Running Multiple Instances of Your App

• 使用kubectl擴展app

Scaling

Scaling是透過改變Deployment的Replicas的數量來實現的

透過Scaling，Deployment將會確保新的Pods會被調度再有可用資源的Node上，且將Pod所需的數量增加到新的要求數目
k8s支援pods的autoscaling，但這部分不會在此解說，但必須知道的是有可能透過autoscaling會將Deployment的所有pods全數終止的情形發生

運行多個app時需要一個方法將流量分配，而service具有一個集成的load-blacer，將會把流量分配給對外開放的Deployment中的pods，且會持續的監控這些pods的endpoints，以確保能夠將流量輸送到這些pods中

而一旦你有了多個app運行，你將能夠使用滾動式更新
這是個重要的概念，將會決定service的部署策略

Interactive Tutorial - Scaling Your App

Step 1: Scaling a deployment

• 首先列出deployments list kubectl get deployments 接著會顯示幾項資訊

  • NAME lists the names of the Deployments in the cluster.
  • READY shows the ratio of CURRENT/DESIRED replicas
  • UP-TO-DATE displays the number of replicas that have been updated to achieve the desired state.
  • AVAILABLE displays how many replicas of the application are available to your users.
  • AGE displays the amount of time that the application has been running.

• 接著列出透過這個deployment所創建的ReplicaSet kubectl get rs ，而ReplicaSet的格式為 [DEPLOYMENT-NAME]-[RANDOM-STRING] 隨機字串的部分是使用pod-template-hash作為seed產生的，列出的ReplicaSet資訊有兩個重要的部分

  • DESIRED displays the desired number of replicas of the application, which you define when you create the Deployment. This is the desired state.
  • CURRENT displays how many replicas are currently running.

• 再來利用 kubectl scale 指令scale Deployment變成四個ReplicaSet kubectl scale deployments/kubernetes-bootcamp --replicas=4

• 再次取得deployments list觀察變化 kubectl get deployments

• 取得pods的資訊 kubectl get pods -o wide

• 觀察deployment得資訊 kubectl describe deployments/kubernetes-bootcamp

Step 2: Load Balancing

• 首先檢查Service是否在對流量進行負載平衡，然後利用describe找出對外裸露的IP & Ports kubectl describe services/kubernetes-bootcamp

• 將NodePort導出為環境變數 export NODE_PORT=$(kubectl get services/kubernetes-bootcamp -o go-template='{{(index .spec.ports 0).nodePort}}') echo NODE_PORT=$NODE_PORT

• 接著利用curl多呼叫幾次API，會發現透過不同的pod response curl $(minikube ip):$NODE_PORT

Step 3: Scale Down

• 將Service縮減為2個replicas kubectl scale deployments/kubernetes-bootcamp --replicas=2

• 取得deployments列表確認改動成功 kubectl get deployments

• 確認pods的數量 kubectl get pods -o wide

參考

• https://kubernetes.io/docs/tutorials/kubernetes-basics/scale/