前言

昨天在介紹ReplicaSet時有提及，目前官方文檔中建議使用Deployment，來取代ReplicationController和ReplicaSet。

We recommend using Deployments instead of directly using ReplicaSets, unless you require custom update orchestration or don't require updates at all

Deployment

實際上Depolyment是一種高階抽象(high-level abstraction)，可以將其理解為: 當建立一個Deployment資源，K8s會自動建立好ReplicaSet和所對應的Pods。

Deployment is a higher-level concept that manages ReplicaSets and provides declarative updates to Pods along with a lot of other useful features.

透過設定期望狀態，Deployment Controller會不斷監控並使其狀態符合預期，而以下列出常見的應用情境:

• 建立Deployment來部屬ReplicaSet
• 擴展部屬以利承受更多負載
• 透過更新PodTemplateSpec來聲明Pod的新狀態
• 當前版本狀態不穩定，透過Rollback至早期的版本
• 透過HorizontalPodAutoscaler更新Deployment可因應負載來增加或減少 replication。

以下為Deployment設定檔範例

# deployment-definition.yml
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:1.14.2
        ports:
        - containerPort: 80