

apiVersion: v1
kind: Pod
metadata:
  name: test-post-start
spec:
  containers:
  - name: test-post-start-container
    image: busybox
    command: ["/bin/sh", "-c", "sleep 15 && echo $(date) 'written by entrypoint' >> log.log && sleep 600"]
    lifecycle:
      postStart:
        exec:
          command: ["/bin/sh", "-c", "sleep 10 && echo $(date) 'written by post start' >> log.log"]

如果 PostStart 不是异步执行,则日志文件内容应该是:

(时间点 T)written by entrypoint
(时间点 T + 约 5 秒)written by post start
```log

否则内容应该是:
```log
(时间点 T)written by post start
(时间点 T + 约 5 秒)written by entrypoint

实验结果:

[root@master k8s]# kubectl exec -it test-post-start sh
/ # cat log.log
Thu Jun 4 06:17:54 UTC 2020 written by post start
Thu Jun 4 06:17:59 UTC 2020 written by entrypoint
/ # 

实验结论

  • PostStart不会挡住主进程的启动
  • PostStart是异步执行
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