Container lifeCycle中的postStart和preStop两种hook小结
Contents
一 postStart hook小结
1 应用场景是什么
对于那些在Container启动之后，在正式对外提供服务之后，需要预热的场景，可以通过在Container级别配置postStart hook来实现。比如：一个NGINX服务启动之后，在正式对外提供之前，可以先在Container内部做一个预热，通过执行某个命令请求，使得服务可以提前缓存一些数据。这样，正式对外提供服务时，可以直接使用
2 postStart类型
postStart只支持httpGet和exec类型的probe，不支持tcpSocket。
3 postStart工作机制
它随着Container中的主进程的运行而运行。如果postStart hook执行失败了，那么它会认为整个Container失败。Container将会被重启。
4 postStart作用范围
它是作用在pod中的Container级别的。
5 postStart举例
[root@master-node Chapter06]# pwd
/root/kubernetes-in-action-2nd-edition/Chapter06
[root@master-node Chapter06]# cat pod.quote-poststart.yaml
apiVersion: v1
kind: Pod
metadata:
name: quote-poststart
spec:
containers:
- name: nginx
image: nginx:alpine
ports:
- name: http
containerPort: 80
lifecycle:
postStart:
exec:
command:
- sh
- -c
- |
apk add fortune && \
curl -O https://luksa.github.io/kiada/book-quotes.txt && \
curl -O https://luksa.github.io/kiada/book-quotes.txt.dat && \
fortune book-quotes.txt > /usr/share/nginx/html/quote
[root@master-node Chapter06]# 执行，并启动：
[root@master-node Chapter06]# kubectl apply -f pod.quote-poststart.yaml pod/quote-poststart created [root@master-node Chapter06]# =================================== [root@master-node ~]# kubectl get events -w LAST SEEN TYPE REASON OBJECT MESSAGE 21s Normal Scheduled pod/quote-poststart Successfully assigned default/quote-poststart to node-1 19s Normal Pulled pod/quote-poststart Container image "nginx:alpine" already present on machine 19s Normal Created pod/quote-poststart Created container nginx 19s Normal Started pod/quote-poststart Started container nginx ... 0s Warning FailedPostStartHook pod/quote-poststart Exec lifecycle hook ([sh -c apk add fortune && \... 0s Normal Killing pod/quote-poststart FailedPostStartHook 0s Normal Pulled pod/quote-poststart Container image "nginx:alpine" already present on machine 0s Normal Created pod/quote-poststart Created container nginx 0s Normal Started pod/quote-poststart Started container nginx ================================== [root@master-node Chapter06]# kubectl get pods -w NAME READY STATUS RESTARTS AGE quote-poststart 0/1 PostStartHookError 0 (91s ago) 6m18s ... quote-poststart 1/1 Running 1 (4m32s ago) 9m19s
同时，从kubectl describe pod quote-poststart 可以看到postStart报错：
[root@master-node Chapter06]# kubectl describe pod quote-poststart Name: quote-poststart Namespace: default Priority: 0 Node: node-1/172.16.11.148 Start Time: Mon, 30 May 2022 09:11:19 +0800 ... Events: Type Reason Age From Message ---- ------ ---- ---- ------- Normal Scheduled 5m34s default-scheduler Successfully assigned default/quote-poststart to node-1 Normal Pulled 47s (x2 over 5m32s) kubelet Container image "nginx:alpine" already present on machine Warning FailedPostStartHook 47s kubelet Exec lifecycle hook ([sh -c apk add fortune && \ curl -O https://luksa.github.io/kiada/book-quotes.txt && \ curl -O https://luksa.github.io/kiada/book-quotes.txt.dat && \ fortune book-quotes.txt > /usr/share/nginx/html/quote ]) for Container "nginx" in Pod "quote-poststart_default(0a375afc-e51b-4033-bd56-d5c932a4171b)" failed - error: command 'sh -c apk add fortune && \ curl -O https://luksa.github.io/kiada/book-quotes.txt && \ curl -O https://luksa.github.io/kiada/book-quotes.txt.dat && \ fortune book-quotes.txt > /usr/share/nginx/html/quote ' exited with 2: ERROR: https://dl-cdn.alpinelinux.org/alpine/v3.15/main: BAD signature WARNING: Ignoring https://dl-cdn.alpinelinux.org/alpine/v3.15/main: No such file or directory ERROR: unable to select packages: , message: "fetch https://dl-cdn.alpinelinux.org/alpine/v3.15/main/x86_64/APKINDEX.tar.gz\nfetch https://dl-cdn.alpinelinux.org/alpine/v3.15/community/x86_64/APKINDEX.tar.gz\n so:libbsd.so.0 (no such package):\n required by: fortune-0.1-r1[so:libbsd.so.0]\nERROR: https://dl-cdn.alpinelinux.org/alpine/v3.15/main: BAD signature\nWARNING: Ignoring https://dl-cdn.alpinelinux.org/alpine/v3.15/main: No such file or directory\nERROR: unable to select packages:\n" Normal Killing 47s kubelet FailedPostStartHook Normal Created 46s (x2 over 5m32s) kubelet Created container nginx Normal Started 46s (x2 over 5m32s) kubelet Started container nginx [root@master-node Chapter06]#
原来报错是，postStart hook中的下载命令失败了，由于网络原因。此时，我们进入pod中的Container里，却是可以看到NGINX已经正常运行的：
[root@master-node Chapter06]# kubectl exec -it quote-poststart -- /bin/sh
/ # ps aux
PID USER TIME COMMAND
1 root 0:00 nginx: master process nginx -g daemon off;
31 nginx 0:00 nginx: worker process
32 nginx 0:00 nginx: worker process
33 root 0:00 sh -c apk add fortune && \ curl -O https://luksa.github.io/kiada/book-quotes.txt && \ curl -O https://luksa.github.io/kiada/book-quotes.txt.dat && \ fortune
38 root 0:00 apk add fortune
44 root 0:00 /bin/sh
49 root 0:00 ps aux
/ # exit
[root@master-node Chapter06]#二 preStop hook小结
1 应用场景
如果想要在停止Container中的应用之前，做一些“压轴收尾”工作。那么，可以通过在Container中指定lifecycle字段，并指定preStop类型的探针来实现。比如：想要优雅的停止一个NGINX的服务，可以通过在NGINX Container里指定preStop的类型为exec，并执行nginx -s quit命令，这个命令会告知NGINX，对于新起的请求就不响应了，对于已经建立会话的连接，直到client正常退出，而不是直接杀掉所有会话连接。
2 preStop类型
httpGet和exec类型
3 preStop工作机制
它会在Container退出之前，执行该preStop hook。需要注意的是 ，即使preStop hook执行失败了，它也不会导致Container进入terminate状态。
4 preStop作用范围
Container级别
5 preStop举例
[root@master-node Chapter06]# pwd
/root/kubernetes-in-action-2nd-edition/Chapter06
[root@master-node Chapter06]# cat pod.quote-prestop.yaml
apiVersion: v1
kind: Pod
metadata:
name: quote-prestop
spec:
containers:
- name: nginx
image: nginx:alpine
ports:
- name: http
containerPort: 80
lifecycle:
postStart:
exec:
command:
- sh
- -c
- |
apk add fortune && \
curl -O https://luksa.github.io/kiada/book-quotes.txt && \
curl -O https://luksa.github.io/kiada/book-quotes.txt.dat && \
fortune book-quotes.txt > /usr/share/nginx/html/quote
preStop:
exec:
command:
- nginx
- -s
- quit
[root@master-node Chapter06]# 三 小结和参考
Container中的lifecycle hook包含postStart和preStop两种类型。见名知意，postStart是在Container启动后，马上执行，其实它是跟Container main process同时执行的；preStop则是在Container停止前，执行的hook，通常用于优雅的关闭容器。
参考：
《Kubernetes in Action Second Edition》 Marko luksa Chapter 06 managing the lifecycle of the Pod’s containers P160–P169.
