为了服务升级过程中提供可持续的不中断的服务，K8S提供了Rolling Update机制，它可以使得服务近乎无缝地平滑升级，即在不停止对外服务的前提下完成应用的更新。滚动更新采用渐进的方式逐步替换旧版本Pod，如果更新不如预期，那么也可以通过回滚操作恢复到更新前的状态。

1.1 实际使用

下面我们部署三副本应用，初始镜像为httpd:2.2.31，然后将其更新到httpd:2.2.32。
vi httpd.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: httpd
spec:
  replicas: 3
  selector:  
    matchLabels:
      run: httpd
  template:
    metadata:
      labels:
        run: httpd
    spec:
      containers:
      - name: httpd
        image: httpd:2.2.31
        ports:
        - containerPort: 80

通过kubectl apply部署

[root@k8s-master ~]# vi httpd.yaml
[root@k8s-master ~]#  kubectl apply -f httpd.yaml
deployment.apps/httpd configured
[root@k8s-master ~]#  kubectl get deployment httpd -o wide
NAME    READY   UP-TO-DATE   AVAILABLE   AGE   CONTAINERS   IMAGES         SELECTOR
httpd   3/3     2            3           3d    httpd        httpd:2.2.31   run=httpd

将配置⽂件中的httpd:2.2.31替换为httpd:2.2.32，再次执行kubectl apply

[root@k8s-master ~]# kubectl get deployment httpd -o wide
NAME    READY   UP-TO-DATE   AVAILABLE   AGE   CONTAINERS   IMAGES         SELECTOR
httpd   3/3     3            3           3d    httpd        httpd:2.2.32   run=httpd

[root@k8s-master ~]#  kubectl get replicaset -o wide
NAME                          DESIRED   CURRENT   READY   AGE     CONTAINERS   IMAGES         SELECTOR
httpd-6847bf7784              0         0         0       3m19s   httpd        httpd:2.2.31   pod-template-hash=6847bf7784,run=httpd
httpd-6c864b4669              3         3         3       59s     httpd        httpd:2.2.32   pod-template-hash=6c864b4669,run=httpd

Deployment httpd的镜像更新为httpd:2.2.32。

[root@k8s-master ~]# kubectl describe deployment httpd
Name:                   httpd
Namespace:              default
CreationTimestamp:      Sat, 27 Jan 2024 18:00:17 +0800
Labels:                 <none>
Annotations:            deployment.kubernetes.io/revision: 3
Selector:               run=httpd
Replicas:               3 desired | 3 updated | 3 total | 3 available | 0 unavailable
StrategyType:           RollingUpdate
MinReadySeconds:        0
RollingUpdateStrategy:  25% max unavailable, 25% max surge
Pod Template:
  Labels:  run=httpd
  Containers:
   httpd:
    Image:        httpd:2.2.32
    Port:         80/TCP
    Host Port:    0/TCP
    Environment:  <none>
    Mounts:       <none>
  Volumes:        <none>
Conditions:
  Type           Status  Reason
  ----           ------  ------
  Available      True    MinimumReplicasAvailable
  Progressing    True    NewReplicaSetAvailable
OldReplicaSets:  <none>
NewReplicaSet:   httpd-6c864b4669 (3/3 replicas created)
Events:
  Type    Reason             Age                From                   Message
  ----    ------             ----               ----                   -------
  Normal  ScalingReplicaSet  3m41s              deployment-controller  Scaled up replica set httpd-6847bf7784 to 1
  Normal  ScalingReplicaSet  3m13s              deployment-controller  Scaled down replica set httpd-ff8d77b9b to 2
  Normal  ScalingReplicaSet  3m13s              deployment-controller  Scaled up replica set httpd-6847bf7784 to 2
  Normal  ScalingReplicaSet  2m42s              deployment-controller  Scaled down replica set httpd-ff8d77b9b to 1
  Normal  ScalingReplicaSet  2m42s              deployment-controller  Scaled up replica set httpd-6847bf7784 to 3
  Normal  ScalingReplicaSet  2m25s              deployment-controller  Scaled down replica set httpd-ff8d77b9b to 0
  Normal  ScalingReplicaSet  81s                deployment-controller  Scaled up replica set httpd-6c864b4669 to 1
  Normal  ScalingReplicaSet  48s                deployment-controller  Scaled down replica set httpd-6847bf7784 to 2
  Normal  ScalingReplicaSet  48s                deployment-controller  Scaled up replica set httpd-6c864b4669 to 2
  Normal  ScalingReplicaSet  27s (x3 over 30s)  deployment-controller  (combined from similar events): Scaled down replica set httpd-6847bf7784 to 0

1.2 回滚

kubectlapply每次更新应用时，Kubernetes都会记录下当前的配置，保存为⼀个revision（版次），这样就可以回滚到某个特定revision。
下⾯实践回滚功能。应用有三个配置⽂件，即httpd.v1.yml、 httpd.v2.yml和httpd.v3.yml，分别对应不同的httpd镜像2.4.16、2.4.17和2.4.18。
vi httpd.v1.yml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: httpd
spec:
  revisionHistoryLimit: 10
  replicas: 3
  selector:
    matchLabels:
      run: httpd
  template:
    metadata:
      labels:
        run: httpd
    spec:
      containers:
      - name: httpd
        image: httpd:2.4.16
        ports:
        - containerPort: 80

vi httpd.v2.yml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: httpd
spec:
  revisionHistoryLimit: 10
  replicas: 3
  selector:
    matchLabels:
      run: httpd
  template:
    metadata:
      labels:
        run: httpd
    spec:
      containers:
      - name: httpd
        image: httpd:2.4.17
        ports:
        - containerPort: 80

vi httpd.v3.yml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: httpd
spec:
  revisionHistoryLimit: 10
  replicas: 3
  selector:
    matchLabels:
      run: httpd
  template:
    metadata:
      labels:
        run: httpd
    spec:
      containers:
      - name: httpd
        image: httpd:2.4.18
        ports:
        - containerPort: 80

通过kubectl apply部署并更新应用。

[root@k8s-master ~]#  kubectl apply -f httpd.v1.yml --record
deployment.apps/httpd configured
[root@k8s-master ~]#  kubectl apply -f httpd.v2.yml --record
deployment.apps/httpd configured
[root@k8s-master ~]#  kubectl apply -f httpd.v3.yml --record
deployment.apps/httpd configured
[root@k8s-master ~]# kubectl get deployment httpd -o wide
NAME    READY   UP-TO-DATE   AVAILABLE   AGE    CONTAINERS   IMAGES         SELECTOR
httpd   3/3     2            3           3d1h   httpd        httpd:2.4.18   run=httpd

–record的作用是将当前命令记录到revision记录中，这样我们就可以知道每个revison对应的是哪个配置文件了。

[root@k8s-master ~]# kubectl rollout history deployment httpd
deployment.apps/httpd
REVISION  CHANGE-CAUSE
7         kubectl apply --filename=httpd.v1.yml --record=true
8         kubectl apply --filename=httpd.v2.yml --record=true
9         kubectl apply --filename=httpd.v3.yml --record=true

如果要回滚到某个版本，比如revision 7，可以执行命令kubectl rollout undo deployment httpd --to-revision=7

[root@k8s-master ~]# kubectl rollout undo deployment httpd --to-revision=7
deployment.apps/httpd rolled back
[root@k8s-master ~]# kubectl rollout history deployment httpd
deployment.apps/httpd
REVISION  CHANGE-CAUSE
8         kubectl apply --filename=httpd.v2.yml --record=true
9         kubectl apply --filename=httpd.v3.yml --record=true
10        kubectl apply --filename=httpd.v1.yml --record=true

httpd.v1.yml 的revison 7变成了revison 10。不过我们可以通过CHANGE-CAUSE知
道每个revison的具体含义，所以⼀定要在执行kubectl apply时加–record参数。