Kubernetes 101

體驗 Kubernetes 最簡單的方法是跑一個 nginx 容器，然後使用 kubectl 操作該容器。Kubernetes 提供了一個類似於 docker run 的命令 kubectl run，可以方便的創建一個容器（實際上創建的是一個由 deployment 來管理的 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

等到容器變成 Running 後，就可以用 kubectl 命令來操作它了，比如

kubectl get - 類似於 docker ps，查詢資源列表
kubectl describe - 類似於 docker inspect，獲取資源的詳細信息
kubectl logs - 類似於 docker logs，獲取容器的日誌
kubectl exec - 類似於 docker exec，在容器內執行一個命令

$ 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 定義 Pod

上面是通過 kubectl run 來啓動了第一個 Pod，但是 kubectl run 並不支持所有的功能。在 Kubernetes 中，更經常使用 yaml 文件來定義資源，並通過 kubectl create -f file.yaml 來創建資源。比如，一個簡單的 nginx Pod 可以定義爲：

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 並不是直接創建一個 Pod，而是先創建一個 Deployment 資源（replicas=1），再由與 Deployment 關聯的 ReplicaSet 來自動創建 Pod，這等價於這樣一個配置：

apiVersion: apps/v1
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 的生命週期通常比較短，只要出現了異常，就會創建一個新的 Pod 來代替它。那容器產生的數據呢？容器內的數據會隨着 Pod 消亡而自動消失。Volume 就是爲了持久化容器數據而生，比如可以爲 redis 容器指定一個 hostPath 來存儲 redis 數據：

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，可以根據實際需要來選擇：

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

使用 Service

前面雖然創建了 Pod，但是在 kubernetes 中，Pod 的 IP 地址會隨着 Pod 的重啓而變化，並不建議直接拿 Pod 的 IP 來交互。那如何來訪問這些 Pod 提供的服務呢？使用 Service。Service 爲一組 Pod（通過 labels 來選擇）提供一個統一的入口，併爲它們提供負載均衡和自動服務發現。比如，可以爲前面的 nginx-app 創建一個 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.

這樣，在 cluster 內部就可以通過 http://10.0.0.66 和 http://node-ip:30772 來訪問 nginx-app。而在 cluster 外面，則只能通過 http://node-ip:30772 來訪問。

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$ 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

$ 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 定義 Pod

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

apiVersion: apps/v1
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

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，可以根據實際需要來選擇：

emptyDir

hostPath

gcePersistentDisk

awsElasticBlockStore

nfs

iscsi

flocker

glusterfs

rbd

cephfs

gitRepo

secret

persistentVolumeClaim

downwardAPI

azureFileVolume

vsphereVolume

使用 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.

這樣，在 cluster 內部就可以通過 http://10.0.0.66 和 http://node-ip:30772 來訪問 nginx-app。而在 cluster 外面，則只能通過 http://node-ip:30772 來訪問。