1. Kubernetes 简介 - 1.2 Kubernetes 101

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2023-12-01

体验Kubernetes最简单的方法是跑一个nginx容器，然后使用kubectl操作该容器。Kubernetes提供了一个类似于docker run的命令kubectl run，可以方便的创建一个容器（实际上创建的是一个由deployment来管理的Pod）：

$ kubectl run --image=nginx:alpine nginx-app --port=80               
deployment "nginx-app" created                                         
$ kubectl get pods
NAME                         READY     STATUS    RESTARTS   AGE
nginx-app-4028413181-cnt1i   1/1       Running   0          52s

等到容器变成Running后，就可以用kubectl命令来操作它了，比如

kubectl get - 类似于docker ps，查询资源列表
kubectl describe - 类似于docker inspect，获取资源的详细信息
kubectl logs - 类似于docker logs，获取容器的日志
kubectl exec - 类似于docker exec，在容器内执行一个命令

$ kubectl get pods
NAME                         READY     STATUS    RESTARTS   AGE
nginx-app-4028413181-cnt1i   1/1       Running   0          6m
$ kubectl exec nginx-app-4028413181-cnt1i ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.5  31736  5108 ?        Ss   00:19   0:00 nginx: master process nginx -g daemon off;
nginx        5  0.0  0.2  32124  2844 ?        S    00:19   0:00 nginx: worker process
root        18  0.0  0.2  17500  2112 ?        Rs   00:25   0:00 ps aux
$ kubectl describe pod nginx-app-4028413181-cnt1i
Name:          nginx-app-4028413181-cnt1i
Namespace:         default
Node:          boot2docker/192.168.64.12
Start Time:        Tue, 06 Sep 2016 08:18:41 +0800
Labels:        pod-template-hash=4028413181
               run=nginx-app
Status:        Running
IP:            172.17.0.3
Controllers:       ReplicaSet/nginx-app-4028413181
Containers:
  nginx-app:
    Container ID:              docker://4ef989b57d0a7638ad9c5bbc22e16d5ea5b459281c77074fc982eba50973107f
    Image:                 nginx
    Image ID:              docker://sha256:4efb2fcdb1ab05fb03c9435234343c1cc65289eeb016be86193e88d3a5d84f6b
    Port:                  80/TCP
    State:                 Running
      Started:             Tue, 06 Sep 2016 08:19:30 +0800
    Ready:                 True
    Restart Count:             0
    Environment Variables:         <none>
Conditions:
  Type         Status
  Initialized      True
  Ready            True
  PodScheduled     True
Volumes:
  default-token-9o8ks:
    Type:          Secret (a volume populated by a Secret)
    SecretName:    default-token-9o8ks
QoS Tier:          BestEffort
Events:
  FirstSeen        LastSeen           Count      From               SubobjectPath              Type           Reason         Message
  ---------        --------           -----      ----               -------------              --------           ------         -------
  8m           8m             1          {default-scheduler }                       Normal         Scheduled          Successfully assigned nginx-app-4028413181-cnt1i to boot2docker
  8m           8m             1          {kubelet boot2docker}      spec.containers{nginx-app}         Normal         Pulling        pulling image "nginx"
  7m           7m             1          {kubelet boot2docker}      spec.containers{nginx-app}         Normal         Pulled         Successfully pulled image "nginx"
  7m           7m             1          {kubelet boot2docker}      spec.containers{nginx-app}         Normal         Created        Created container with docker id 4ef989b57d0a
  7m           7m             1          {kubelet boot2docker}      spec.containers{nginx-app}         Normal         Started        Started container with docker id 4ef989b57d0a
$ curl http://172.17.0.3
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
<style>
    body {
        width: 35em;
        margin: 0 auto;
        font-family: Tahoma, Verdana, Arial, sans-serif;
    }
</style>
</head>
<body>
<h1>Welcome to nginx!</h1>
<p>If you see this page, the nginx web server is successfully installed and
working. Further configuration is required.</p>
<p>For online documentation and support please refer to
<a href="http://nginx.org/">nginx.org</a>.<br/>
Commercial support is available at
<a href="http://nginx.com/">nginx.com</a>.</p>
<p><em>Thank you for using nginx.</em></p>
</body>
</html>
$ kubectl logs nginx-app-4028413181-cnt1i
127.0.0.1 - - [06/Sep/2016:00:27:13 +0000] "GET / HTTP/1.0 " 200 612 "-" "-" "-"

使用yaml定义Pod

上面是通过kubectl run来启动了第一个Pod，但是kubectl run并不支持所有的功能。在Kubernetes中，更经常使用yaml文件来定义资源，并通过kubectl create -f file.yaml来创建资源。比如，一个简单的nginx Pod可以定义为：

apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:
    app: nginx
spec:
  containers:
  - name: nginx
    image: nginx
    ports:
    - containerPort: 80

前面提到，kubectl run并不是直接创建一个Pod，而是先创建一个Deployment资源（replicas=1），再由与Deployment关联的ReplicaSet来自动创建Pod，这等价于这样一个配置：

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  labels:
    run: nginx-app
  name: nginx-app
  namespace: default
spec:
  replicas: 1
  selector:
    matchLabels:
      run: nginx-app
  strategy:
    rollingUpdate:
      maxSurge: 1
      maxUnavailable: 1
    type: RollingUpdate
  template:
    metadata:
      labels:
        run: nginx-app
    spec:
      containers:
      - image: nginx
        name: nginx-app
        ports:
        - containerPort: 80
          protocol: TCP
      dnsPolicy: ClusterFirst
      restartPolicy: Always

使用Volume

Pod的生命周期通常比较短，只要出现了异常，就会创建一个新的Pod来代替它。那容器产生的数据呢？容器内的数据会随着Pod消亡而自动消失。Volume就是为了持久化容器数据而生，比如可以为redis容器指定一个hostPath来存储redis数据：

apiVersion: v1
kind: Pod
metadata:
  name: redis
spec:
  containers:
  - name: redis
    image: redis
    volumeMounts:
    - name: redis-persistent-storage
      mountPath: /data/redis
  volumes:
  - name: redis-persistent-storage
    hostPath:
      path: /data/

Kubernetes volume支持非常多的插件，可以根据实际需要来选择：

emptyDir
hostPath
gcePersistentDisk
awsElasticBlockStore
nfs
iscsi
flocker
glusterfs
rbd
cephfs
gitRepo
secret
persistentVolumeClaim
downwardAPI
azureFileVolume
vsphereVolume

使用Service

前面虽然创建了Pod，但是在kubernetes中，Pod的IP地址会随着Pod的重启而变化，并不建议直接拿Pod的IP来交互。那如何来访问这些Pod提供的服务呢？使用Service。Service为一组Pod（通过labels来选择）提供一个统一的入口，并为它们提供负载均衡和自动服务发现。比如，可以为前面的nginx-app创建一个service：

$ kubectl expose deployment nginx-app --port=80 --target-port=80 --type=NodePort
service "nginx-app" exposed
$ kubectl describe service nginx-app
Name:              nginx-app
Namespace:             default
Labels:            run=nginx-app
Selector:              run=nginx-app
Type:              ClusterIP
IP:                10.0.0.66
Port:              <unset>    80/TCP
NodePort:              <unset>    30772/TCP
Endpoints:             172.17.0.3:80
Session Affinity:          None
No events.

这样，在cluster内部就可以通过http://10.0.0.66和http://node-ip:30772来访问nginx-app。而在cluster外面，则只能通过http://node-ip:30772来访问。