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KubernetesDeployment滚动更新机制是什么

本篇内容介绍了“Kubernetes Deployment滚动更新机制是什么”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！

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定义Deployment时与rolling update的相关项

以下面的frontend Deployment为例，重点关注.spec.minReadySeconds,.spec.strategy.rollingUpdate.maxSurge,.spec.strategy.rollingUpdate. maxUnavailable。

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: frontend
spec:
  minReadySeconds: 5
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxSurge: 3
      maxUnavailable: 2
  replicas: 25
  template:
    metadata:
      labels:
        app: guestbook
        tier: frontend
    spec:
      containers:
      - name: php-redis
        image: gcr.io/google-samples/gb-frontend:v4
        resources:
          requests:
            cpu: 100m
            memory: 100Mi
        env:
        - name: GET_HOSTS_FROM
          value: DNS
          # If your cluster config does not include a dns service, then to
          # instead access environment variables to find service host
          # info, comment out the 'value: dns' line above, and uncomment the
          # line below:
          # value: env
        ports:
        - containerPort: 80

  • .spec.minReadySeconds: 新创建的Pod状态为Ready持续的时间至少为.spec.minReadySeconds才认为Pod Available(Ready)。

  • .spec.strategy.rollingUpdate.maxSurge: specifies the maximum number of Pods that can be created over the desired number of Pods. The value cannot be 0 if MaxUnavailable is 0. 可以为整数或者百分比，默认为desired Pods数的25%. Scale Up新的ReplicaSet时，按照比例计算出允许的MaxSurge，计算时向上取整(比如3.4，取4)。

  • .spec.strategy.rollingUpdate.maxUnavailable: specifies the maximum number of Pods that can be unavailable during the update process. The value cannot be 0 if maxSurge is 0.可以为整数或者百分比，默认为desired Pods数的25%. Scale Down旧的ReplicaSet时，按照比例计算出允许的maxUnavailable，计算时向下取整(比如3.6，取3)。

因此，在Deployment rollout时，需要保证Available(Ready) Pods数不低于 desired pods number - maxUnavailable; 保证所有的Pods数不多于 desired pods number + maxSurge。

滚动更新的流程

Note: A Deployment’s rollout is triggered if and only if the Deployment’s pod template (that is, .spec.template) is changed, for example if the labels or container images of the template are updated. Other updates, such as scaling the Deployment, do not trigger a rollout.

我们继续以上面的Deployment为例子，并考虑最常用的情况--更新image（发布新版本）:

kubectl set image deploy frontend php-redis=gcr.io/google-samples/gb-frontend:v3 --record

set image之后，导致Deployment's Pod Template发生变化，就会触发rollout。我们只考虑RollingUpdate策略（Kubernetes还支持ReCreate更新策略）。通过kubectl get rs -w来watch ReplicaSet的变化。

[root@master03 ~]# kubectl get rs -w
NAME                  DESIRED   CURRENT   READY     AGE
frontend-3114648124   25        25        25        14m
frontend-3099797709   0         0         0         1h
frontend-3099797709   0         0         0         1h
frontend-3099797709   3         0         0         1h
frontend-3114648124   23        25        25        17m
frontend-3099797709   5         0         0         1h
frontend-3114648124   23        25        25        17m
frontend-3114648124   23        23        23        17m
frontend-3099797709   5         0         0         1h
frontend-3099797709   5         3         0         1h
frontend-3099797709   5         5         0         1h
frontend-3099797709   5         5         1         1h
frontend-3114648124   22        23        23        17m
frontend-3099797709   5         5         2         1h
frontend-3114648124   22        23        23        17m
frontend-3114648124   22        22        22        17m
frontend-3099797709   6         5         2         1h
frontend-3114648124   21        22        22        17m
frontend-3099797709   6         5         2         1h
frontend-3114648124   21        22        22        17m
frontend-3099797709   7         5         2         1h
frontend-3099797709   7         6         2         1h
frontend-3114648124   21        21        21        17m
frontend-3099797709   7         6         2         1h
frontend-3099797709   7         7         2         1h
frontend-3099797709   7         7         2         1h
frontend-3099797709   7         7         3         1h
frontend-3099797709   7         7         4         1h
frontend-3114648124   20        21        21        17m
frontend-3099797709   8         7         4         1h
frontend-3114648124   20        21        21        17m
frontend-3114648124   20        20        20        17m
frontend-3099797709   8         7         4         1h
frontend-3099797709   8         8         4         1h
frontend-3099797709   8         8         5         1h
frontend-3114648124   19        20        20        17m
frontend-3099797709   9         8         5         1h
frontend-3114648124   19        20        20        17m
frontend-3099797709   9         8         5         1h
frontend-3099797709   9         9         5         1h
frontend-3114648124   19        19        19        17m
frontend-3099797709   9         9         5         1h
frontend-3114648124   18        19        19        18m
frontend-3099797709   10        9         5         1h
frontend-3114648124   18        19        19        18m
frontend-3099797709   10        9         5         1h
frontend-3114648124   18        18        18        18m
frontend-3099797709   10        10        5         1h
frontend-3099797709   10        10        5         1h
frontend-3114648124   18        18        18        18m
frontend-3099797709   10        10        6         1h
frontend-3099797709   10        10        6         1h
frontend-3114648124   17        18        18        18m
frontend-3114648124   17        18        18        18m
frontend-3099797709   11        10        6         1h
frontend-3099797709   11        10        6         1h
frontend-3114648124   17        17        17        18m
frontend-3099797709   11        11        6         1h

说明：

  1. frontend-3114648124为原来的RS（成为OldRS），frontend-3099797709为新建的RS（成为NewRS，当然也可能是Old RS，如果之前执行过这个一样的内容）。

  2. maxSurge:3, maxUnavailable=2, desired replicas=25

  • NewRS创建maxSurge(3)个Pods，这时达到pods数的上限值desired replicas + maxSurge (28个）

  • 不会等NewRS创建的Pods Ready，而是马上delete OldRS maxUnavailable(2)个Pods，这时Ready的Pods number最差也能保证desired replicas - maxUnavailable(23个)

  • 接下来的流程是不固定，只要新建的Pods有几个返回Ready，则意味着可以接着删除几个旧的Pods了。只要有几个删除成功的Pods返回，就会创建一定数量的Pods，只要All pods数量与上限值desired replicas + maxSurge有差值空间，就会接着创建新的Pods。

  • 如此进行滚动更新， 直到创建的新Pods个数达到desired replicas，并等待它们都Ready，然后再删除所有剩余的旧的Pods。至此，滚动流程结束。

对同一个Deployment先后触发滚动更新，逻辑如何？

我们考虑这个情况，但用户执行某个滚动更新后，未等待此次滚动更新结束，就继续执行了一次新的滚动更新请求，这时后台滚动流程会怎么样呢？会乱成一锅粥么？

我们继续以这个例子来看：

# deploy frontend 稳定运行在v2(frontend-888714875)时：
[root@master03 ~]# kubectl get rs -w
NAME                DESIRED   CURRENT   READY     AGE

====执行 kubectl set image deploy frontend php-redis=gcr.io/google-samples/gb-frontend:v3 --record
----备注： v3 --> frontend-776431694
frontend-776431694   0         0         0         6h
frontend-776431694   0         0         0         6h
frontend-776431694   3         0         0         6h
frontend-888714875   23        25        25        5h
frontend-776431694   5         0         0         6h
frontend-888714875   23        25        25        5h
frontend-888714875   23        23        23        5h
frontend-776431694   5         0         0         6h
frontend-776431694   5         3         0         6h
frontend-776431694   5         5         0         6h
frontend-776431694   5         5         1         6h
frontend-776431694   5         5         2         6h
frontend-776431694   5         5         3         6h
frontend-776431694   5         5         4         6h
frontend-776431694   5         5         4         6h
frontend-888714875   22        23        23        5h
frontend-776431694   6         5         4         6h
frontend-888714875   22        23        23        5h
frontend-888714875   22        22        22        5h
frontend-776431694   6         5         4         6h
frontend-776431694   6         6         4         6h
frontend-776431694   6         6         4         6h
frontend-888714875   19        22        22        5h
frontend-776431694   9         6         4         6h
frontend-888714875   19        22        22        5h
frontend-776431694   9         6         4         6h
frontend-888714875   19        19        19        5h
frontend-776431694   9         9         4         6h
frontend-888714875   19        19        19        5h

==== 执行 kubectl set image deploy frontend php-redis=gcr.io/google-samples/gb-frontend:v4 --record ====
----- 备注：v4 --> frontend-3099797709 ----

frontend-3099797709   0         0         0         6h
frontend-3099797709   0         0         0         6h
frontend-776431694   4         9         4         6h
frontend-3099797709   5         0         0         6h
frontend-3099797709   5         0         0         6h
frontend-3099797709   5         5         0         6h
frontend-776431694   4         9         4         6h
frontend-776431694   4         4         4         6h
frontend-3099797709   5         5         0         6h
frontend-3099797709   5         5         1         6h
frontend-3099797709   5         5         2         6h
frontend-3099797709   5         5         3         6h
frontend-3099797709   5         5         4         6h
frontend-3099797709   5         5         4         6h
frontend-776431694   2         4         4         6h
frontend-3099797709   7         5         4         6h
frontend-776431694   2         4         4         6h
frontend-776431694   2         2         2         6h
frontend-776431694   2         2         2         6h
frontend-3099797709   7         5         4         6h
frontend-776431694   0         2         2         6h
frontend-3099797709   7         7         4         6h
frontend-776431694   0         2         2         6h
frontend-3099797709   9         7         4         6h
frontend-776431694   0         0         0         6h
frontend-3099797709   9         7         4         6h
frontend-3099797709   9         9         4         6h
frontend-776431694   0         0         0         6h
frontend-3099797709   9         9         4         6h
frontend-3099797709   9         9         5         6h
frontend-3099797709   9         9         6         6h
frontend-3099797709   9         9         7         6h
frontend-888714875   17        19        19        5h
frontend-3099797709   11        9         7         6h
frontend-888714875   17        19        19        5h
frontend-888714875   17        17        17        5h
frontend-3099797709   11        9         7         6h
frontend-888714875   16        17        17        5h
frontend-3099797709   11        11        7         6h
frontend-3099797709   12        11        7         6h
frontend-888714875   16        17        17        5h
frontend-888714875   16        16        16        5h
frontend-3099797709   12        11        7         6h
frontend-3099797709   12        12        7         6h
frontend-3099797709   12        12        8         6h
frontend-3099797709   12        12        8         6h
frontend-888714875   15        16        16        5h
frontend-3099797709   13        12        8         6h
frontend-888714875   15        16        16        5h
frontend-888714875   15        15        15        5h
frontend-3099797709   13        12        8         6h
frontend-3099797709   13        13        8         6h
frontend-3099797709   13        13        8         6h
frontend-3099797709   13        13        9         6h
frontend-3099797709   13        13        10        6h
frontend-888714875   14        15        15        5h
frontend-3099797709   14        13        10        6h
frontend-888714875   14        15        15        5h
frontend-888714875   14        14        14        5h
frontend-3099797709   14        13        10        6h
frontend-888714875   14        14        14        5h
frontend-3099797709   14        14        11        6h
frontend-3099797709   14        14        12        6h
frontend-3099797709   14        14        12        6h
frontend-3099797709   14        14        12        6h
frontend-888714875   11        14        14        5h
frontend-3099797709   17        14        12        6h
frontend-888714875   11        14        14        5h
frontend-3099797709   17        14        12        6h
frontend-888714875   11        11        11        5h
frontend-3099797709   17        17        12        6h
frontend-888714875   11        11        11        5h
frontend-3099797709   17        17        12        6h
frontend-3099797709   17        17        13        6h
frontend-3099797709   17        17        14        6h
frontend-3099797709   17        17        14        6h
frontend-888714875   10        11        11        5h
frontend-3099797709   18        17        14        6h
frontend-888714875   10        11        11        5h
frontend-888714875   10        10        10        5h
frontend-3099797709   18        17        14        6h
frontend-3099797709   18        18        14        6h
frontend-3099797709   18        18        15        6h
frontend-888714875   9         10        10        5h
frontend-3099797709   18        18        16        6h
frontend-888714875   9         10        10        5h
frontend-3099797709   19        18        16        6h
frontend-3099797709   19        18        16        6h
frontend-888714875   9         9         9         5h
frontend-888714875   7         9         9         5h
frontend-3099797709   19        18        16        6h
frontend-888714875   7         9         9         5h
frontend-3099797709   21        18        16        6h
frontend-888714875   7         9         9         5h
frontend-3099797709   21        19        16        6h
frontend-888714875   7         7         7         5h
frontend-3099797709   21        21        16        6h
frontend-888714875   7         7         7         5h
frontend-3099797709   21        21        17        6h
frontend-3099797709   21        21        18        6h
frontend-3099797709   21        21        18        6h
frontend-888714875   5         7         7         5h
frontend-888714875   5         7         7         5h
frontend-3099797709   23        21        18        6h
frontend-888714875   5         5         5         5h
frontend-3099797709   23        21        18        6h
frontend-3099797709   23        23        18        6h
frontend-3099797709   23        23        18        6h
frontend-3099797709   23        23        19        6h
frontend-3099797709   23        23        20        6h
frontend-3099797709   23        23        20        6h
frontend-888714875   3         5         5         5h
frontend-3099797709   25        23        20        6h
frontend-888714875   3         5         5         5h
frontend-888714875   3         3         3         5h
frontend-3099797709   25        23        20        6h
frontend-888714875   3         3         3         5h
frontend-3099797709   25        25        20        6h
frontend-3099797709   25        25        21        6h
frontend-3099797709   25        25        22        6h
frontend-3099797709   25        25        22        6h
frontend-888714875   2         3         3         5h
frontend-888714875   2         3         3         5h
frontend-888714875   2         2         2         5h
frontend-888714875   2         2         2         5h
frontend-3099797709   25        25        23        6h
frontend-888714875   1         2         2         5h
frontend-888714875   1         2         2         5h
frontend-888714875   1         1         1         5h
frontend-3099797709   25        25        23        6h
frontend-888714875   0         1         1         5h
frontend-888714875   0         1         1         5h
frontend-888714875   0         0         0         5h
frontend-3099797709   25        25        24        6h
frontend-3099797709   25        25        25        6h
frontend-3099797709   25        25        25        6h

说明：
deployment frontend稳定运行在v2版本(RS：frontend-888714875)，然后执行kubectl set image触发滚动更新到v3版本（RS: frontend-776431694）, 当v3 RS的desired个数scale up到9个，ready个数为4个时，用户又执行kubectl set image触发滚动更新到v4版本（RS: frontend-3099797709）。

  • v2到v3的滚动流程同上一小节的描述；

  • 当新的滚动流程触发后，最老的v2的RS保持不动，不会继续scale down。

  • 然后v4将通过滚动更新的方式把已经scale up的9个v3 RS的pods替换掉，将所有v3的Pods升级到v4。

  • 最后再接着v4 RS滚动更新把v2的RS所有的旧Pods都升级到v4。

  • 整个完整的滚动流程中，都必须遵守maxSurge和maxUnavailable的约束，不能越雷池半步。

设想一个更复杂的场景：如果在上述v4滚动更新替换到半吊子的v3 RS过程中，用户又触发了一个滚动更新到v5版本，流程会怎么样呢？ 不要怕，原理是一样的，Deployment rolling update总是先把最新的RS滚动更新替换掉，然后逐步把旧的RS滚动更新替换掉，直到最最老的那个RS scale down为0，流程就结束了，可以简要概括如下：

  • 剩余的v2, v3停止scale down；

  • v5把v4通过滚动更新的方式替换掉;

  • v5再把剩余v3通过滚动更新的方式替换掉;

  • v5再把剩余v2通过滚动更新的方式替换掉;

最后的RS会按照RS从新到旧排序的方式，逐步把旧的RS Pods scale down to 0， 自己 scale up to desired pods number。

理解rollout pause和resume

或许很多人至今还会这么觉得：整个滚动更新的过程中，一旦用户执行了kubectl rollout pause deploy/frontend后，正在执行的滚动流程就会立刻停止，然后用户执行kubectl rollout resume deploy/frontend就会继续未完成的滚动更新。那你就大错特错了！

kubectl rollout pause只会用来停止触发下一次rollout。什么意思呢？ 上面描述的这个场景，正在执行的滚动历程是不会停下来的，而是会继续正常的进行滚动，直到完成。等下一次，用户再次触发rollout时，Deployment就不会真的去启动执行滚动更新了，而是等待用户执行了kubectl rollout resume，流程才会真正启动执行。

ReplicaSet和rollout history的关系

前提，你要知道关于--record：
Setting the kubectl flag --record to true allows you to record current command in the annotations of the resources being created or updated.

默认情况下，所有通过kubectl xxxx --record都会被kubernetes记录到etcd进行持久化，这无疑会占用资源，最重要的是，时间久了，当你kubectl get rs时，会有成百上千的垃圾RS返回给你，那时你可能就眼花缭乱了。

上生产时，我们最好通过设置Deployment的.spec.revisionHistoryLimit来限制最大保留的revision number，比如15个版本，回滚的时候一般只会回滚到最近的几个版本就足够了。

执行下面的命令，可以返回某个Deployment的所有record记录：

$ kubectl rollout history deployment/nginx-deployment
deployments "nginx-deployment"
REVISION    CHANGE-CAUSE
1           kubectl create -f docs/user-guide/nginx-deployment.yaml --record
2           kubectl set image deployment/nginx-deployment nginx=nginx:1.9.1
3           kubectl set image deployment/nginx-deployment nginx=nginx:1.91

然后执行rollout undo命令就可以回滚到to-revision指定的版本。

kubectl rollout undo deployment/nginx-deployment --to-revision=2
deployment "nginx-deployment" rolled back

其实rollout history中记录的revision都和ReplicaSets一一对应。如果手动delete某个ReplicaSet，对应的rollout history就会被删除，也就是还说你无法回滚到这个revison了。

roolout history和ReplicaSet的对应关系，可以在kubectl describe rs $RSNAME返回的revision字段中得到，这里的revision就对应着roolout history返回的revison。

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