Detailed explanation of kubernetes-pod (1)

pod introduction

pod structure

Each Pod can contain one or more containers, which can be divided into two categories:

• The number of containers where user programs reside can be large or small

• The Pause container, which is a root container that every Pod will have , has two functions:

  • It can be used as a basis to evaluate the health status of the entire Pod

  • You can set the Ip address on the root container, and other containers have this Ip (Pod IP) to achieve network communication within the Pod

    这里是Pod内部的通讯，Pod的之间的通讯采用虚拟二层网络技术来实现，我们当前环境用的是Flannel
    

pod definition

The following is the resource list of Pod:

apiVersion: v1     #必选，版本号，例如v1
kind: Pod       　 #必选，资源类型，例如 Pod
metadata:       　 #必选，元数据
  name: string     #必选，Pod名称
  namespace: string  #Pod所属的命名空间,默认为"default"
  labels:       　　  #自定义标签列表
    - name: string      　          
spec:  #必选，Pod中容器的详细定义
  containers:  #必选，Pod中容器列表
  - name: string   #必选，容器名称
    image: string  #必选，容器的镜像名称
    imagePullPolicy: [ Always|Never|IfNotPresent ]  #获取镜像的策略 
    command: [string]   #容器的启动命令列表，如不指定，使用打包时使用的启动命令
    args: [string]      #容器的启动命令参数列表
    workingDir: string  #容器的工作目录
    volumeMounts:       #挂载到容器内部的存储卷配置
    - name: string      #引用pod定义的共享存储卷的名称，需用volumes[]部分定义的的卷名
      mountPath: string #存储卷在容器内mount的绝对路径，应少于512字符
      readOnly: boolean #是否为只读模式
    ports: #需要暴露的端口库号列表
    - name: string        #端口的名称
      containerPort: int  #容器需要监听的端口号
      hostPort: int       #容器所在主机需要监听的端口号，默认与Container相同
      protocol: string    #端口协议，支持TCP和UDP，默认TCP
    env:   #容器运行前需设置的环境变量列表
    - name: string  #环境变量名称
      value: string #环境变量的值
    resources: #资源限制和请求的设置
      limits:  #资源限制的设置
        cpu: string     #Cpu的限制，单位为core数，将用于docker run --cpu-shares参数
        memory: string  #内存限制，单位可以为Mib/Gib，将用于docker run --memory参数
      requests: #资源请求的设置
        cpu: string    #Cpu请求，容器启动的初始可用数量
        memory: string #内存请求,容器启动的初始可用数量
    lifecycle: #生命周期钩子
        postStart: #容器启动后立即执行此钩子,如果执行失败,会根据重启策略进行重启
        preStop: #容器终止前执行此钩子,无论结果如何,容器都会终止
    livenessProbe:  #对Pod内各容器健康检查的设置，当探测无响应几次后将自动重启该容器
      exec:       　 #对Pod容器内检查方式设置为exec方式
        command: [string]  #exec方式需要制定的命令或脚本
      httpGet:       #对Pod内个容器健康检查方法设置为HttpGet，需要制定Path、port
        path: string
        port: number
        host: string
        scheme: string
        HttpHeaders:
        - name: string
          value: string
      tcpSocket:     #对Pod内个容器健康检查方式设置为tcpSocket方式
         port: number
       initialDelaySeconds: 0       #容器启动完成后首次探测的时间，单位为秒
       timeoutSeconds: 0    　　    #对容器健康检查探测等待响应的超时时间，单位秒，默认1秒
       periodSeconds: 0     　　    #对容器监控检查的定期探测时间设置，单位秒，默认10秒一次
       successThreshold: 0
       failureThreshold: 0
       securityContext:
         privileged: false
  restartPolicy: [Always | Never | OnFailure]  #Pod的重启策略
  nodeName: <string> #设置NodeName表示将该Pod调度到指定到名称的node节点上
  nodeSelector: obeject #设置NodeSelector表示将该Pod调度到包含这个label的node上
  imagePullSecrets: #Pull镜像时使用的secret名称，以key：secretkey格式指定
  - name: string
  hostNetwork: false   #是否使用主机网络模式，默认为false，如果设置为true，表示使用宿主机网络
  volumes:   #在该pod上定义共享存储卷列表
  - name: string    #共享存储卷名称 （volumes类型有很多种）
    emptyDir: {
    
    }       #类型为emtyDir的存储卷，与Pod同生命周期的一个临时目录。为空值
    hostPath: string   #类型为hostPath的存储卷，表示挂载Pod所在宿主机的目录
      path: string      　　        #Pod所在宿主机的目录，将被用于同期中mount的目录
    secret:       　　　#类型为secret的存储卷，挂载集群与定义的secret对象到容器内部
      scretname: string  
      items:     
      - key: string
        path: string
    configMap:         #类型为configMap的存储卷，挂载预定义的configMap对象到容器内部
      name: string
      items:
      - key: string
        path: string

#小提示：
#   在这里，可通过一个命令来查看每种资源的可配置项
#   kubectl explain 资源类型         查看某种资源可以配置的一级属性
#   kubectl explain 资源类型.属性     查看属性的子属性
[root@k8s-master01 ~]# kubectl explain pod
KIND:     Pod
VERSION:  v1
FIELDS:
   apiVersion   <string>
   kind <string>
   metadata     <Object>
   spec <Object>
   status       <Object>

[root@k8s-master01 ~]# kubectl explain pod.metadata
KIND:     Pod
VERSION:  v1
RESOURCE: metadata <Object>
FIELDS:
   annotations  <map[string]string>
   clusterName  <string>
   creationTimestamp    <string>
   deletionGracePeriodSeconds   <integer>
   deletionTimestamp    <string>
   finalizers   <[]string>
   generateName <string>
   generation   <integer>
   labels       <map[string]string>
   managedFields        <[]Object>
   name <string>
   namespace    <string>
   ownerReferences      <[]Object>
   resourceVersion      <string>
   selfLink     <string>
   uid  <string>

In kubernetes, the first-level attributes of basically all resources are the same, mainly including 5 parts:

• apiVersion version, defined internally by kubernetes, the version number must be queried with kubectl api-versions
• kind type, defined internally by kubernetes, the version number must be queried with kubectl api-resources
• metadata Metadata, mainly resource identification and description, commonly used are name, namespace, labels, etc.
• spec description, which is the most important part of the configuration, contains detailed descriptions of various resource configurations
• status status information, the content inside does not need to be defined, it is automatically generated by kubernetes

Among the above attributes, spec is the focus of the next research, continue to look at its common sub-attributes:

• containers <[]Object> container list, used to define the details of the container
• nodeName dispatches the pod to the specified Node node according to the value of nodeName
• nodeSelector <map[]> According to the information defined in NodeSelector, select to schedule the Pod to the Node containing these labels
• hostNetwork Whether to use the host network mode, the default is false, if set to true, it means to use the host network
• volumes <[]Object> Storage volume, used to define the storage information hung on the Pod
• restartPolicy Restart strategy, indicating the processing strategy of the Pod when it encounters a failure

pod configuration

This section mainly studies pod.spec.containersattributes, which is also the most critical configuration in pod configuration.

[root@k8s-master01 ~]# kubectl explain pod.spec.containers
KIND:     Pod
VERSION:  v1
RESOURCE: containers <[]Object>   # 数组，代表可以有多个容器
FIELDS:
   name  <string>     # 容器名称
   image <string>     # 容器需要的镜像地址
   imagePullPolicy  <string> # 镜像拉取策略 
   command  <[]string> # 容器的启动命令列表，如不指定，使用打包时使用的启动命令
   args     <[]string> # 容器的启动命令需要的参数列表
   env      <[]Object> # 容器环境变量的配置
   ports    <[]Object>     # 容器需要暴露的端口号列表
   resources <Object>      # 资源限制和资源请求的设置

basic configuration

Create a pod-base.yaml file with the following content:

apiVersion: v1
kind: Pod
metadata:
  name: pod-base
  namespace: dev
  labels:
    user: heima
spec:
  containers:
  - name: nginx
    image: nginx:1.17.1
  - name: busybox
    image: busybox:1.30

The configuration of a relatively simple Pod is defined above, which contains two containers:

• nginx: Created with the nginx image of version 1.17.1, (nginx is a lightweight web container)
• busybox: Created with the busybox image version 1.30, (busybox is a small collection of linux commands)

# 创建Pod
[root@k8s-master01 pod]# kubectl apply -f pod-base.yaml
pod/pod-base created

# 查看Pod状况
# READY 1/2 : 表示当前Pod中有2个容器，其中1个准备就绪，1个未就绪
# RESTARTS  : 重启次数，因为有1个容器故障了，Pod一直在重启试图恢复它
[root@k8s-master01 pod]# kubectl get pod -n dev
NAME       READY   STATUS    RESTARTS   AGE
pod-base   1/2     Running   4          95s

# 可以通过describe查看内部的详情
# 此时已经运行起来了一个基本的Pod，虽然它暂时有问题
[root@k8s-master01 pod]# kubectl describe pod pod-base -n dev

mirror pull

Create a pod-imagepullpolicy.yaml file with the following content:

apiVersion: v1
kind: Pod
metadata:
  name: pod-imagepullpolicy
  namespace: dev
spec:
  containers:
  - name: nginx
    image: nginx:1.17.1
    imagePullPolicy: Never # 用于设置镜像拉取策略
  - name: busybox
    image: busybox:1.30

imagePullPolicy, used to set the image pull policy, kubernetes supports configuration of three pull policies:

• Always: Always pull the image from the remote warehouse (always download remotely)
• IfNotPresent: If there is a local mirror, use the local mirror, if there is no local mirror, pull the mirror from the remote warehouse (if there is a local mirror, there is no remote download locally)
• Never: Only use the local image, never go to the remote warehouse to pull, if there is no local image, an error will be reported (always use the local image)

Description of default values:

If the image tag is a specific version number, the default policy is: IfNotPresent

If the image tag is: latest (final version), the default policy is always

# 创建Pod
[root@k8s-master01 pod]# kubectl create -f pod-imagepullpolicy.yaml
pod/pod-imagepullpolicy created

# 查看Pod详情
# 此时明显可以看到nginx镜像有一步Pulling image "nginx:1.17.1"的过程
[root@k8s-master01 pod]# kubectl describe pod pod-imagepullpolicy -n dev
......
Events:
  Type     Reason     Age               From               Message
  ----     ------     ----              ----               -------
  Normal   Scheduled  <unknown>         default-scheduler  Successfully assigned dev/pod-imagePullPolicy to node1
  Normal   Pulling    32s               kubelet, node1     Pulling image "nginx:1.17.1"
  Normal   Pulled     26s               kubelet, node1     Successfully pulled image "nginx:1.17.1"
  Normal   Created    26s               kubelet, node1     Created container nginx
  Normal   Started    25s               kubelet, node1     Started container nginx
  Normal   Pulled     7s (x3 over 25s)  kubelet, node1     Container image "busybox:1.30" already present on machine
  Normal   Created    7s (x3 over 25s)  kubelet, node1     Created container busybox
  Normal   Started    7s (x3 over 25s)  kubelet, node1     Started container busybox

start command

In the previous case, there has been a problem that has not been resolved, that is, the busybox container has not been successfully run, so what caused the failure of this container?

It turns out that busybox is not a program, but a collection of tools. After the kubernetes cluster is started and managed, it will automatically shut down. The solution is to keep it running, which uses the command configuration.

Create a pod-command.yaml file with the following content:

apiVersion: v1
kind: Pod
metadata:
  name: pod-command
  namespace: dev
spec:
  containers:
  - name: nginx
    image: nginx:1.17.1
  - name: busybox
    image: busybox:1.30
    command: ["/bin/sh","-c","touch /tmp/hello.txt;while true;do /bin/echo $(date +%T) >> /tmp/hello.txt; sleep 3; done;"]

command, used to run a command after the container in the pod is initialized.

Explain the meaning of the above command a little bit:

"/bin/sh", "-c", use sh to execute the command

touch /tmp/hello.txt; create a /tmp/hello.txt file

while true;do /bin/echo $(date +%T) >> /tmp/hello.txt; sleep 3; done; Write the current time to the file every 3 seconds

# 创建Pod
[root@k8s-master01 pod]# kubectl create  -f pod-command.yaml
pod/pod-command created

# 查看Pod状态
# 此时发现两个pod都正常运行了
[root@k8s-master01 pod]# kubectl get pods pod-command -n dev
NAME          READY   STATUS   RESTARTS   AGE
pod-command   2/2     Runing   0          2s

# 进入pod中的busybox容器，查看文件内容
# 补充一个命令: kubectl exec  pod名称 -n 命名空间 -it -c 容器名称 /bin/sh  在容器内部执行命令
# 使用这个命令就可以进入某个容器的内部，然后进行相关操作了
# 比如，可以查看txt文件的内容
[root@k8s-master01 pod]# kubectl exec pod-command -n dev -it -c busybox /bin/sh
/ # tail -f /tmp/hello.txt
14:44:19
14:44:22
14:44:25

特别说明：
    通过上面发现command已经可以完成启动命令和传递参数的功能，为什么这里还要提供一个args选项，用于传递参数呢?这其实跟docker有点关系，kubernetes中的command、args两项其实是实现覆盖Dockerfile中ENTRYPOINT的功能。
 1 如果command和args均没有写，那么用Dockerfile的配置。
 2 如果command写了，但args没有写，那么Dockerfile默认的配置会被忽略，执行输入的command
 3 如果command没写，但args写了，那么Dockerfile中配置的ENTRYPOINT的命令会被执行，使用当前args的参数
 4 如果command和args都写了，那么Dockerfile的配置被忽略，执行command并追加上args参数

environment variable

Create a pod-env.yaml file with the following content:

apiVersion: v1
kind: Pod
metadata:
  name: pod-env
  namespace: dev
spec:
  containers:
  - name: busybox
    image: busybox:1.30
    command: ["/bin/sh","-c","while true;do /bin/echo $(date +%T);sleep 60; done;"]
    env: # 设置环境变量列表
    - name: "username"
      value: "admin"
    - name: "password"
      value: "123456"

env, environment variable, is used to set environment variables in the container in the pod.

# 创建Pod
[root@k8s-master01 ~]# kubectl create -f pod-env.yaml
pod/pod-env created

# 进入容器，输出环境变量
[root@k8s-master01 ~]# kubectl exec pod-env -n dev -c busybox -it /bin/sh
/ # echo $username
admin
/ # echo $password
123456

This method is not very recommended. It is recommended to store these configurations separately in the configuration file, which will be introduced later.

port settings

This section introduces the port settings of the container, that is, the ports option of containers.

First look at the sub-options supported by ports:

[root@k8s-master01 ~]# kubectl explain pod.spec.containers.ports
KIND:     Pod
VERSION:  v1
RESOURCE: ports <[]Object>
FIELDS:
   name         <string>  # 端口名称，如果指定，必须保证name在pod中是唯一的		
   containerPort<integer> # 容器要监听的端口(0<x<65536)
   hostPort     <integer> # 容器要在主机上公开的端口，如果设置，主机上只能运行容器的一个副本(一般省略) 
   hostIP       <string>  # 要将外部端口绑定到的主机IP(一般省略)
   protocol     <string>  # 端口协议。必须是UDP、TCP或SCTP。默认为“TCP”。

Next, write a test case, create pod-ports.yaml

apiVersion: v1
kind: Pod
metadata:
  name: pod-ports
  namespace: dev
spec:
  containers:
  - name: nginx
    image: nginx:1.17.1
    ports: # 设置容器暴露的端口列表
    - name: nginx-port
      containerPort: 80
      protocol: TCP

# 创建Pod
[root@k8s-master01 ~]# kubectl create -f pod-ports.yaml
pod/pod-ports created

# 查看pod
# 在下面可以明显看到配置信息
[root@k8s-master01 ~]# kubectl get pod pod-ports -n dev -o yaml
......
spec:
  containers:
  - image: nginx:1.17.1
    imagePullPolicy: IfNotPresent
    name: nginx
    ports:
    - containerPort: 80
      name: nginx-port
      protocol: TCP
......

To access the program in the container, you need to use thePodip:containerPort

resource quota

To run a program in a container, it must occupy certain resources, such as CPU and memory. If the resources of a certain container are not limited, it may consume a large amount of resources, causing other containers to fail to run. In response to this situation, kubernetes provides a mechanism for quotas on memory and cpu resources. This mechanism is mainly implemented through the resources option, which has two sub-options:

• limits: Used to limit the maximum resource usage of the runtime container. When the container resource usage exceeds the limits, it will be terminated and restarted
• requests : Used to set the minimum resources required by the container. If the environment resources are not enough, the container will fail to start

The upper and lower limits of resources can be set through the above two options.

Next, write a test case, create pod-resources.yaml

apiVersion: v1
kind: Pod
metadata:
  name: pod-resources
  namespace: dev
spec:
  containers:
  - name: nginx
    image: nginx:1.17.1
    resources: # 资源配额
      limits:  # 限制资源（上限）
        cpu: "2" # CPU限制，单位是core数
        memory: "10Gi" # 内存限制
      requests: # 请求资源（下限）
        cpu: "1"  # CPU限制，单位是core数
        memory: "10Mi"  # 内存限制

Here is a description of the units of cpu and memory:

• cpu: core number, can be integer or decimal
• memory: memory size, you can use Gi, Mi, G, M, etc.

# 运行Pod
[root@k8s-master01 ~]# kubectl create  -f pod-resources.yaml
pod/pod-resources created

# 查看发现pod运行正常
[root@k8s-master01 ~]# kubectl get pod pod-resources -n dev
NAME            READY   STATUS    RESTARTS   AGE  
pod-resources   1/1     Running   0          39s   

# 接下来，停止Pod
[root@k8s-master01 ~]# kubectl delete  -f pod-resources.yaml
pod "pod-resources" deleted

# 编辑pod，修改resources.requests.memory的值为10Gi
[root@k8s-master01 ~]# vim pod-resources.yaml

# 再次启动pod
[root@k8s-master01 ~]# kubectl create  -f pod-resources.yaml
pod/pod-resources created

# 查看Pod状态，发现Pod启动失败
[root@k8s-master01 ~]# kubectl get pod pod-resources -n dev -o wide
NAME            READY   STATUS    RESTARTS   AGE          
pod-resources   0/1     Pending   0          20s    

# 查看pod详情会发现，如下提示
[root@k8s-master01 ~]# kubectl describe pod pod-resources -n dev
......
Warning  FailedScheduling  35s   default-scheduler  0/3 nodes are available: 1 node(s) had taint {
    
    node-role.kubernetes.io/master: }, that the pod didn't tolerate, 2 Insufficient memory.(内存不足)