Kubernetes 101

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）：

  1. $ kubectl run --image=nginx:alpine nginx-app --port=80               
  2. deployment "nginx-app" created                                         
  3. $ kubectl get pods
  4. NAME                         READY     STATUS    RESTARTS   AGE
  5. 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
  1. $ kubectl get pods
  2. NAME                         READY     STATUS    RESTARTS   AGE
  3. nginx-app-4028413181-cnt1i   1/1       Running   0          6m
  5. $ kubectl exec nginx-app-4028413181-cnt1i ps aux
  6. USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
  7. root         1  0.0  0.5  31736  5108 ?        Ss   00:19   0:00 nginx: master process nginx -g daemon off;
  8. nginx        5  0.0  0.2  32124  2844 ?        S    00:19   0:00 nginx: worker process
  9. root        18  0.0  0.2  17500  2112 ?        Rs   00:25   0:00 ps aux
  11. $ kubectl describe pod nginx-app-4028413181-cnt1i
  12. Name:          nginx-app-4028413181-cnt1i
  13. Namespace:         default
  14. Node:          boot2docker/192.168.64.12
  15. Start Time:        Tue, 06 Sep 2016 08:18:41 +0800
  16. Labels:        pod-template-hash=4028413181
  17.                run=nginx-app
  18. Status:        Running
  19. IP:            172.17.0.3
  20. Controllers:       ReplicaSet/nginx-app-4028413181
  21. Containers:
  22.   nginx-app:
  23.     Container ID:              docker://4ef989b57d0a7638ad9c5bbc22e16d5ea5b459281c77074fc982eba50973107f
  24.     Image:                 nginx
  25.     Image ID:              docker://sha256:4efb2fcdb1ab05fb03c9435234343c1cc65289eeb016be86193e88d3a5d84f6b
  26.     Port:                  80/TCP
  27.     State:                 Running
  28.       Started:             Tue, 06 Sep 2016 08:19:30 +0800
  29.     Ready:                 True
  30.     Restart Count:             0
  31.     Environment Variables:         <none>
  32. Conditions:
  33.   Type         Status
  34.   Initialized      True
  35.   Ready            True
  36.   PodScheduled     True
  37. Volumes:
  38.   default-token-9o8ks:
  39.     Type:          Secret (a volume populated by a Secret)
  40.     SecretName:    default-token-9o8ks
  41. QoS Tier:          BestEffort
  42. Events:
  43.   FirstSeen        LastSeen           Count      From               SubobjectPath              Type           Reason         Message
  44.   ---------        --------           -----      ----               -------------              --------           ------         -------
  45.   8m           8m             1          {default-scheduler }                       Normal         Scheduled          Successfully assigned nginx-app-4028413181-cnt1i to boot2docker
  46.   8m           8m             1          {kubelet boot2docker}      spec.containers{nginx-app}         Normal         Pulling        pulling image "nginx"
  47.   7m           7m             1          {kubelet boot2docker}      spec.containers{nginx-app}         Normal         Pulled         Successfully pulled image "nginx"
  48.   7m           7m             1          {kubelet boot2docker}      spec.containers{nginx-app}         Normal         Created        Created container with docker id 4ef989b57d0a
  49.   7m           7m             1          {kubelet boot2docker}      spec.containers{nginx-app}         Normal         Started        Started container with docker id 4ef989b57d0a
  51. $ curl http://172.17.0.3
  52. <!DOCTYPE html>
  53. <html>
  54. <head>
  55. <title>Welcome to nginx!</title>
  56. <style>
  57.     body {
  58.         width: 35em;
  59.         margin: 0 auto;
  60.         font-family: Tahoma, Verdana, Arial, sans-serif;
  61.     }
  62. </style>
  63. </head>
  64. <body>
  65. <h1>Welcome to nginx!</h1>
  66. <p>If you see this page, the nginx web server is successfully installed and
  67. working. Further configuration is required.</p>
  68. <p>For online documentation and support please refer to
  69. <a href="http://nginx.org/">nginx.org</a>.<br/>
  70. Commercial support is available at
  71. <a href="http://nginx.com/">nginx.com</a>.</p>
  72. <p><em>Thank you for using nginx.</em></p>
  73. </body>
  74. </html>
  76. $ kubectl logs nginx-app-4028413181-cnt1i
  77. 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：

  1. apiVersion: v1
  2. kind: Pod
  3. metadata:
  4.   name: nginx
  5.   labels:
  6.     app: nginx
  7. spec:
  8.   containers:
  9.   - name: nginx
  10.     image: nginx
  11.     ports:
  12.     - 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：

  1. apiVersion: extensions/v1beta1
  2. kind: Deployment
  3. metadata:
  4.   labels:
  5.     run: nginx-app
  6.   name: nginx-app
  7.   namespace: default
  8. spec:
  9.   replicas: 1
  10.   selector:
  11.     matchLabels:
  12.       run: nginx-app
  13.   strategy:
  14.     rollingUpdate:
  15.       maxSurge: 1
  16.       maxUnavailable: 1
  17.     type: RollingUpdate
  18.   template:
  19.     metadata:
  20.       labels:
  21.         run: nginx-app
  22.     spec:
  23.       containers:
  24.       - image: nginx
  25.         name: nginx-app
  26.         ports:
  27.         - containerPort: 80
  28.           protocol: TCP
  29.       dnsPolicy: ClusterFirst
  30.       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：

  1. apiVersion: v1
  2. kind: Pod
  3. metadata:
  4.   name: redis
  5. spec:
  6.   containers:
  7.   - name: redis
  8.     image: redis
  9.     volumeMounts:
  10.     - name: redis-persistent-storage
  11.       mountPath: /data/redis
  12.   volumes:
  13.   - name: redis-persistent-storage
  14.     hostPath:
  15.       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：

  1. $ kubectl expose deployment nginx-app --port=80 --target-port=80 --type=NodePort
  2. service "nginx-app" exposed
  3. $ kubectl describe service nginx-app
  4. Name:              nginx-app
  5. Namespace:             default
  6. Labels:            run=nginx-app
  7. Selector:              run=nginx-app
  8. Type:              ClusterIP
  9. IP:                10.0.0.66
  10. Port:              <unset>    80/TCP
  11. NodePort:              <unset>    30772/TCP
  12. Endpoints:             172.17.0.3:80
  13. Session Affinity:          None
  14. 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。