# Deployment strategy
* Kubernetes 在計算 availableReplicas 數值時不考慮終止過程中的 Pod， availableReplicas 的值一定介於 replicas - maxUnavailable 和 replicas + maxSurge 之間。 因此，你可能在上線期間看到 Pod 個數比預期的多，Deployment 所消耗的總的資源也大於 replicas + maxSurge 個 Pod 所用的資源，直到被終止的 Pod 所設定的 terminationGracePeriodSeconds 到期為止。

# maxSurge、maxUnavailable 可以用數字和 % 宣告，如果使用數字宣告，單位就是 pod 的個數 
# maxSurge 代表是在 rolling update 過程中可以創建的額外 Pod 的最大數量或百分比
# maxUnavailable 代表是在 rolling update 過程中允許不可用的 Pod 的最大數量或百分比
$ echo 'apiVersion: apps/v1
kind: Deployment
metadata:
  creationTimestamp: null
  labels:
    app: test
  name: test
spec:
  replicas: 5
  selector:
    matchLabels:
      app: test
  strategy:
    rollingUpdate:
      maxSurge: 1                # 可以多出一個 pod
      maxUnavailable: 1          # 允許一個不可提供服務的 pod
  template:
    metadata:
      creationTimestamp: null
      labels:
        app: test
    spec:
      containers:
      - image: quay.io/cloudwalker/nginx
        name: nginx
        resources: {}
status: {}' | kubectl apply -f -

* 使用 rollout 更新 deploy

$ kubectl rollout restart deploy test

* deployment 會去呼叫新的 replica 多長出一個 pod，之後就再叫舊的 replica 減少一個 pod，以此類推

$ kubectl describe deploy test
......
Events:
  Type    Reason             Age                From                   Message
  ----    ------             ----               ----                   -------
  Normal  ScalingReplicaSet  54s                deployment-controller  Scaled up replica set test-6477f8d8fd to 5
  Normal  ScalingReplicaSet  33s                deployment-controller  Scaled up replica set test-6d7487958d to 1
  Normal  ScalingReplicaSet  33s                deployment-controller  Scaled down replica set test-6477f8d8fd to 4 from 5
  Normal  ScalingReplicaSet  32s                deployment-controller  Scaled up replica set test-6d7487958d to 2 from 1
  Normal  ScalingReplicaSet  30s                deployment-controller  Scaled down replica set test-6477f8d8fd to 3 from 4
  Normal  ScalingReplicaSet  30s                deployment-controller  Scaled up replica set test-6d7487958d to 3 from 2
  Normal  ScalingReplicaSet  28s                deployment-controller  Scaled down replica set test-6477f8d8fd to 2 from 3
  Normal  ScalingReplicaSet  28s                deployment-controller  Scaled up replica set test-6d7487958d to 4 from 3
  Normal  ScalingReplicaSet  26s                deployment-controller  Scaled down replica set test-6477f8d8fd to 1 from 2
  Normal  ScalingReplicaSet  22s (x2 over 26s)  deployment-controller  (combined from similar events): Scaled down replica set test-6477f8d8fd to 0 from 1

* 刪除 deployment

$ kubectl delete deploy test

### 測試 maxSurge 設為 2

$ echo 'apiVersion: apps/v1
kind: Deployment
metadata:
  creationTimestamp: null
  labels:
    app: test
  name: test
spec:
  replicas: 5
  selector:
    matchLabels:
      app: test
  strategy:
    rollingUpdate:
      maxSurge: 2
      maxUnavailable: 1
  template:
    metadata:
      creationTimestamp: null
      labels:
        app: test
    spec:
      containers:
      - image: quay.io/cloudwalker/nginx
        name: nginx
        resources: {}
status: {}' | kubectl apply -f -

* 使用 rollout 更新 deploy

$ kubectl rollout restart deploy test

* 可以發現 deployment 第一次呼叫新的 replica 會直接多兩個 pod，但是由於刪除舊的 pod 只有一個，因此第二次新的 replica 只有長出一個 pod。
* pod 數量需要符合 replicas - maxUnavailable 和 replicas + maxSurge 之間，以此題為例， pod 數量就是 4~7 之間。

$ kubectl describe deploy test
......
Events:
  Type    Reason             Age   From                   Message
  ----    ------             ----  ----                   -------
  Normal  ScalingReplicaSet  46s   deployment-controller  Scaled up replica set test-6477f8d8fd to 5
  Normal  ScalingReplicaSet  18s   deployment-controller  Scaled up replica set test-77b9c86d to 2
  Normal  ScalingReplicaSet  18s   deployment-controller  Scaled down replica set test-6477f8d8fd to 4 from 5
  Normal  ScalingReplicaSet  17s   deployment-controller  Scaled up replica set test-77b9c86d to 3 from 2
  Normal  ScalingReplicaSet  15s   deployment-controller  Scaled down replica set test-6477f8d8fd to 3 from 4
  Normal  ScalingReplicaSet  15s   deployment-controller  Scaled up replica set test-77b9c86d to 4 from 3
  Normal  ScalingReplicaSet  11s   deployment-controller  Scaled down replica set test-6477f8d8fd to 2 from 3
  Normal  ScalingReplicaSet  11s   deployment-controller  Scaled up replica set test-77b9c86d to 5 from 4
  Normal  ScalingReplicaSet  11s   deployment-controller  Scaled down replica set test-6477f8d8fd to 1 from 2
  Normal  ScalingReplicaSet  9s    deployment-controller  (combined from similar events): Scaled down replica set test-6477f8d8fd to 0 from 1

* 刪除 deployment

$ kubectl delete deploy test

### 測試 maxSurge 設為 1 maxUnavailable 設為 0

$ echo 'apiVersion: apps/v1
kind: Deployment
metadata:
  creationTimestamp: null
  labels:
    app: test
  name: test
spec:
  replicas: 5
  selector:
    matchLabels:
      app: test
  strategy:
    rollingUpdate:
      maxSurge: 1
      maxUnavailable: 0
  template:
    metadata:
      creationTimestamp: null
      labels:
        app: test
    spec:
      containers:
      - image: quay.io/cloudwalker/nginx
        name: nginx
        resources: {}
status: {}' | kubectl apply -f -

$ kubectl rollout restart deploy test

$ kubectl describe deploy test
......
Events:
  Type    Reason             Age   From                   Message
  ----    ------             ----  ----                   -------
  Normal  ScalingReplicaSet  22s   deployment-controller  Scaled up replica set test-86f864c84c to 5
  Normal  ScalingReplicaSet  10s   deployment-controller  Scaled up replica set test-6c754cf5f6 to 1
  Normal  ScalingReplicaSet  8s    deployment-controller  Scaled down replica set test-86f864c84c to 4 from 5
  Normal  ScalingReplicaSet  8s    deployment-controller  Scaled up replica set test-6c754cf5f6 to 2 from 1
  Normal  ScalingReplicaSet  5s    deployment-controller  Scaled down replica set test-86f864c84c to 3 from 4
  Normal  ScalingReplicaSet  5s    deployment-controller  Scaled up replica set test-6c754cf5f6 to 3 from 2
  Normal  ScalingReplicaSet  3s    deployment-controller  Scaled down replica set test-86f864c84c to 2 from 3
  Normal  ScalingReplicaSet  3s    deployment-controller  Scaled up replica set test-6c754cf5f6 to 4 from 3
  Normal  ScalingReplicaSet  1s    deployment-controller  Scaled down replica set test-86f864c84c to 1 from 2
  Normal  ScalingReplicaSet  1s    deployment-controller  (combined from similar events): Scaled up replica set test-6c754cf5f6 to 5 from 4

* 刪除 deployment

$ kubectl delete deploy test

### 測試 maxSurge 設為 0 maxUnavailable 設為 1

$ echo 'apiVersion: apps/v1
kind: Deployment
metadata:
  creationTimestamp: null
  labels:
    app: test
  name: test
spec:
  replicas: 1
  selector:
    matchLabels:
      app: test
  strategy:
    rollingUpdate:
      maxSurge: 0
      maxUnavailable: 1
  template:
    metadata:
      creationTimestamp: null
      labels:
        app: test
    spec:
      containers:
      - image: quay.io/cloudwalker/nginx
        name: nginx
        resources: {}
status: {}' | kubectl apply -f -

$ kubectl rollout restart deploy test

* 會先刪除原本的 pod 在建立新的 pod

$ kubectl describe deploy test
......
Events:
  Type    Reason             Age                From                   Message
  ----    ------             ----               ----                   -------
  Normal  ScalingReplicaSet  37s                deployment-controller  Scaled up replica set test-86f864c84c to 5
  Normal  ScalingReplicaSet  23s                deployment-controller  Scaled down replica set test-86f864c84c to 4 from 5
  Normal  ScalingReplicaSet  23s                deployment-controller  Scaled up replica set test-5b97f75c85 to 1 from 0
  Normal  ScalingReplicaSet  20s                deployment-controller  Scaled down replica set test-86f864c84c to 3 from 4
  Normal  ScalingReplicaSet  20s                deployment-controller  Scaled up replica set test-5b97f75c85 to 2 from 1
  Normal  ScalingReplicaSet  18s                deployment-controller  Scaled down replica set test-86f864c84c to 2 from 3
  Normal  ScalingReplicaSet  18s                deployment-controller  Scaled up replica set test-5b97f75c85 to 3 from 2
  Normal  ScalingReplicaSet  16s                deployment-controller  Scaled down replica set test-86f864c84c to 1 from 2
  Normal  ScalingReplicaSet  16s                deployment-controller  Scaled up replica set test-5b97f75c85 to 4 from 3
  Normal  ScalingReplicaSet  14s (x2 over 14s)  deployment-controller  (combined from similar events): Scaled up replica set test-5b97f75c85 to 5 from 4

## 參考文件
https://medium.com/@bubu.tripathy/understanding-maxsurge-and-maxunavailable-4966dfafc8ba

https://kubernetes.io/zh-cn/docs/concepts/workloads/controllers/deployment/