目录

• 一、背景
• 二、官网代码示例
• 三、锁的实现

一、背景

在kubernetes的世界中，很多组件仅仅需要一个实例在运行，比如controller-manager或第三方的controller，但是为了高可用性，需要组件有多个副本，在发生故障的时候需要自动切换。因此，需要利用leader election的机制多副本部署，单实例运行的模式。应用程序可以使用外部的组件比如ZooKeeper或Etcd等中间件进行leader eleaction， ZooKeeper的实现是采用临时节点的方案，临时节点存活与客户端与ZooKeeper的会话期间，在会话结束后，临时节点会被立刻删除，临时节点被删除后，其他处于被动状态的服务实例会竞争生成临时节点，生成临时节点的客户端（服务实例）就变成Leader，从而保证整个集群中只有一个活跃的实例，在发生故障的时候，也能快速的实现主从之间的迁移。Etcd是一个分布式的kv存储组件，利用Raft协议维护副本的状态服务，Etcd的Revision机制可以实现分布式锁的功能，Etcd的concurrency利用的分布式锁的能力实现了选Leader的功能（本文更多关注的是k8s本身的能力，Etcd的concurrency机制不做详细介绍）。

kubernetes使用的Etcd作为底层的存储组件，因此我们是不是有可能利用kubernetes的API实现选leader的功能呢？其实kubernetes的SIG已经提供了这方面的能力，主要是通过configmap/lease/endpoint的资源实现选Leader的功能。

二、官网代码示例

kubernetes官方提供了一个使用的例子，源码在：github.com/kubernetes/…

选举的过程中，每个实例的状态有可能是：

• 选择成功->运行业务代码
• 等待状态，有其他实例成为了leader。当leader放弃锁后，此状态的实例有可能会成为新的leader
• 释放leader的锁，在运行的业务代码退出

在稳定的环境中，实例一旦成为了leader，通常情况是不会释放锁的，会保持一直运行的状态，这样有利于业务的稳定和Controller快速的对资源的状态变化做成相应的操作。只有在网络不稳定或误操作删除实例的情况下，才会触发leader的重新选举。

kubernetes官方提供的选举例子详解如下：

package main
import (
  "context"
  "flag"
  "os"
  "os/signal"
  "syscall"
  "time"
  "github.com/google/uuid"
  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
  // kubeconfig 指定了kubernetes集群的配置文文件路径
  flag.StringVar(&kubeconfig, "kubeconfig", "", "absolute path to the kubeconfig file")
  // 锁的拥有者的ID，如果没有传参数进来，就随机生成一个
  flag.StringVar(&id, "id", uuid.New().String(), "the holder identity name")
  // 锁的ID，对应kubernetes中资源的name
  flag.StringVar(&leaseLockName, "lease-lock-name", "", "the lease lock resource name")
  // 锁的命名空间
  flag.StringVar(&leaseLockNamespace, "lease-lock-namespace", "", "the lease lock resource namespace")
  // 解析命令行参数
  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).")
  }
  // leader election uses the Kubernetes API by writing to a
  // lock object, which can be a LeaseLock object (preferred),
  // a ConfigMap, or an Endpoints (deprecated) object.
  // Conflicting writes are detected and each client handles those actions
  // independently.
  config, err := buildConfig(kubeconfig)
  if err != nil {
    klog.Fatal(err)
  }
  // 获取kubernetes集群的客户端，如果获取不到，就抛异常退出
  client := clientset.NewForConfigOrDie(config)
  // 模拟Controller的逻辑代码
  run := func(ctx context.Context) {
    // complete your controller loop here
    klog.Info("Controller loop...")
    // 不退出
    select {}
  }
  // 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()
  // listen for interrupts or the Linux SIGTERM signal and cancel
  // our context, which the leader election code will observe and
  // step down
  // 处理系统的系统，收到SIGTERM信号后，会退出进程
  ch := make(chan os.Signal, 1)
  signal.Notify(ch, os.Interrupt, syscall.SIGTERM)
  go func() {
    <-ch
    klog.Info("Received termination, signaling shutdown")
    cancel()
  }()
  // we use the Lease lock type since edits to Leases are less common
  // and fewer objects in the cluster watch "all Leases".
  
  // 根据参数，生成锁。这里使用的Lease这种类型资源作为锁
  lock := &resourcelock.LeaseLock{
    LeaseMeta: metav1.ObjectMeta{
      Name:      leaseLockName,
      Namespace: leaseLockNamespace,
    },
    // 跟kubernetes集群关联起来
    Client: client.CoordinationV1(),
    LockConfig: resourcelock.ResourceLockConfig{
      Identity: id,
    },
  }
  // start the leader election code loop
  
  // 注意，选举逻辑启动时候，会传入ctx参数，如果ctx对应的cancel函数被调用，那么选举也会结束
  leaderelection.RunOrDie(ctx, leaderelection.LeaderElectionConfig{
    // 选举使用的锁
    Lock: lock,
    // IMPORTANT: you MUST ensure that any code you have that
    // is protected by the lease must terminate **before**
    // you call cancel. Otherwise, you could have a background
    // loop still running and another process could
    // get elected before your background loop finished, violating
    // the stated goal of the lease.
    //主动放弃leader，当ctx canceled的时候
    ReleaseOnCancel: true,
    LeaseDuration:   60 * time.Second,  // 选举的任期，60s一个任期，如果在60s后没有renew，那么leader就会释放锁，重新选举
    RenewDeadline:   15 * time.Second,  // renew的请求的超时时间
    RetryPeriod:     5 * time.Second, // leader获取到锁后，renew leadership的间隔。非leader，抢锁成为leader的间隔（有1.2的jitter因子，详细看代码）
    
    // 回调函数的注册
    Callbacks: leaderelection.LeaderCallbacks{
      
      // 成为leader的回调
      OnStartedLeading: func(ctx context.Context) {
        // we're notified when we start - this is where you would
        // usually put your code
        // 运行controller的逻辑
        run(ctx)
      },
      OnStoppedLeading: func() {
        // we can do cleanup here
        // 退出leader的
        klog.Infof("leader lost: %s", id)
        os.Exit(0)
      },
      OnNewLeader: func(identity string) {
        // 有新的leader当选
        // we're notified when new leader elected
        if identity == id {
          // I just got the lock
          return
        }
        klog.Infof("new leader elected: %s", identity)
      },
    },
  })
}

启动一个实例，观察日志输出和kubernetes集群上的lease资源,启动命令

go run main.go --kubeconfig=/tmp/test-kubeconfig.yaml -logtostderr=true -lease-lock-name=example -lease-lock-namespace=default -id=1

可以看到，日志有输出，id=1的实例获取到资源了。

go run main.go --kubeconfig=/tmp/test-kubeconfig.yaml -logtostderr=true -lease-lock-name=example -lease-lock-namespace=default -id=1 I1023 17:00:21.670298 94227 leaderelection.go:248] attempting to acquire leader lease default/example... I1023 17:00:21.784234 94227 leaderelection.go:258] successfully acquired lease default/example I1023 17:00:21.784316 94227 main.go:78] Controller loop...

在kubernetes的集群上，看到

我们接着启动一个实例，id=2，日志中输出

go run main.go --kubeconfig=/tmp/test-kubeconfig.yaml -logtostderr=true -lease-lock-name=example -lease-lock-namespace=default -id=2 I1023 17:05:00.555145 95658 leaderelection.go:248] attempting to acquire leader lease default/example... I1023 17:05:00.658202 95658 main.go:151] new leader elected: 1

可以看出，id=2的实例，没有获取到锁，并且观察到id=1的锁获取到了实例。接着我们尝试退出id=1的实例，观察id=2的实例是否会成为新的leader

三、锁的实现

kubernets的资源都可以实现Get/Create/Update的操作，因此，理论上所有的资源都可以作为锁的底层。kubernetes 提供了Lease/Configmap/Endpoint作为锁的底层。

锁的状态转移如下：

锁需要实现以下的接口

type Interface interface {
  // Get returns the LeaderElectionRecord
  Get(ctx context.Context) (*LeaderElectionRecord, []byte, error)
  // Create attempts to create a LeaderElectionRecord
  Create(ctx context.Context, ler LeaderElectionRecord) error
  // Update will update and existing LeaderElectionRecord
  Update(ctx context.Context, ler LeaderElectionRecord) error
  // RecordEvent is used to record events
  RecordEvent(string)
  // Identity will return the locks Identity
  Identity() string
  // Describe is used to convert details on current resource lock
  // into a string
  Describe() string
}

理论上，有Get/Create/Update三个方法，就可以实现锁的机制了。但是，需要保证update和create操作的原子性，这个就是kuberenetes的机制保证了。第二章的官网代码例子中，leaderelection.RunOrDie使用的RunOrDie接口，其实就是调用Run接口，而Run接口实现非常简单:

func (le *LeaderElector) Run(ctx context.Context) {
  defer runtime.HandleCrash()
  defer func() {
    le.config.Callbacks.OnStoppedLeading()
  }()
  // 获取锁，如果没有获取到，就一直等待
  if !le.acquire(ctx) {
    return // ctx signalled done
  }
  ctx, cancel := context.WithCancel(ctx)
  defer cancel()
  // 获取到锁后，需要调用回调函数中的OnStartedLeading，运行controller的代码
  go le.config.Callbacks.OnStartedLeading(ctx)
  
  // 获取到锁后，需要不断地进行renew操作
  le.renew(ctx)
}

LeaderElector关键是需要acquire和renew的操作，acquire和renew操作代码如下：

func (le *LeaderElector) acquire(ctx context.Context) bool {
  ctx, cancel := context.WithCancel(ctx)
  defer cancel()
  succeeded := false
  desc := le.config.Lock.Describe()
  klog.Infof("attempting to acquire leader lease %v...", desc)
  // 此接口会阻塞，利用定时的机制，获取锁，如果获取不到一直循环，除非ctx被取消。
  wait.JitterUntil(func() {
    // 获取锁
    succeeded = le.tryAcquireOrRenew(ctx)
    le.maybeReportTransition()
    if !succeeded {
      klog.V(4).Infof("failed to acquire lease %v", desc)
      return
    }
    le.config.Lock.RecordEvent("became leader")
    le.metrics.leaderOn(le.config.Name)
    klog.Infof("successfully acquired lease %v", desc)
    cancel()
  }, le.config.RetryPeriod, JitterFactor, true, ctx.Done())
  return succeeded
}
// renew loops calling tryAcquireOrRenew and returns immediately when tryAcquireOrRenew fails or ctx signals done.
func (le *LeaderElector) renew(ctx context.Context) {
  ctx, cancel := context.WithCancel(ctx)
  defer cancel()
  // 循环renew机制，renew成功，不会返回true，导致Until会不断循环
  wait.Until(func() {
    //RenewDeadline的实现在这里，如果renew超过了RenewDeadline，会导致renew失败，主退出
    timeoutCtx, timeoutCancel := context.WithTimeout(ctx, le.config.RenewDeadline)
    defer timeoutCancel()
    err := wait.PollImmediateUntil(le.config.RetryPeriod, func() (bool, error) {
      // renew锁
      return le.tryAcquireOrRenew(timeoutCtx), nil
    }, timeoutCtx.Done())
    le.maybeReportTransition()
    desc := le.config.Lock.Describe()
    if err == nil {
      klog.V(5).Infof("successfully renewed lease %v", desc)
      // renew成功
      return
    }
    le.config.Lock.RecordEvent("stopped leading")
    le.metrics.leaderOff(le.config.Name)
    klog.Infof("failed to renew lease %v: %v", desc, err)
    cancel()
  }, le.config.RetryPeriod, ctx.Done())
  // if we hold the lease, give it up
  if le.config.ReleaseOnCancel {
    le.release()
  }
}

关键的实现在于tryAcquireOrRenew，而tryAcquireOrRenew就是依赖锁的状态转移机制完成核心逻辑。

func (le *LeaderElector) tryAcquireOrRenew(ctx context.Context) bool {
  now := metav1.Now()
  leaderElectionRecord := rl.LeaderElectionRecord{
    HolderIdentity:       le.config.Lock.Identity(),
    LeaseDurationSeconds: int(le.config.LeaseDuration / time.Second),
    RenewTime:            now,
    AcquireTime:          now,
  }
  // 1. obtain or create the ElectionRecord
  // 检查锁有没有
  oldLeaderElectionRecord, oldLeaderElectionRawRecord, err := le.config.Lock.Get(ctx)
  if err != nil {
    // 没有锁的资源，就创建一个
    if !errors.IsNotFound(err) {
      klog.Errorf("error retrieving resource lock %v: %v", le.config.Lock.Describe(), err)
      return false
    }
    if err = le.config.Lock.Create(ctx, leaderElectionRecord); err != nil {
      klog.Errorf("error initially creating leader election record: %v", err)
      return false
    }
    //对外宣称自己成为了leader
    le.setObservedRecord(&leaderElectionRecord)
    return true
  }
  // 2. Record obtained, check the Identity & Time
  if !bytes.Equal(le.observedRawRecord, oldLeaderElectionRawRecord) {
    // 这个机制很重要，会如果leader会不断正常renew这个锁，oldLeaderElectionRawRecord会一直发生变化，发生变化会更新le.observedTime
    le.setObservedRecord(oldLeaderElectionRecord)
    le.observedRawRecord = oldLeaderElectionRawRecord
  }
  // 如果还没超时并且此实例不是leader（leader是其他实例），那么就直接退出
  if len(oldLeaderElectionRecord.HolderIdentity) > 0 &&
    le.observedTime.Add(le.config.LeaseDuration).After(now.Time) &&
    !le.IsLeader() {
    klog.V(4).Infof("lock is held by %v and has not yet expired", oldLeaderElectionRecord.HolderIdentity)
    return false
  }
  // 3. We're going to try to update. The leaderElectionRecord is set to it's default
  // here. Let's correct it before updating.
  // 如果是leader，就更新时间RenewTime，保证其他实例（非主）可以观察到：主还活着
  if le.IsLeader() {
    leaderElectionRecord.AcquireTime = oldLeaderElectionRecord.AcquireTime
    leaderElectionRecord.LeaderTransitions = oldLeaderElectionRecord.LeaderTransitions
  } else {
  // 不是leader，那么锁就发生了转移
    leaderElectionRecord.LeaderTransitions = oldLeaderElectionRecord.LeaderTransitions + 1
  }
  // 更新锁
  // update the lock itself
  if err = le.config.Lock.Update(ctx, leaderElectionRecord); err != nil {
    klog.Errorf("Failed to update lock: %v", err)
    return false
  }
  le.setObservedRecord(&leaderElectionRecord)
  return true
}