Kubernetes Replica Sets

ReplicaSet（RS）是Replication Controller（RC）的升级版本。ReplicaSet 和  Replication Controller之间的唯一区别是对选择器的支持。ReplicaSet支持labels user guide中描述的set-based选择器要求， 而Replication Controller仅支持equality-based的选择器要求。

如何使用ReplicaSet

大多数kubectl 支持Replication Controller 命令的也支持ReplicaSets。rolling-update命令除外，如果要使用rolling-update，请使用Deployments来实现。

虽然ReplicaSets可以独立使用，但它主要被 Deployments用作pod 机制的创建、删除和更新。当使用Deployment时，你不必担心创建pod的ReplicaSets，因为可以通过Deployment实现管理ReplicaSets。

何时使用ReplicaSet

ReplicaSet能确保运行指定数量的pod。然而，Deployment 是一个更高层次的概念，它能管理ReplicaSets，并提供对pod的更新等功能。因此，我们建议你使用Deployment来管理ReplicaSets，除非你需要自定义更新编排。

这意味着你可能永远不需要操作ReplicaSet对象，而是使用Deployment替代管理 。

示例

frontend.yaml

apiVersion: extensions/v1beta1
kind: ReplicaSet
metadata:
  name: frontend
  # these labels can be applied automatically
  # from the labels in the pod template if not set
  # labels:
    # app: guestbook
    # tier: frontend
spec:
  # this replicas value is default
  # modify it according to your case
  replicas: 3
  # selector can be applied automatically
  # from the labels in the pod template if not set,
  # but we are specifying the selector here to
  # demonstrate its usage.
  selector:
    matchLabels:
      tier: frontend
    matchExpressions:
      - {key: tier, operator: In, values: [frontend]}
  template:
    metadata:
      labels:
        app: guestbook
        tier: frontend
    spec:
      containers:
      - name: php-redis
        image: gcr.io/google_samples/gb-frontend:v3
        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

将此配置保存到（frontend.yaml）并提交到Kubernetes集群时，将创建定义的ReplicaSet及其管理的pod。

$ kubectl create -f frontend.yaml
replicaset "frontend" created
$ kubectl describe rs/frontend
Name:          frontend
Namespace:     default
Image(s):      gcr.io/google_samples/gb-frontend:v3
Selector:      tier=frontend,tier in (frontend)
Labels:        app=guestbook,tier=frontend
Replicas:      3 current / 3 desired
Pods Status:   3 Running / 0 Waiting / 0 Succeeded / 0 Failed
No volumes.
Events:
  FirstSeen    LastSeen    Count    From                SubobjectPath    Type        Reason            Message
  ---------    --------    -----    ----                -------------    --------    ------            -------
  1m           1m          1        {replicaset-controller }             Normal      SuccessfulCreate  Created pod: frontend-qhloh
  1m           1m          1        {replicaset-controller }             Normal      SuccessfulCreate  Created pod: frontend-dnjpy
  1m           1m          1        {replicaset-controller }             Normal      SuccessfulCreate  Created pod: frontend-9si5l
$ kubectl get pods
NAME             READY     STATUS    RESTARTS   AGE
frontend-9si5l   1/1       Running   0          1m
frontend-dnjpy   1/1       Running   0          1m
frontend-qhloh   1/1       Running   0          1m

ReplicaSet as an Horizontal Pod Autoscaler target

ReplicaSet也可以作为 Horizontal Pod Autoscalers (HPA)的目标 。也就是说，一个ReplicaSet可以由一个HPA来自动伸缩。以下是针对我们在上一个示例中创建的ReplicaSet的HPA示例。

hpa-rs.yaml
apiVersion: autoscaling/v1 kind: HorizontalPodAutoscaler metadata: name: frontend-scaler spec: scaleTargetRef: kind: ReplicaSet name: frontend minReplicas: 3 maxReplicas: 10 targetCPUUtilizationPercentage: 50

kubectl create -f hpa-rs.yaml