概述
在容器的部署過程中，有的時候需要在容器執行之前進行一些預配置的工作，比如下載配置，判斷某些服務是否啟動，修改配置等一些準備的工作，想要實現這些功能，在k8s中可以使用初始化容器，在應用容器執行之前進行一些預處理的工作。
本文件介紹在k8s中初始化容器的使用方法。
使用方法
以下的例子使用初始化容器，在nginx容器啟動之前下載一個index.html檔案
kubectl apply -f - <<EOF
apiVersion: v1
kind: Pod
metadata:
name: initnginx
spec:
initContainers:
- name: install
image: busybox
command:
- wget
- "-O"
- "/var/index.html"
- "https://www.baidu.com"
containers:
- name: nginx
image: 172.20.45.174:81/base/nginx:1.15-alpine
ports:
- containerPort: 80
dnsPolicy: Default
EOF檢視pod的狀態
[root@nccztsjb-node-11 ~]# kubectl get pod
NAME READY STATUS RESTARTS AGE
cm-test-app 1/1 Running 0 20h
cm-test-nginx 1/1 Running 0 175m
cm-test-pod 1/1 Running 0 20h
initnginx 0/1 Init:0/1 0 16s #在初始化階段
nginx-test-5c7f8fc697-lrvpm 1/1 Running 0 32d
nginx-test-5c7f8fc697-whx4g 1/1 Running 0 32d
web-0 1/1 Running 0 32d
web-1 1/1 Running 0 32d
web-2 1/1 Running 0 32d
# 初始化容器執行完成後，pod狀態變為Running
[root@nccztsjb-node-11 ~]# kubectl get pod
NAME READY STATUS RESTARTS AGE
cm-test-app 1/1 Running 0 20h
cm-test-nginx 1/1 Running 0 178m
cm-test-pod 1/1 Running 0 20h
initnginx 1/1 Running 0 9s
nginx-test-5c7f8fc697-lrvpm 1/1 Running 0 32d
nginx-test-5c7f8fc697-whx4g 1/1 Running 0 32d
web-0 1/1 Running 0 32d
web-1 1/1 Running 0 32d
web-2 1/1 Running 0 32d
[root@nccztsjb-node-11 ~]# 檢視初始化容器的日誌
[root@nccztsjb-node-11 ~]# kubectl logs initnginx -c install
Connecting to www.baidu.com (103.235.46.39:443)
wget: note: TLS certificate validation not implemented
saving to '/var/index.html'
index.html 100% |********************************| 2443 0:00:00 ETA
'/var/index.html' saved
[root@nccztsjb-node-11 ~]#說明執行命令是成功的。
進入容器中，檢視是否存在該檔案
[root@nccztsjb-node-11 ~]# kubectl exec -it initnginx -- bash
[initnginx root:~]# cd /var/
[initnginx root:/var]# ls
cache empty git lib local lock log opt run spool tmp
[initnginx root:/var]# 發現並沒有該檔案！
問題在於install容器和nginx容器不是同一個容器所以，下載的/var/index.html只是在install容器的/var目錄下。
為了實現nginx可以訪問初始化容器中的內容，可以掛載一個資料捲進行2個容器的共享，修改pod定義如下：
kubectl apply -f - <<EOF
apiVersion: v1
kind: Pod
metadata:
name: initnginx
spec:
initContainers:
- name: install
image: busybox
command:
- wget
- "-O"
- "/work-dir/index.html"
- "https://www.baidu.com"
volumeMounts:
- name: workdir
mountPath: /work-dir
containers:
- name: nginx
image: 172.20.45.174:81/base/nginx:1.15-alpine
ports:
- containerPort: 80
volumeMounts:
- name: workdir
mountPath: /usr/share/nginx/html
dnsPolicy: Default
volumes:
- name: workdir
emptyDir: {}
EOF這樣的化，2個容器中就都掛載了workdir這個資料卷，初始化容器中下載的檔案，就可以被nginx應用容器來使用了。
建立pod後檢視nginx容器中的檔案
[root@nccztsjb-node-11 ~]# kubectl exec -it initnginx -- bash
[initnginx root:~]# cd /usr/share/nginx/
[initnginx root:/usr/share/nginx]# ls
html
[initnginx root:/usr/share/nginx]# cd html/
[initnginx root:/usr/share/nginx/html]# ls
index.html
[initnginx root:/usr/share/nginx/html]# cat index.html
<!DOCTYPE html>
<!--STATUS OK--><html> <head><meta http-equiv=content-type content=text/html;charset=utf-8><meta http-equiv=X-UA-Compatible content=IE=Edge><meta content=always name=referrer><link rel=stylesheet type=text/css href=https://ss1.bdstatic.com/5eN1bjq8AAUYm2zgoY3K/r/www/cache/bdorz/baidu.min.css><title>百度一下，你就知道</title></head> <body link=#0000cc> <div id=wrapper> <div id=head> <div class=head_wrapper> <div class=s_form> <div class=s_form_wrapper> <div id=lg> <img hidefocus=true src=//www.baidu.com/img/bd_logo1.png width=270 height=129> </div> <form id=form name=f action=//www.baidu.com/s class=fm> <input type=hidden name=bdorz_come value=1> <input type=hidden name=ie value=utf-8> <input type=hidden name=f value=8> <input type=hidden name=rsv_bp value=1> <input type=hidden name=rsv_idx value=1> <input type=hidden name=tn value=baidu><span class="bg s_ipt_wr"><input id=kw name=wd class=s_ipt value maxlength=255 autocomplete=off autofocus=autofocus></span><span class="bg s_btn_wr"><input type=submit id=su value=百度一下 class="bg s_btn" autofocus></span> </form> </div> </div> <div id=u1> <a href=http://news.baidu.com name=tj_trnews class=mnav>新聞</a> <a href=https://www.hao123.com name=tj_trhao123 class=mnav>hao123</a> <a href=http://map.baidu.com name=tj_trmap class=mnav>地圖</a> <a href=http://v.baidu.com name=tj_trvideo class=mnav>視訊</a> <a href=http://tieba.baidu.com name=tj_trtieba class=mnav>貼吧</a> <noscript> <a href=http://www.baidu.com/bdorz/login.gif?login&tpl=mn&u=http%3A%2F%2Fwww.baidu.com%2f%3fbdorz_come%3d1 name=tj_login class=lb>登入</a> </noscript> <script>document.write('<a href="http://www.baidu.com/bdorz/login.gif?login&tpl=mn&u='+ encodeURIComponent(window.location.href+ (window.location.search === "" ? "?" : "&")+ "bdorz_come=1")+ '" name="tj_login" class="lb">登入</a>');
</script> <a href=//www.baidu.com/more/ name=tj_briicon class=bri style="display: block;">更多產品</a> </div> </div> </div> <div id=ftCon> <div id=ftConw> <p id=lh> <a href=http://home.baidu.com>關於百度</a> <a href=http://ir.baidu.com>About Baidu</a> </p> <p id=cp>©2017 Baidu <a href=http://www.baidu.com/duty/>使用百度前必讀</a> <a href=http://jianyi.baidu.com/ class=cp-feedback>意見反饋</a> 京ICP證030173號 <img src=//www.baidu.com/img/gs.gif> </p> </div> </div> </div> </body> </html>
[initnginx root:/usr/share/nginx/html]# 發現index.html檔案已經生成了，就是在初始化容器中建立的檔案被掛載到nginx中來使用。
檢視初始化容器的日誌
[root@nccztsjb-node-11 ~]# kubectl logs initnginx -c install
Connecting to www.baidu.com (103.235.46.39:443)
wget: note: TLS certificate validation not implemented
saving to '/work-dir/index.html'
index.html 100% |********************************| 2443 0:00:00 ETA
'/work-dir/index.html' saved
[root@nccztsjb-node-11 ~]# OK，通過日誌發現是向/work-dir/目錄中寫了index.html檔案。
以上就是初始化容器的基本的用法。
檢視pod的事件也可以看到pod中容器執行的順序
[root@nccztsjb-node-11 ~]# kubectl describe pod initnginx
Name: initnginx
Namespace: default
Priority: 0
Node: 172.20.59.57/172.20.59.57
Start Time: Tue, 11 Jan 2022 13:49:57 +0800
Labels: <none>
Annotations: cni.projectcalico.org/podIP: 172.23.29.31/32
cni.projectcalico.org/podIPs: 172.23.29.31/32
Status: Running
IP: 172.23.29.31
IPs:
IP: 172.23.29.31
Init Containers:
install:
Container ID: docker://9d2aec37255a02a2b13abced64717057eb100dd329bc6b6529211e30595f1c93
Image: busybox
Image ID: docker-pullable://busybox@sha256:5acba83a746c7608ed544dc1533b87c737a0b0fb730301639a0179f9344b1678
Port: <none>
Host Port: <none>
Command:
wget
-O
/work-dir/index.html
https://www.baidu.com
State: Terminated
Reason: Completed
Exit Code: 0
Started: Tue, 11 Jan 2022 13:50:05 +0800
Finished: Tue, 11 Jan 2022 13:50:08 +0800
Ready: True
Restart Count: 0
Environment: <none>
Mounts:
/var/run/secrets/kubernetes.io/serviceaccount from default-token-8ss8s (ro)
/work-dir from workdir (rw)
Containers:
nginx:
Container ID: docker://9f088e80c72e6691ea48bf3a9edf3983ccf8e8d32dd17e27425916efdfec4ff4
Image: 172.20.45.174:81/base/nginx:1.15-alpine
Image ID: docker-pullable://172.20.45.174:81/base/nginx@sha256:478a73bcec93acc3e814ddd6fb2f95c6f6b4b0d0f168a4feaa039513d260a5d9
Port: 80/TCP
Host Port: 0/TCP
State: Running
Started: Tue, 11 Jan 2022 13:50:08 +0800
Ready: True
Restart Count: 0
Environment: <none>
Mounts:
/usr/share/nginx/html from workdir (rw)
/var/run/secrets/kubernetes.io/serviceaccount from default-token-8ss8s (ro)
Conditions:
Type Status
Initialized True
Ready True
ContainersReady True
PodScheduled True
Volumes:
workdir:
Type: EmptyDir (a temporary directory that shares a pod's lifetime)
Medium:
SizeLimit: <unset>
default-token-8ss8s:
Type: Secret (a volume populated by a Secret)
SecretName: default-token-8ss8s
Optional: false
QoS Class: BestEffort
Node-Selectors: <none>
Tolerations: node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal Scheduled 6m27s default-scheduler Successfully assigned default/initnginx to 172.20.59.57
Normal Pulling 6m26s kubelet Pulling image "busybox"
Normal Pulled 6m20s kubelet Successfully pulled image "busybox" in 6.378365555s
Normal Created 6m20s kubelet Created container install
Normal Started 6m19s kubelet Started container install
Normal Pulled 6m16s kubelet Container image "172.20.45.174:81/base/nginx:1.15-alpine" already present on machine
Normal Created 6m16s kubelet Created container nginx
Normal Started 6m16s kubelet Started container nginx
[root@nccztsjb-node-11 ~]# 即先建立初始化容器，執行結束後，才是執行nginx這個應用容器。
初始化容器使用的說明
關於初始化容器的使用，有幾點說明
- 在pod中可以有一個或者多個初始化容器
- 初始化容器執行成功後，應用的容器才能被執行
- 初始化容器是僅執行一次的任務