1.Kubernetes的controller pattern
需要认识到Kubernetes操作Pod的逻辑，都是由控制器来完成的。
查看之前写过的nginx-deployment的YAML文件
[root@kubernetes01 ~]# cat nginx-deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-deployment
labels:
app: nginx
spec:
replicas: 2
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx:1.8.1
ports:
- containerPort: 80
Kubernetes中的kube-controller-manager组件执行了上面的Deployment定义的编排动作。
Deployment定义的template字段，这种字段在Kubernetes中叫做PodTemplate（Pod模版）。
而这个YAML文件描述的就是Deployment这样一个控制器，template以上的部分是对控制器的定义，template以下的部分属于对被控制的对象的定义。
2.Kubernetes Pod 的自动伸缩
Pod水平扩展和水平收缩的能力依赖于Kubernetes中一个非常重要的API对象，ReplicaSet也就是副本集。
2.1.Pod的水平扩展和伸缩
查看例子的YAML文件
[root@kubernetes01 ~]# cat nginx-replicaset.yaml
apiVersion: apps/v1
kind: ReplicaSet
metadata:
name: nginx-replicaset
labels:
app: nginx
spec:
replicas: 3
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx:1.8.0
上面这个例子中，ReplicaSet对象是由一个Pod模版和副本数目的多少定义组成的。
再看这个YAML文件
[root@kubernetes01 ~]# cat nginx-deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-deployment
labels:
app: nginx
spec:
replicas: 3
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx:1.8.1
ports:
- containerPort: 80
Deployment控制器实际操作的是ReplicaSet对象，而不是Pod对象。
这个定义replicas=3的Deployment与他的ReplicaSet以及Pod的关系可以理解成是一种分层控制层层控制的关系，replicas的值决定是扩展还是收缩，当你去扩展或是收缩的时候还伴随着滚动更新。
2.2.Pod的滚动更新
1.首先修改镜像版本
kubectl edit deployment/nginx-deployment
spec:
containers:
- image: nginx:1.9.1
imagePullPolicy: IfNotPresent
name: nginx
ports:
- containerPort: 80
protocol: TCP
resources: {}
terminationMessagePath: /dev/termination-log
terminationMessagePolicy: File
dnsPolicy: ClusterFirst
restartPolicy: Always
schedulerName: default-scheduler
securityContext: {}
terminationGracePeriodSeconds: 30
这块儿直接把image中nginx的版本改成了1.9.1然后保存退出，文件有了新的变动会触发更新。
2.查看滚动更新的效果
[root@kubernetes01 ~]# kubectl describe deployment nginx-deployment | tail -n 10
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal ScalingReplicaSet 30m deployment-controller Scaled up replica set nginx-deployment-7f987f7889 to 3
Normal ScalingReplicaSet 8m55s deployment-controller Scaled up replica set nginx-deployment-6987cdb55b to 1
Normal ScalingReplicaSet 8m54s deployment-controller Scaled down replica set nginx-deployment-7f987f7889 to 2
Normal ScalingReplicaSet 8m54s deployment-controller Scaled up replica set nginx-deployment-6987cdb55b to 2
Normal ScalingReplicaSet 8m33s deployment-controller Scaled down replica set nginx-deployment-7f987f7889 to 1
Normal ScalingReplicaSet 8m33s deployment-controller Scaled up replica set nginx-deployment-6987cdb55b to 3
Normal ScalingReplicaSet 8m12s deployment-controller Scaled down replica set nginx-deployment-7f987f7889 to 0
首先通过Events输出的内容我们可以看到这个deployment滚动更新的Message。
[root@kubernetes01 ~]# kubectl get rs
NAME DESIRED CURRENT READY AGE
nginx-deployment-5c689d88bb 0 0 0 14d
nginx-deployment-5cd6d46846 0 0 0 14d
nginx-deployment-67d57d6df9 0 0 0 9d
nginx-deployment-6987cdb55b 3 3 3 10m
nginx-deployment-7f987f7889 0 0 0 9d
nginx-replicaset 0 0 0 37m
然后查看这个Deployment控制的ReplicaSet的最终状态，这里等待了一会儿所以已经是最终的状态了，配合前边Events的信息，nginx-deployment-6987cdb55b up to 1，旧的nginx-deployment-7f987f7889 down to 2，通过交替的逐一的升级和降级“滚动更新”，最终
nginx-deployment-6987cdb55b 的DESIRED，CURRENT，READY都变成了期望值3！
状态字段的含义：
DESIRED：用户期望的Pod副本个数
CURRENT：当前处于Running状态的Pod的个数
UP-TO-DATE：当前处于最新版本的Pod的个数
READY：处于健康检查正确状态的Pod的个数，Running状态的新版本
我们还可以通过 kubectl rollout status deployment/nginx-deployment这条命令实时的查看Deployment对象的状态变化。应用的版本和ReplicaSet是一一对应的，这也是Kubernetes实现对多个应用版本描述的设计思想。
3.Kubernetes Deployment对应用的版本控制
1.首先通过命令直接这个Deployment的镜像修改成错误的
[root@kubernetes01 ~]# kubectl set image deployment/nginx-deployment nginx=nginx:1.99
deployment.extensions/nginx-deployment image updated
2.查看Deployment的状态
[root@kubernetes01 ~]# kubectl get rs
NAME DESIRED CURRENT READY AGE
nginx-deployment-5476c5bdd 1 1 0 12s
nginx-deployment-5c689d88bb 0 0 0 15d
nginx-deployment-5cd6d46846 0 0 0 14d
nginx-deployment-67d57d6df9 0 0 0 10d
nginx-deployment-6987cdb55b 3 3 3 46m
nginx-deployment-7f987f7889 0 0 0 10d
nginx-replicaset 0 0 0 73m
nginx-staticwebsite-648bc64544 2 2 2 9d
nginx-staticwebsite-8479f8997f 0 0 0 9d
tomcat-deployment-001-84d957dc97 2 2 2 6d1h
tomcat-test-6cc4d85cf6 2 2 2 6d18h
可以看到nginx-deployment-5476c5bdd这个有问题的Deployment的状态。
3.回退到上个版本
[root@kubernetes01 ~]# kubectl rollout undo deployment/nginx-deployment
deployment.extensions/nginx-deployment
4.回退之后查看Deployment的状态
[root@kubernetes01 ~]# kubectl get rs
NAME DESIRED CURRENT READY AGE
nginx-deployment-5476c5bdd 0 0 0 90s
nginx-deployment-5c689d88bb 0 0 0 15d
nginx-deployment-5cd6d46846 0 0 0 14d
nginx-deployment-67d57d6df9 0 0 0 10d
nginx-deployment-6987cdb55b 3 3 3 47m
nginx-deployment-7f987f7889 0 0 0 10d
nginx-replicaset 0 0 0 74m
nginx-staticwebsite-648bc64544 2 2 2 9d
nginx-staticwebsite-8479f8997f 0 0 0 9d
tomcat-deployment-001-84d957dc97 2 2 2 6d1h
tomcat-test-6cc4d85cf6 2 2 2 6d18h
5.查看这个Deployment的历史版本记录
[root@kubernetes01 ~]# kubectl rollout history deployment/nginx-deployment
deployment.extensions/nginx-deployment
REVISION CHANGE-CAUSE
1 <none>
2 <none>
3 <none>
4 <none>
6 <none>
7 <none>
6.查看第一个版本的信息
[root@kubernetes01 ~]# kubectl rollout history deployment/nginx-deployment --revision=1
deployment.extensions/nginx-deployment with revision #1
Pod Template:
Labels: app=nginx
pod-template-hash=5c689d88bb
Containers:
nginx:
Image: nginx:1.7.9
Port: 80/TCP
Host Port: 0/TCP
Environment: <none>
Mounts: <none>
Volumes: <none>
7.切换到第一个版本
[root@kubernetes01 ~]# kubectl rollout undo deployment/nginx-deployment --to-revision=1
deployment.extensions/nginx-deployment
8.查看Deployment的状态
[root@kubernetes01 ~]# kubectl get rs
NAME DESIRED CURRENT READY AGE
nginx-deployment-5476c5bdd 0 0 0 19m
nginx-deployment-5c689d88bb 3 3 3 15d
nginx-deployment-5cd6d46846 0 0 0 14d
nginx-deployment-67d57d6df9 0 0 0 10d
nginx-deployment-6987cdb55b 0 0 0 65m
nginx-deployment-7f987f7889 0 0 0 10d
nginx-replicaset 0 0 0 92m
nginx-staticwebsite-648bc64544 2 2 2 9d
nginx-staticwebsite-8479f8997f 0 0 0 9d
tomcat-deployment-001-84d957dc97 2 2 2 6d1h
tomcat-test-6cc4d85cf6 2 2 2 6d18h
这块儿可以看到副本集nginx-deployment-5c689d88bb已经变成了我们期望的状态。
4.总结
Deployment是一个俩层控制器，首先通过控制ReplicaSet来控制版本，ReplicaSet则控制Pod。我们可以通过使用kubectl rollout命令控制应用的版本。
PS:文中服务器使用的是国内某☁️的机器
欢迎大家留言讨论哦，欢迎大家和我一起学习Kubernetes～～～
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