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利用kubernetes中的leader选举机制来完成自己的HA应用

2022-06-29 22:17:00 【Hermokrates】

Backgroud

前一章中，对kubernetes的选举原理进行了深度剖析，下面就通过一个example来实现一个，利用kubernetes提供的选举机制完成的高可用应用。

对于此章需要提前对一些概念有所了解后才可以继续看下去

leader election mechanism
RBCA
Pod runtime mechanism

Implementation

代码实现

如果仅仅是使用Kubernetes中的锁，实现的代码也只有几行而已。

package main

import (
	"context"
	"flag"
	"fmt"
	"os"
	"os/signal"
	"syscall"
	"time"

	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	clientset "k8s.io/client-go/kubernetes"
	"k8s.io/client-go/rest"
	"k8s.io/client-go/tools/clientcmd"
	"k8s.io/client-go/tools/leaderelection"
	"k8s.io/client-go/tools/leaderelection/resourcelock"
	"k8s.io/klog/v2"
)

func buildConfig(kubeconfig string) (*rest.Config, error) {
    
	if kubeconfig != "" {
    
		cfg, err := clientcmd.BuildConfigFromFlags("", kubeconfig)
		if err != nil {
    
			return nil, err
		}
		return cfg, nil
	}

	cfg, err := rest.InClusterConfig()
	if err != nil {
    
		return nil, err
	}
	return cfg, nil
}

func main() {
    
	klog.InitFlags(nil)

	var kubeconfig string
	var leaseLockName string
	var leaseLockNamespace string
	var id string
	// 初始化客户端的部分
	flag.StringVar(&kubeconfig, "kubeconfig", "", "absolute path to the kubeconfig file")
	flag.StringVar(&id, "id", "", "the holder identity name")
	flag.StringVar(&leaseLockName, "lease-lock-name", "", "the lease lock resource name")
	flag.StringVar(&leaseLockNamespace, "lease-lock-namespace", "", "the lease lock resource namespace")
	flag.Parse()

	if leaseLockName == "" {
    
		klog.Fatal("unable to get lease lock resource name (missing lease-lock-name flag).")
	}
	if leaseLockNamespace == "" {
    
		klog.Fatal("unable to get lease lock resource namespace (missing lease-lock-namespace flag).")
	}
	config, err := buildConfig(kubeconfig)
	if err != nil {
    
		klog.Fatal(err)
	}
	client := clientset.NewForConfigOrDie(config)

	run := func(ctx context.Context) {
    
		// 实现的业务逻辑，这里仅仅为实验，就直接打印了
		klog.Info("Controller loop...")

		for {
    
			fmt.Println("I am leader, I was working.")
			time.Sleep(time.Second * 5)
		}
	}

	// use a Go context so we can tell the leaderelection code when we
	// want to step down
	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()

	// 监听系统中断
	ch := make(chan os.Signal, 1)
	signal.Notify(ch, os.Interrupt, syscall.SIGTERM)
	go func() {
    
		<-ch
		klog.Info("Received termination, signaling shutdown")
		cancel()
	}()

	// 创建一个资源锁
	lock := &resourcelock.LeaseLock{
    
		LeaseMeta: metav1.ObjectMeta{
    
			Name:      leaseLockName,
			Namespace: leaseLockNamespace,
		},
		Client: client.CoordinationV1(),
		LockConfig: resourcelock.ResourceLockConfig{
    
			Identity: id,
		},
	}

	// 开启一个选举的循环
	leaderelection.RunOrDie(ctx, leaderelection.LeaderElectionConfig{
    
		Lock:            lock,
		ReleaseOnCancel: true,
		LeaseDuration:   60 * time.Second,
		RenewDeadline:   15 * time.Second,
		RetryPeriod:     5 * time.Second,
		Callbacks: leaderelection.LeaderCallbacks{
    
			OnStartedLeading: func(ctx context.Context) {
    
				// 当选举为leader后所运行的业务逻辑
				run(ctx)
			},
			OnStoppedLeading: func() {
    
				// we can do cleanup here
				klog.Infof("leader lost: %s", id)
				os.Exit(0)
			},
			OnNewLeader: func(identity string) {
     // 申请一个选举时的动作
				if identity == id {
    
					return
				}
				klog.Infof("new leader elected: %s", identity)
			},
		},
	})
}

注：这种lease锁只能在in-cluster模式下运行，如果需要类似二进制部署的程序，可以选择endpoint类型的资源锁。

生成镜像

这里已经制作好了镜像并上传到dockerhub（cylonchau/leaderelection:v0.0.2）上了，如果只要学习运行原理，则忽略此步骤

FROM golang:alpine AS builder
MAINTAINER cylon
WORKDIR /election
COPY . /election
ENV GOPROXY https://goproxy.cn,direct
RUN GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go build -o elector main.go

FROM alpine AS runner
WORKDIR /go/elector
COPY --from=builder /election/elector .
VOLUME ["/election"]
ENTRYPOINT ["./elector"]

准备资源清单

默认情况下，Kubernetes运行的pod在请求Kubernetes集群内资源时，默认的账户是没有权限的，默认服务帐户无权访问协调 API，因此我们需要创建另一个serviceaccount并相应地设置对应的RBAC权限绑定；在清单中配置上这个sa，此时所有的pod就会有协调锁的权限了

apiVersion: v1
kind: ServiceAccount
metadata:
  name: sa-leaderelection
---
apiVersion: rbac.authorization.k8s.io/v1
kind: Role
metadata:
  name: leaderelection
rules:
  - apiGroups:
      - coordination.k8s.io
    resources:
      - leases
    verbs:
      - '*'
---
apiVersion: rbac.authorization.k8s.io/v1
kind: RoleBinding
metadata:
  name: leaderelection
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: Role
  name: leaderelection
subjects:
  - kind: ServiceAccount
    name: sa-leaderelection
---
apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app: leaderelection
  name: leaderelection
  namespace: default
spec:
  replicas: 3
  selector:
    matchLabels:
      app: leaderelection
  template:
    metadata:
      labels:
        app: leaderelection
    spec:
      containers:
        - image: cylonchau/leaderelection:v0.0.2
          imagePullPolicy: IfNotPresent
          command: ["./elector"]
          args:
          - "-id=$(POD_NAME)"
          - "-lease-lock-name=test"
          - "-lease-lock-namespace=default"
          env:
          - name: POD_NAME
            valueFrom:
              fieldRef:
                apiVersion: v1
                fieldPath: metadata.name
          name: elector
      serviceAccountName: sa-leaderelection

集群中运行

执行完清单后，当pod启动后，可以看到会创建出一个 lease

$ kubectl get lease
NAME   HOLDER                            AGE
test   leaderelection-5644c5f84f-frs5n   1s


$ kubectl describe lease
Name:         test
Namespace:    default
Labels:       <none>
Annotations:  <none>
API Version:  coordination.k8s.io/v1
Kind:         Lease
Metadata:
  Creation Timestamp:  2022-06-28T16:39:45Z
  Managed Fields:
    API Version:  coordination.k8s.io/v1
    Fields Type:  FieldsV1
    fieldsV1:
      f:spec:
        f:acquireTime:
        f:holderIdentity:
        f:leaseDurationSeconds:
        f:leaseTransitions:
        f:renewTime:
    Manager:         elector
    Operation:       Update
    Time:            2022-06-28T16:39:45Z
  Resource Version:  131693
  Self Link:         /apis/coordination.k8s.io/v1/namespaces/default/leases/test
  UID:               bef2b164-a117-44bd-bad3-3e651c94c97b
Spec:
  Acquire Time:            2022-06-28T16:39:45.931873Z
  Holder Identity:         leaderelection-5644c5f84f-frs5n
  Lease Duration Seconds:  60
  Lease Transitions:       0
  Renew Time:              2022-06-28T16:39:55.963537Z
Events:                    <none>