k8s与HPA--基于Kubernetes的事件驱动自动缩放

KEDA能够对事件驱动的Kubernetes工做负载进行细粒度的自动缩放（包括从零到零的自动缩放）。 KEDA充当Kubernetes Metrics服务器，容许用户使用专用的Kubernetes自定义资源定义来定义自动缩放规则。

KEDA能够在云和边缘上运行，能够与Kubernetes组件（例如Horizo​​ntal Pod Autoscaler）进行本地集成，而且没有外部依赖性。

工做原理

KEDA在Kubernetes中扮演着两个关键角色。首先，它充当代理来激活和停用部署，以在无事件的状况下从零扩展到零。其次，它充当Kubernetes指标服务器，将丰富的事件数据（例如队列长度或流滞后）暴露给水平Pod自动缩放器以推进横向扩展。而后由部署决定是否直接从源中使用事件。这样能够保留丰富的事件集成，并使完成或放弃队列消息之类的手势能够当即使用。

Event sources and scalers

KEDA有许多“scalers”，它们既能够检测是否应激活或停用部署，也能够提供特定事件源的自定义指标。今天，对如下内容提供了缩放器支持：

• AWS CloudWatch
• AWS Simple Queue Service
• Azure Event Hub†
• Azure Service Bus Queues and Topics
• Azure Storage Queues
• GCP PubSub
• Kafka
• Liiklus
• Prometheus
• RabbitMQ
• Redis Lists

固然其余事件源正在增长中，以下：

规划中

• Azure IoT Hub#214
• Azure Storage Blobs#154
• Azure Cosmos DB#232
• Azure Monitor
• Azure Durable Functions

待规划

• AWS Kinesis
• Kubernetes Events
• MongoDB
• CockroachDB
• MQTT

ScaledObject自定义资源定义

为了使部署与事件源同步，须要部署ScaledObject自定义资源。 ScaledObjects包含有关要扩展的部署的信息，事件源的元数据（例如，链接字符串密钥，队列名称），轮询间隔和冷却时间。 ScaledObject将产生相应的自动扩展资源（HPA定义）以扩展部署。删除ScaledObjects时，将清除相应的HPA定义。

例如：

apiVersion: keda.k8s.io/v1alpha1
kind: ScaledObject
metadata:
  name: kafka-scaledobject
  namespace: default
  labels:
    deploymentName: azure-functions-deployment
spec:
  scaleTargetRef:
    deploymentName: azure-functions-deployment
  pollingInterval: 30
  triggers:
  - type: kafka
    metadata:
      # Required
      brokerList: localhost:9092
      consumerGroup: my-group       # Make sure that this consumer group name is the same one as the one that is consuming topics
      topic: test-topic
      lagThreshold: "50"

部署

可使用helm部署，也能够yaml部署。利用yaml部署能够执行以下操做：

kubectl apply -f KedaScaleController.yaml

KedaScaleController.yaml 以下：

apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  name: scaledobjects.keda.k8s.io
spec:
  group: keda.k8s.io
  version: v1alpha1
  names:
    kind: ScaledObject
    singular: scaledobject
    plural: scaledobjects
    shortNames:
      - sco
    categories:
      - keda
  scope: Namespaced
  additionalPrinterColumns:
    - name: Deployment
      type: string
      JSONPath: .spec.scaleTargetRef.deploymentName
    - name: Triggers
      type: string
      JSONPath: .spec.triggers[*].type
    - name: Age
      type: date
      JSONPath: .metadata.creationTimestamp
  validation:
    openAPIV3Schema:
      properties:
        spec:
          required: [triggers]
          type: object
          properties:
            scaleType:
              type: string
              enum: [deployment, job]
            pollingInterval:
              type: integer
            cooldownPeriod:
              type: integer
            minReplicaCount:
              type: integer
            maxReplicaCount:
              type: integer
            scaleTargetRef:
              required: [deploymentName]
              type: object
              properties:
                deploymentName:
                  type: string
                containerName:
                  type: string
            triggers:
              type: array
              items:
                type: object
                required: [type, metadata]
                properties:
                  type:
                    type: string
                  authenticationRef:
                    type: object
                    properties:
                      name:
                        type: string
                  metadata:
                    type: object
                    additionalProperties:
                      type: string
---
apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  name: triggerauthentications.keda.k8s.io
spec:
  group: keda.k8s.io
  version: v1alpha1
  names:
    kind: TriggerAuthentication
    singular: triggerauthentication
    plural: triggerauthentications
    shortNames:
      - ta
      - triggerauth
    categories:
      - keda
  scope: Namespaced
---
apiVersion: v1
kind: Namespace
metadata:
  name: keda
---
kind: ServiceAccount
apiVersion: v1
metadata:
  name: keda-operator
  namespace: keda
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: keda-operator-service-account-role-binding
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: cluster-admin
subjects:
  - kind: ServiceAccount
    name: keda-operator
    namespace: keda
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: keda:system:auth-delegator
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: system:auth-delegator
subjects:
  - kind: ServiceAccount
    name: keda-operator
    namespace: keda
---
apiVersion: rbac.authorization.k8s.io/v1
kind: RoleBinding
metadata:
  name: keda-auth-reader
  namespace: kube-system
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: Role
  name: extension-apiserver-authentication-reader
subjects:
  - kind: ServiceAccount
    name: keda-operator
    namespace: keda
---
apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app: keda-operator
  name: keda-operator
  namespace: keda
spec:
  replicas: 1
  selector:
    matchLabels:
      app: keda-operator
  template:
    metadata:
      labels:
        app: keda-operator
      name: keda-operator
    spec:
      serviceAccountName: keda-operator
      containers:
        - name: keda-operator
          image: kedacore/keda:latest
          args:
            - /adapter
            - --secure-port=6443
            - --logtostderr=true
            - --v=2
          ports:
            - containerPort: 6443
              name: https
            - containerPort: 8080
              name: http
          volumeMounts:
            - mountPath: /tmp
              name: temp-vol
      volumes:
        - name: temp-vol
          emptyDir: {}
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: custom-metrics-resource-reader
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: custom-metrics-resource-reader
subjects:
  - kind: ServiceAccount
    name: keda-operator
    namespace: keda
---
apiVersion: v1
kind: Service
metadata:
  name: keda-operator
  namespace: keda
spec:
  ports:
    - name: https
      port: 443
      targetPort: 6443
    - name: http
      port: 80
      targetPort: 8080
  selector:
    app: keda-operator
---
apiVersion: apiregistration.k8s.io/v1beta1
kind: APIService
metadata:
  name: v1beta1.external.metrics.k8s.io
spec:
  service:
    name: keda-operator
    namespace: keda
  group: external.metrics.k8s.io
  version: v1beta1
  insecureSkipTLSVerify: true
  groupPriorityMinimum: 100
  versionPriority: 100
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: custom-metrics-resource-reader
rules:
  - apiGroups:
      - ""
    resources:
      - namespaces
      - pods
      - services
      - external
    verbs:
      - get
      - list
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: keda-hpa-controller-custom-metrics
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: cluster-admin
subjects:
  - kind: ServiceAccount
    name: horizontal-pod-autoscaler
    namespace: kube-system

代码架构解读

关键代码在pkg文件夹下，以下图：

• adapter和provider 主要实现了一个custom metrics的adapter，基本规范参照github.com/kubernetes-incubator/custom-metrics-apiserver。若是是外部metrics ，那么主要是实现GetExternalMetric和ListAllExternalMetrics两个方法便可。
• apis 和 client 均为k8s架手架生成的。apis主要存放 crd --ScaledObject对象的定义，而client 为keda client和 informer 等。经过crd 扩展过k8s的应该对此比较熟悉。
• controller 即为一个k8s 针对ScaledObject的控制器。实际k8s 的开发中，crd 建立了以后，必须编写对应的controller，针对crd的add，update，delete三种事件做出实际操做。
• signals 则比较简单，封装了context.Context。
• kubernetes 比较简单，整体思路就是根据config，建立kdea client 和kube client，供controller 使用。
• handler 比较关键，基本上controller 中的sync 逻辑和metrics-server 提供metrics的接口 均在这里实现的。
• scalers。就是不一样事件源的实现。那么若是咱们想增长一种本身的事件源，在这里实现便可。

举例说明一下，当使用客户端--kubectl 或是client-go部署一个针对deployment A 的ScaledObject crd。想根据kafaka的消息积压数目进行hpa。那么controller会监听到建立了crd，将会对新增动做作出操做。具体就是，根据crd的具体内容建立一个hpa对象，crd 的spec 内容会转换成hpa 。此时官方k8s的hpa就会经过scalers中的kafka scaler 读取kafka指定topic的消息数目，而后最终由hpa controller 作出是否扩缩的决定。

结论

KEDA 目前处于Experimental Phase 阶段。微软和红帽但愿社区共同参与。

KEDA 并无实现了本身的HPA，其实最终起做用的依旧是社区中的HPA，他只是根据crd 内容生成了HPA 对象，只不过这个metrics 是外部metrics。KEDA 主要是集成了各类事件源。