Pod初始化容器之Init Container

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

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;']

[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

[root@k8s-master mnt]# kubectl logs myapp-pod -c init-myservice

Server:         10.96.0.10

Address:        10.96.0.10:53

** server can't find myservice.default.svc.cluster.local: NXDOMAIN

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

*** Can't find myservice.default.svc.cluster.local: No answer

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

waiting for myservice

Server:         10.96.0.10

Address:        10.96.0.10:53

** server can't find myservice.default.svc.cluster.local: NXDOMAIN

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

*** Can't find myservice.default.svc.cluster.local: No answer

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

waiting for myservice

Server:         10.96.0.10

Address:        10.96.0.10:53

** server can't find myservice.default.svc.cluster.local: NXDOMAIN

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

*** Can't find myservice.default.svc.cluster.local: No answer

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

waiting for myservice

Server:         10.96.0.10

Address:        10.96.0.10:53

** server can't find myservice.default.svc.cluster.local: NXDOMAIN

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

*** Can't find myservice.default.svc.cluster.local: No answer

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

waiting for myservice

Server:         10.96.0.10

Address:        10.96.0.10:53

** server can't find myservice.default.svc.cluster.local: NXDOMAIN

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

*** Can't find myservice.default.svc.cluster.local: No answer

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

waiting for myservice

Server:         10.96.0.10

Address:        10.96.0.10:53

** server can't find myservice.default.svc.cluster.local: NXDOMAIN

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

*** Can't find myservice.default.svc.cluster.local: No answer

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

waiting for myservice

Server:         10.96.0.10

Address:        10.96.0.10:53

** server can't find myservice.default.svc.cluster.local: NXDOMAIN

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

*** Can't find myservice.default.svc.cluster.local: No answer

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

waiting for myservice

Server:         10.96.0.10

Address:        10.96.0.10:53

** server can't find myservice.default.svc.cluster.local: NXDOMAIN

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

*** Can't find myservice.default.svc.cluster.local: No answer

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

waiting for myservice

Server:         10.96.0.10

Address:        10.96.0.10:53

** server can't find myservice.default.svc.cluster.local: NXDOMAIN

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

*** Can't find myservice.default.svc.cluster.local: No answer

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

waiting for myservice

Server:         10.96.0.10

Address:        10.96.0.10:53

** server can't find myservice.default.svc.cluster.local: NXDOMAIN

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

*** Can't find myservice.default.svc.cluster.local: No answer

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

waiting for myservice

Server:         10.96.0.10

Address:        10.96.0.10:53

** server can't find myservice.default.svc.cluster.local: NXDOMAIN

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

*** Can't find myservice.default.svc.cluster.local: No answer

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

waiting for myservice

Server:         10.96.0.10

Address:        10.96.0.10:53

** server can't find myservice.default.svc.cluster.local: NXDOMAIN

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

*** Can't find myservice.default.svc.cluster.local: No answer

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

waiting for myservice

Server:         10.96.0.10

Address:        10.96.0.10:53

** server can't find myservice.default.svc.cluster.local: NXDOMAIN

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

*** Can't find myservice.default.svc.cluster.local: No answer

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

waiting for myservice

Server:         10.96.0.10

Address:        10.96.0.10:53

** server can't find myservice.default.svc.cluster.local: NXDOMAIN

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

*** Can't find myservice.default.svc.cluster.local: No answer

*** Can't find myservice.svc.cluster.local: No answer

*** Can't find myservice.cluster.local: No answer

*** Can't find myservice.localdomain: No answer

waiting for myservice

myservice.yaml

[root@k8s-master mnt]# cat myservice.yaml

kind: Service

apiVersion: v1

metadata:

  name: myservice

spec:

  ports:

    - protocol: TCP

      port: 80

      targetPort: 9376

[root@k8s-master mnt]#

[root@k8s-master mnt]# vim myservice.yaml

[root@k8s-master mnt]# kubectl create -f myservice.yaml

service/myservice created

[root@k8s-master mnt]# kubectl get pod

NAME        READY   STATUS     RESTARTS   AGE

myapp-pod   0/1     Init:0/2   0          4m23s

[root@k8s-master mnt]# kubectl get pod

NAME        READY   STATUS     RESTARTS   AGE

myapp-pod   0/1     Init:0/2   0          4m25s

[root@k8s-master mnt]# kubectl get pod

NAME        READY   STATUS     RESTARTS   AGE

myapp-pod   0/1     Init:0/2   0          4m27s

[root@k8s-master mnt]# kubectl get svc

NAME         TYPE        CLUSTER-IP    EXTERNAL-IP   PORT(S)   AGE

kubernetes   ClusterIP   10.96.0.1     <none>        443/TCP   10h

myservice    ClusterIP   10.102.35.5   <none>        80/TCP    49s

[root@k8s-master mnt]# kubectl get pod -n kube-system

NAME                                 READY   STATUS    RESTARTS   AGE

coredns-58cc8c89f4-pzbrd             1/1     Running   23         10h

coredns-58cc8c89f4-vmhl2             1/1     Running   23         10h

etcd-k8s-master                      1/1     Running   4          10h

kube-apiserver-k8s-master            1/1     Running   4          10h

kube-controller-manager-k8s-master   1/1     Running   21         10h

kube-flannel-ds-amd64-c4fs4          1/1     Running   2          9h

kube-flannel-ds-amd64-ct6mc          1/1     Running   2          9h

kube-flannel-ds-amd64-mtzz9          1/1     Running   5          9h

kube-proxy-9bdql                     1/1     Running   2          9h

kube-proxy-cv8lk                     1/1     Running   2          9h

kube-proxy-h8jk8                     1/1     Running   4          10h

kube-scheduler-k8s-master            1/1     Running   21         10h

[root@k8s-master mnt]# kubectl get pod

NAME        READY   STATUS     RESTARTS   AGE

myapp-pod   0/1     Init:1/2   0          5m58s

[root@k8s-master mnt]# vim myservice.yaml

[root@k8s-master mnt]# kubectl create -f myservice.yaml

service/myservice created

[root@k8s-master mnt]# kubectl get pod

NAME        READY   STATUS     RESTARTS   AGE

myapp-pod   0/1     Init:0/2   0          4m23s

[root@k8s-master mnt]# kubectl get pod

NAME        READY   STATUS     RESTARTS   AGE

myapp-pod   0/1     Init:0/2   0          4m25s

[root@k8s-master mnt]# kubectl get pod

NAME        READY   STATUS     RESTARTS   AGE

myapp-pod   0/1     Init:0/2   0          4m27s

[root@k8s-master mnt]# kubectl get svc

NAME         TYPE        CLUSTER-IP    EXTERNAL-IP   PORT(S)   AGE

kubernetes   ClusterIP   10.96.0.1     <none>        443/TCP   10h

myservice    ClusterIP   10.102.35.5   <none>        80/TCP    49s

[root@k8s-master mnt]# kubectl get pod -n kube-system

NAME                                 READY   STATUS    RESTARTS   AGE

coredns-58cc8c89f4-pzbrd             1/1     Running   23         10h

coredns-58cc8c89f4-vmhl2             1/1     Running   23         10h

etcd-k8s-master                      1/1     Running   4          10h

kube-apiserver-k8s-master            1/1     Running   4          10h

kube-controller-manager-k8s-master   1/1     Running   21         10h

kube-flannel-ds-amd64-c4fs4          1/1     Running   2          9h

kube-flannel-ds-amd64-ct6mc          1/1     Running   2          9h

kube-flannel-ds-amd64-mtzz9          1/1     Running   5          9h

kube-proxy-9bdql                     1/1     Running   2          9h

kube-proxy-cv8lk                     1/1     Running   2          9h

kube-proxy-h8jk8                     1/1     Running   4          10h

kube-scheduler-k8s-master            1/1     Running   21         10h

[root@k8s-master mnt]# kubectl get pod

NAME        READY   STATUS     RESTARTS   AGE

myapp-pod   0/1     Init:1/2   0          5m58s

现象：发现变成1/2了

[root@k8s-master mnt]# cat mydb.yaml

kind: Service

apiVersion: v1

metadata:

  name: mydb

spec:

  ports:

    - protocol: TCP

      port: 80

      targetPort: 9377

[root@k8s-master mnt]#

[root@k8s-master mnt]# vim mydb.yaml

[root@k8s-master mnt]# kubectl create -f mydb.yaml

service/mydb created

[root@k8s-master mnt]# kubectl get pod

NAME        READY   STATUS     RESTARTS   AGE

myapp-pod   0/1     Init:1/2   0          11m

[root@k8s-master mnt]# kubectl get pod

NAME        READY   STATUS     RESTARTS   AGE

myapp-pod   0/1     Init:1/2   0          11m

[root@k8s-master mnt]# kubectl get pod

NAME        READY   STATUS    RESTARTS   AGE

myapp-pod   1/1     Running   0          12m

[root@k8s-master mnt]# kubectl get svc

NAME         TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)   AGE

kubernetes   ClusterIP   10.96.0.1       <none>        443/TCP   10h

mydb         ClusterIP   10.104.158.92   <none>        80/TCP    3m24s

myservice    ClusterIP   10.102.35.5     <none>        80/TCP    10m

[root@k8s-master mnt]#

现象：myapp-pod起来了

说明

在 Pod 启动过程中， Init 容器会按顺序在网络和数据卷初始化之后启动。每个容器必须在下一个容器启动之前成功退出
如果由于运行时或失败退出，将导致容器启动失败，它会根据 Pod 的 restartPolicy 指定的策略进行重试。然而，如果 Pod 的 restartPolicy 设置为 Always ， Init 容器失败时会使用RestartPolicy 策略
在所有的 Init 容器没有成功之前， Pod 将不会变成 Ready 状态。 Init 容器的端口将不会在Service 中进行聚集。正在初始化中的 Pod 处于 Pending 状态，但应该会将 Initializing 状态设置为 true
如果 Pod 重启，所有 Init 容器必须重新执行
对 Init 容器 spec 的修改被限制在容器 image 字段，修改其他字段都不会生效。更改 Init容器的 image 字段，等价于重启该 Pod
Init 容器具有应用容器的所有字段。除了 readinessProbe ，因为 Init 容器无法定义不同于完成（ completion ）的就绪（ readiness ）之外的其他状态。这会在验证过程中强制
在 Pod 中的每个 app 和 Init 容器的名称必须唯一；与任何其它容器共享同一个名称，会在验证时抛出错误

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