194 lines
6.6 KiB
Markdown
194 lines
6.6 KiB
Markdown
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# Kubernetes 101
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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）：
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```sh
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$ kubectl run --image=nginx nginx-app --port=80
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deployment "nginx-app" created
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$ kubectl get pods
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NAME READY STATUS RESTARTS AGE
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nginx-app-4028413181-cnt1i 1/1 Running 0 52s
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```
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等到容器变成Running后，就可以各种`kubectl`命令来操作它了，比如
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- `kubectl get` - 类似于`docker ps`，查询资源列表
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- `kubectl describe` - 类似于`docker inspect`，获取资源的详细信息
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- `kubectl logs` - 类似于`docker logs`，获取容器的日志
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- `kubectl exec` - 类似于`docker exec`，在容器内执行一个命令
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```sh
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$ kubectl get pods
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NAME READY STATUS RESTARTS AGE
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nginx-app-4028413181-cnt1i 1/1 Running 0 6m
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$ kubectl exec nginx-app-4028413181-cnt1i ps aux
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USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
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root 1 0.0 0.5 31736 5108 ? Ss 00:19 0:00 nginx: master process nginx -g daemon off;
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nginx 5 0.0 0.2 32124 2844 ? S 00:19 0:00 nginx: worker process
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root 18 0.0 0.2 17500 2112 ? Rs 00:25 0:00 ps aux
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$ kubectl describe pod nginx-app-4028413181-cnt1i
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Name: nginx-app-4028413181-cnt1i
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Namespace: default
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Node: boot2docker/192.168.64.12
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Start Time: Tue, 06 Sep 2016 08:18:41 +0800
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Labels: pod-template-hash=4028413181
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run=nginx-app
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Status: Running
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IP: 172.17.0.3
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Controllers: ReplicaSet/nginx-app-4028413181
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Containers:
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nginx-app:
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Container ID: docker://4ef989b57d0a7638ad9c5bbc22e16d5ea5b459281c77074fc982eba50973107f
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Image: nginx
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Image ID: docker://sha256:4efb2fcdb1ab05fb03c9435234343c1cc65289eeb016be86193e88d3a5d84f6b
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Port: 80/TCP
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State: Running
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Started: Tue, 06 Sep 2016 08:19:30 +0800
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Ready: True
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Restart Count: 0
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Environment Variables: <none>
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Conditions:
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Type Status
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Initialized True
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Ready True
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PodScheduled True
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Volumes:
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default-token-9o8ks:
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Type: Secret (a volume populated by a Secret)
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SecretName: default-token-9o8ks
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QoS Tier: BestEffort
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Events:
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FirstSeen LastSeen Count From SubobjectPath Type Reason Message
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--------- -------- ----- ---- ------------- -------- ------ -------
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8m 8m 1 {default-scheduler } Normal Scheduled Successfully assigned nginx-app-4028413181-cnt1i to boot2docker
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8m 8m 1 {kubelet boot2docker} spec.containers{nginx-app} Normal Pulling pulling image "nginx"
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7m 7m 1 {kubelet boot2docker} spec.containers{nginx-app} Normal Pulled Successfully pulled image "nginx"
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7m 7m 1 {kubelet boot2docker} spec.containers{nginx-app} Normal Created Created container with docker id 4ef989b57d0a
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7m 7m 1 {kubelet boot2docker} spec.containers{nginx-app} Normal Started Started container with docker id 4ef989b57d0a
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$ kubectl logs nginx-app-4028413181-cnt1i
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127.0.0.1 - - [06/Sep/2016:00:27:13 +0000] "GET / HTTP/1.0 " 200 612 "-" "-" "-"
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127.0.0.1 - - [06/Sep/2016:00:27:15 +0000] "GET / HTTP/1.0 " 200 612 "-" "-" "-"
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```
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## 使用yaml定义Pod
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上面是通过`kubectl run`来启动了第一个Pod，但是`kubectl run`并不能支持所有的功能。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：
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```yaml
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apiVersion: v1
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kind: Pod
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metadata:
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name: nginx
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labels:
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app: nginx
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spec:
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containers:
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- name: nginx
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image: nginx
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ports:
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- containerPort: 80
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```
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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Deployment来自动创建Pod，这等价于这样一个配置：
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```yaml
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apiVersion: extensions/v1beta1
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kind: Deployment
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metadata:
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labels:
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run: nginx-app
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name: nginx-app
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namespace: default
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spec:
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replicas: 1
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selector:
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matchLabels:
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run: nginx-app
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strategy:
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rollingUpdate:
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maxSurge: 1
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maxUnavailable: 1
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type: RollingUpdate
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template:
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metadata:
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labels:
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run: nginx-app
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spec:
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containers:
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- image: nginx
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name: nginx-app
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ports:
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- containerPort: 80
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protocol: TCP
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dnsPolicy: ClusterFirst
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restartPolicy: Always
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```
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## 使用Volume
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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：
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```yaml
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apiVersion: v1
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kind: Pod
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metadata:
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name: redis
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spec:
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containers:
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- name: redis
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image: redis
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volumeMounts:
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- name: redis-persistent-storage
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mountPath: /data/redis
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volumes:
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- name: redis-persistent-storage
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hostPath:
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path: /data/
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```
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Kubernetes volume支持非常多的插件，可以根据实际需要来选择：
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* emptyDir
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* hostPath
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* gcePersistentDisk
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* awsElasticBlockStore
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* nfs
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* iscsi
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* flocker
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* glusterfs
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* rbd
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* cephfs
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* gitRepo
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* secret
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* persistentVolumeClaim
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* downwardAPI
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* azureFileVolume
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* vsphereVolume
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## 使用Service
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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：
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```yaml
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$ kubectl expose deployment nginx-app --type=NodePort --port=80 --target-port=80
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service "nginx-app" exposed
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$ kubectl describe service nginx-app
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Name: nginx-app
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Namespace: default
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Labels: run=nginx-app
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Selector: run=nginx-app
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Type: NodePort
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IP: 10.0.0.66
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Port: <unset> 80/TCP
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NodePort: <unset> 30772/TCP
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Endpoints: 172.17.0.3:80
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Session Affinity: None
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No events.
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```
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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，只能通过`http://node-ip:30772`来访问。
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