Posted Updated DevOps9 minutes read (About 1302 words)

Kubernetes 基礎(四) Pod Controller - Deployment

前言

上篇文章 - [Kubernetes] Kubernetes 基礎(三) Pod Controller - ReplicaSet 探討如何透過 ReplicaSet 管理多個 Pod 副本，確保 K8s 在任何時間點能保有特定數量的 Pod 於 Cluster 中運行。

本文接著討論 Pod 的另一種 Controller - Deployment

什麼是 Deployment Controller？

相較於先前討論 ReplicaSet 用於管理多個 Pod 副本（多個相同的 Pod），Deployment 則是利用 ReplicaSet 來更新 Pod 的版本。

Deployment 提供更動版本的功能，可以決定升級為新版或是退回舊版(rollback)，概念像是告訴 K8s Cluster 要什麼版本的 Pod，Deployment 負責變更且維持該版本的 Pod，一般用於處理版本更新與降版的流程。

實作

範例

跟前幾篇文章一樣用 nginx 服務作為範例，撰寫一個 Deployment 的 yaml 檔:
deployment.yaml

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
apiVersion: apps/v1
kind: Deployment
metadata: # 定義 Deployment 的名稱
  name: myapp-deployment-demo
  labels:
    app: myapp-deployment-demo
spec: # 描述 ReplicaSet 的規格
  replicas: 3
  selector:
    matchLabels:
      app: myapp-nginx-demo
  template: # 定義 Pod 的 Template
    metadata:
      labels: # 定義 Pod 名稱
        app: myapp-nginx-demo
    spec:
      containers: # 定義 Pod 裡的 container 資訊
      - name: myapp-nginx-demo
        image: nginx
        resources:
          limits:
            cpu: 200m
            memory: 200Mi
          requests:
              cpu: 100m
              memory: 64Mi

進行 apply

1
kubectl apply -f deployment.yml

下 get 指令查看 Pod 狀態

1
kubectl get deployments myapp-deployment-demo

output

1
2
NAME                        READY   UP-TO-DATE   AVAILABLE   AGE
myapp-deployment-demo   3/3     3            3           2m8s

Deployment 如何更新(Rollout Update)？

前面有提到 Deployment 最主要的功能就是負責變更、維持 Pod 的版本，那 Deployment 是如何對 Pod 做更新的？

Deployment 更新的流程


圖片源

從上面的 gif 圖來看，Deployment 會先創造一個 ReplicaSet，裡面有三個 Pod，若想把 Pod 裡的 Image 由 nginx 改成別的 image，如: httpd，或是想更新 image 版本，原始的 ReplicaSet 底下有多個相同版本的 Pod，當 Deployment 進行更新時採取的策略是盡可能安全：

  • 先起一個新版 ReplicaSet (如圖中的 ReplicaSet v2)
  • ReplicaSet v2 會循序地創造新的 Pod
  • 當新的 Pod 啟動，變成 Runing 的狀態時，舊版對應的 Pod 才會被砍掉，照這個循環去迭代新的 Pod
  • Rollout Update 的過程中有些是新版，有些是舊版
  • 舊的 ReplicaSet (如圖中的 ReplicaSet v1) 可以選擇要不要保留。 保留的好處在於，若有退版需求，將前述步驟反過來，循序地降回舊版

如何觀察 Rollout Update 的變化？

kubectl 有提供相關指令可以觀察 Deployment 的變化。
語法

1
kubectl rollout status deployment <deployment_name>

執行 rollout status 的指令前，先把 myapp-deployment-demo 的 yaml 檔裡的 image 從 nginx 改為 http，接著重新 apply。

建議先開兩個終端機的視窗，一個執行 apply，一個執行 rollout status，於 rollout status 的終端機視窗觀察變化

apply

1
kubectl apply -f deployment.yml

rollout status

1
kubectl rollout status deployment myapp-deployment-demo

看到輸出類似下方結果:

1
2
3
4
5
6
7
8
9
10
Waiting for deployment "myapp-deployment-demo" rollout to finish: 0 out of 3 new replicas have been updated...
Waiting for deployment "myapp-deployment-demo" rollout to finish: 1 out of 3 new replicas have been updated...
Waiting for deployment "myapp-deployment-demo" rollout to finish: 1 out of 3 new replicas have been updated...
Waiting for deployment "myapp-deployment-demo" rollout to finish: 1 out of 3 new replicas have been updated...
Waiting for deployment "myapp-deployment-demo" rollout to finish: 2 out of 3 new replicas have been updated...
Waiting for deployment "myapp-deployment-demo" rollout to finish: 2 out of 3 new replicas have been updated...
Waiting for deployment "myapp-deployment-demo" rollout to finish: 2 out of 3 new replicas have been updated...
Waiting for deployment "myapp-deployment-demo" rollout to finish: 1 old replicas are pending termination...
Waiting for deployment "myapp-deployment-demo" rollout to finish: 1 old replicas are pending termination...
deployment "myapp-deployment-demo" successfully rolled out

如何降版(Roll Back)？

執行 rollout undo 的指令即可降版(Roll Back)回去。
語法

1
kubectl rollout undo deployment <deployment_name>

將剛剛的 Deployment 範例降回上一個版本

1
kubectl rollout undo deployment myapp-deployment-demo

觀察 rollout status 的輸出結果

1
2
3
4
5
6
7
8
9
Waiting for deployment "myapp-deployment-demo" rollout to finish: 1 out of 3 new replicas have been updated...
Waiting for deployment "myapp-deployment-demo" rollout to finish: 1 out of 3 new replicas have been updated...
Waiting for deployment "myapp-deployment-demo" rollout to finish: 1 out of 3 new replicas have been updated...
Waiting for deployment "myapp-deployment-demo" rollout to finish: 2 out of 3 new replicas have been updated...
Waiting for deployment "myapp-deployment-demo" rollout to finish: 2 out of 3 new replicas have been updated...
Waiting for deployment "myapp-deployment-demo" rollout to finish: 2 out of 3 new replicas have been updated...
Waiting for deployment "myapp-deployment-demo" rollout to finish: 1 old replicas are pending termination...
Waiting for deployment "myapp-deployment-demo" rollout to finish: 1 old replicas are pending termination...
deployment "myapp-deployment-demo" successfully rolled out

總結

前面幾個範例中，已透過 kubectl apply 指令去操作 Deployment，更新服務的版本。

大部分的情況下都會採用 Deployment 的方式進行部署，可以滾動式地更新不同環境裡的服務，避免服務因為部署的關係(等待新的 image 下載等等)而突然中斷。

下篇文章會接著討論 Pod 底下的 Controller - DaemonSet

Reference

#

Comments

Recents

×