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