Pod初始化容器之Init Container

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我是靠谱客的博主孝顺鸭子，这篇文章主要介绍Pod初始化容器之Init Container，现在分享给大家，希望可以做个参考。

Init 容器的介绍

Pod能够具有多个容器，应用运行在容器里面，但是它也可能有一个或多个先于应用容器启动的 Init容器
Init 容器与普通的容器非常像，除了如下两点：
c Init 容器总是运行到成功完成为止
2）每个 Init 容器都必须在下一个 Init 容器启动之前成功完成
如果 Pod 的 Init 容器失败， Kubernetes 会不断地重启该 Pod ，直到 Init 容器成功为止。然而，如果 Pod 对应的 restartPolicy 为 Never。
Init 容器的作用

因为 Init 容器具有与应用程序容器分离的单独镜像，所以它们的启动相关代码具有如下优势：
1）它们可以包含并运行实用工具，但是出于安全考虑，是不建议在应用程序容器镜像中包含这些实用工具的
2) 它们可以包含使用工具和定制化代码来安装，但是不能出现在应用程序镜像中。例如，创建镜像没必要 FROM 另一个镜像，只需要在安装过程中使用类似 sed 、 awk 、 python 或 dig
这样的工具。
3) 应用程序镜像可以分离出创建和部署的角色，而没有必要联合它们构建一个单独的镜像。
4) Init 容器使用 Linux Namespace ，所以相对应用程序容器来说具有不同的文件系统视图。因此，它们能够具有访问 Secret 的权限，而应用程序容器则不能。
5) 它们必须在应用程序容器启动之前运行完成，而应用程序容器是并行运行的，所以 Init 容器能够提供了一种简单的阻塞或延迟应用容器的启动的方法，直到满足了一组先决条件。

测试

说明：主要是在启动Pod,有2个initc，一开始是没有准备的，所以现象会显示初始化0/2

init.yaml

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kind: Pod
metadata:
  name: myapp-pod
  labels:
    app: myapp
spec:
  containers:
  - name: myapp-container   
    image: busybox
    command: ['sh', '-c', 'echo The app is running! && sleep 3600']
  initContainers:
  - name: init-myservice
    image: busybox
    command: ['sh', '-c', 'until nslookup myservice; do echo waiting for myservice; sleep 2; done;']
  - name: init-mydb
    image: busybox
    command: ['sh', '-c', 'until nslookup mydb; do echo waiting for mydb; sleep 2; done;']

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[root@k8s-master mnt]# kubectl create -f init.yaml
pod/myapp-pod created
[root@k8s-master mnt]# kubectl get pod
NAME        READY   STATUS     RESTARTS   AGE
myapp-pod   0/1     Init:0/2   0          8s
[root@k8s-master mnt]# kubectl describe myapp-pod
error: the server doesn't have a resource type "myapp-pod"
[root@k8s-master mnt]# kubectl describe pod myapp-pod
Name:         myapp-pod
Namespace:    default
Priority:     0
Node:         k8s-node02/192.168.180.134
Start Time:   Wed, 18 Dec 2019 22:02:57 +0800
Labels:       app=myapp
Annotations:  <none>
Status:       Pending
IP:           10.244.1.9
IPs:
  IP:  10.244.1.9
Init Containers:
  init-myservice:
    Container ID:  docker://3c0e850042efab506f95737adfd3dc6ef2da9218ce51eb5eb4e94573a657fd2b
    Image:         busybox
    Image ID:      docker-pullable://busybox@sha256:1828edd60c5efd34b2bf5dd3282ec0cc04d47b2ff9caa0b6d4f07a21d1c08084
    Port:          <none>
    Host Port:     <none>
    Command:
      sh
      -c
      until nslookup myservice; do echo waiting for myservice; sleep 2; done;
    State:          Running
      Started:      Wed, 18 Dec 2019 22:03:03 +0800
    Ready:          False
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-gx2h8 (ro)
  init-mydb:
    Container ID:
    Image:         busybox
    Image ID:
    Port:          <none>
    Host Port:     <none>
    Command:
      sh
      -c
      until nslookup mydb; do echo waiting for mydb; sleep 2; done;
    State:          Waiting
      Reason:       PodInitializing
    Ready:          False
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-gx2h8 (ro)
Containers:
  myapp-container:
    Container ID:
    Image:         busybox
    Image ID:
    Port:          <none>
    Host Port:     <none>
    Command:
      sh
      -c
      echo The app is running! && sleep 3600
    State:          Waiting
      Reason:       PodInitializing
    Ready:          False
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-gx2h8 (ro)
Conditions:
  Type              Status
  Initialized       False
  Ready             False
  ContainersReady   False
  PodScheduled      True
Volumes:
  default-token-gx2h8:
    Type:        Secret (a volume populated by a Secret)
    SecretName:  default-token-gx2h8
    Optional:    false
QoS Class:       BestEffort
Node-Selectors:  <none>
Tolerations:     node.kubernetes.io/not-ready:NoExecute for 300s
                 node.kubernetes.io/unreachable:NoExecute for 300s
Events:
  Type    Reason     Age        From                 Message
  ----    ------     ----       ----                 -------
  Normal  Scheduled  <unknown>  default-scheduler    Successfully assigned default/myapp-pod to k8s-node02
  Normal  Pulling    22s        kubelet, k8s-node02  Pulling image "busybox"
  Normal  Pulled     18s        kubelet, k8s-node02  Successfully pulled image "busybox"
  Normal  Created    18s        kubelet, k8s-node02  Created container init-myservice
  Normal  Started    17s        kubelet, k8s-node02  Started container init-myservice

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[root@k8s-master mnt]# kubectl create -f init.yaml
pod/myapp-pod created
[root@k8s-master mnt]# kubectl get pod
NAME        READY   STATUS     RESTARTS   AGE
myapp-pod   0/1     Init:0/2   0          8s
[root@k8s-master mnt]# kubectl describe myapp-pod
error: the server doesn't have a resource type "myapp-pod"
[root@k8s-master mnt]# kubectl describe pod myapp-pod
Name:         myapp-pod
Namespace:    default
Priority:     0
Node:         k8s-node02/192.168.180.134
Start Time:   Wed, 18 Dec 2019 22:02:57 +0800
Labels:       app=myapp
Annotations:  <none>
Status:       Pending
IP:           10.244.1.9
IPs:
  IP:  10.244.1.9
Init Containers:
  init-myservice:
    Container ID:  docker://3c0e850042efab506f95737adfd3dc6ef2da9218ce51eb5eb4e94573a657fd2b
    Image:         busybox
    Image ID:      docker-pullable://busybox@sha256:1828edd60c5efd34b2bf5dd3282ec0cc04d47b2ff9caa0b6d4f07a21d1c08084
    Port:          <none>
    Host Port:     <none>
    Command:
      sh
      -c
      until nslookup myservice; do echo waiting for myservice; sleep 2; done;
    State:          Running
      Started:      Wed, 18 Dec 2019 22:03:03 +0800
    Ready:          False
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-gx2h8 (ro)
  init-mydb:
    Container ID:
    Image:         busybox
    Image ID:
    Port:          <none>
    Host Port:     <none>
    Command:
      sh
      -c
      until nslookup mydb; do echo waiting for mydb; sleep 2; done;
    State:          Waiting
      Reason:       PodInitializing
    Ready:          False
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-gx2h8 (ro)
Containers:
  myapp-container:
    Container ID:
    Image:         busybox
    Image ID:
    Port:          <none>
    Host Port:     <none>
    Command:
      sh
      -c
      echo The app is running! && sleep 3600
    State:          Waiting
      Reason:       PodInitializing
    Ready:          False
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-gx2h8 (ro)
Conditions:
  Type              Status
  Initialized       False
  Ready             False
  ContainersReady   False
  PodScheduled      True
Volumes:
  default-token-gx2h8:
    Type:        Secret (a volume populated by a Secret)
    SecretName:  default-token-gx2h8
    Optional:    false
QoS Class:       BestEffort
Node-Selectors:  <none>
Tolerations:     node.kubernetes.io/not-ready:NoExecute for 300s
                 node.kubernetes.io/unreachable:NoExecute for 300s
Events:
  Type    Reason     Age        From                 Message
  ----    ------     ----       ----                 -------
  Normal  Scheduled  <unknown>  default-scheduler    Successfully assigned default/myapp-pod to k8s-node02
  Normal  Pulling    22s        kubelet, k8s-node02  Pulling image "busybox"
  Normal  Pulled     18s        kubelet, k8s-node02  Successfully pulled image "busybox"
  Normal  Created    18s        kubelet, k8s-node02  Created container init-myservice
  Normal  Started    17s        kubelet, k8s-node02  Started container init-myservice

查看myservice

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[root@k8s-master mnt]# kubectl logs myapp-pod -c init-myservice
Server:         10.96.0.10
Address:        10.96.0.10:53