1、概述
在Kubernetes中，为了实现组件高可用，同一个组件需要部署多个副本，例如多个apiserver、scheduler、controller-manager等，其中apiserver是无状态的，每个组件都可以工作，而scheduler与controller-manager是有状态的，同一时刻只能存在一个活跃的，需要进行选主。
Kubernetes中是通过leaderelection来实现组件的高可用的。在Kubernetes本身的组件中，kube-scheduler和kube-manager-controller两个组件是有leader选举的，这个选举机制是Kubernetes对于这两个组件的高可用保障。即正常情况下kube-scheduler或kube-manager-controller组件的多个副本只有一个是处于业务逻辑运行状态，其它副本则不断的尝试去获取锁，去竞争leader，直到自己成为leader。如果正在运行的leader因某种原因导致当前进程退出，或者锁丢失，则由其它副本去竞争新的leader，获取leader继而执行业务逻辑。
不光是Kubernetes本身组件用到了这个选举策略，我们自己定义的服务同样可以用这个算法去实现选主。在Kubernetes client-go包中就提供了接口供用户使用。代码路径在client-go/tools/leaderelection下。
2、leaderelection使用示例
以下是一个简单使用的例子（例子来源于client-go中的example包中），编译完成之后同时启动多个进程，但是只有一个进程在工作，当把leader进程kill掉之后，会重新选举出一个leader进行工作，即执行其中的 run 方法：
//代码路径：client-go/examples/leader-election/main.go
package main
import (
"context"
"flag"
"os"
"os/signal"
"syscall"
"time"
"/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
flag.StringVar(&kubeconfig, "kubeconfig", "", "absolute path to the kubeconfig file")
flag.StringVar(&id, "id", uuid.New().String(), "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).")
}
// 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)
}
client := clientset.NewForConfigOrDie(config)
//业务逻辑
run := func(ctx context.Context) {
// complete your controller loop here
klog.Info("Controller loop...")
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
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".
// 指定锁的资源对象，这里使用了Lease资源，还支持configmap，endpoint，或者multilock(即多种配合使用)
lock := &resourcelock.LeaseLock{
LeaseMeta: metav1.ObjectMeta{
Name: leaseLockName,
Namespace: leaseLockNamespace,
},
Client: client.CoordinationV1(),
LockConfig: resourcelock.ResourceLockConfig{
Identity: id,
},
}
// 进行选举
// start the leader election code loop
leaderelection.RunOrDie(ctx, leaderelection.LeaderElectionConfig{
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.
ReleaseOnCancel: true,
LeaseDuration: 60 * time.Second, //租约时长，非主候选者用来判断资源锁是否过期
RenewDeadline: 15 * time.Second, //leader刷新资源锁超时时间
RetryPeriod: 5 * time.Second, //调用资源锁间隔
//回调函数，根据选举不同事件触发
Callbacks: leaderelection.LeaderCallbacks{
OnStartedLeading: func(ctx context.Context) {
//变为leader执行的业务代码
// we're notified when we start - this is where you would
// usually put your code
run(ctx)
},
OnStoppedLeading: func() {
// 进程退出
// we can do cleanup here
klog.Infof("leader lost: %s", id)
os.Exit(0)
},
OnNewLeader: func(identity string) {
//当产生新的leader后执行的方法
// we're notified when new leader elected
if identity == id {
// I just got the lock
return
}
klog.Infof("new leader elected: %s", identity)
},
},
})
}关键启动参数说明：
kubeconfig: 指定kubeconfig文件地址
lease-lock-name：指定lock的名称
lease-lock-namespace：指定lock的namespace
id: 例子中提供的区别参数，用于区分实例
logtostderr：klog提供的参数，指定log输出到控制台
v: 指定日志输出级别2.1 同时启动三个进程：
启动进程1：
go run main.go -kubeconfig=/Users/apple/.kube/config148 -logtostderr=true -lease-lock-name=example -lease-lock-namespace=default -id=1 -v=4输出：
apple@appledeMacBook-Pro test$ go run main.go -kubeconfig=/Users/apple/.kube/config148 -logtostderr=true -lease-lock-name=example -lease-lock-namespace=default -id=1 -v=4
I0126 13:46:59.753974 35080 leaderelection.go:243] attempting to acquire leader lease default/example...
I0126 13:47:00.660260 35080 leaderelection.go:253] successfully acquired lease default/example
I0126 13:47:00.660368 35080 main.go:75] Controller loop...这里可以看出来id=1的进程持有锁，并且运行的程序。
启动进程2：
go run main.go -kubeconfig=/Users/apple/.kube/config148 -logtostderr=true -lease-lock-name=example -lease-lock-namespace=default -id=2 -v=4输出：
apple@appledeMacBook-Pro test$ go run main.go -kubeconfig=/Users/apple/.kube/config148 -logtostderr=true -lease-lock-name=example -lease-lock-namespace=default -id=2 -v=4
I0126 13:47:05.066516 35096 leaderelection.go:243] attempting to acquire leader lease default/example...
I0126 13:47:05.451887 35096 leaderelection.go:346] lock is held by 1 and has not yet expired
I0126 13:47:05.451909 35096 leaderelection.go:248] failed to acquire lease default/example
I0126 13:47:05.451918 35096 main.go:145] new leader elected: 1
I0126 13:47:14.188160 35096 leaderelection.go:346] lock is held by 1 and has not yet expired
I0126 13:47:14.188188 35096 leaderelection.go:248] failed to acquire lease default/example
I0126 13:47:24.929607 35096 leaderelection.go:346] lock is held by 1 and has not yet expired
I0126 13:47:24.929636 35096 leaderelection.go:248] failed to acquire lease default/example
.......这里可以看出来id=1的进程持有锁，并且运行的程序，而id=2的进程表示无法获取到锁，在不断的进行尝试。
启动进程3：
go run main.go -kubeconfig=/Users/apple/.kube/config148 -logtostderr=true -lease-lock-name=example -lease-lock-namespace=default -id=3 -v=4输出：
apple@appledeMacBook-Pro test$ go run main.go -kubeconfig=/Users/apple/.kube/config148 -logtostderr=true -lease-lock-name=example -lease-lock-namespace=default -id=3 -v=4
I0126 13:47:12.431518 35112 leaderelection.go:243] attempting to acquire leader lease default/example...
I0126 13:47:12.776614 35112 leaderelection.go:346] lock is held by 1 and has not yet expired
I0126 13:47:12.776649 35112 leaderelection.go:248] failed to acquire lease default/example
I0126 13:47:12.776663 35112 main.go:145] new leader elected: 1
I0126 13:47:21.499295 35112 leaderelection.go:346] lock is held by 1 and has not yet expired
I0126 13:47:21.499325 35112 leaderelection.go:248] failed to acquire lease default/example
I0126 13:47:32.241544 35112 leaderelection.go:346] lock is held by 1 and has not yet expired
I0126 13:47:32.241572 35112 leaderelection.go:248] failed to acquire lease default/example
.......这里可以看出来id=1的进程持有锁，并且运行的程序，而id=3的进程表示无法获取到锁，在不断的进行尝试。
2.2 停掉进程1并观察进程2和进程3竞争新的leader
apple@appledeMacBook-Pro test$ go run main.go -kubeconfig=/Users/apple/.kube/config148 -logtostderr=true -lease-lock-name=example -lease-lock-namespace=default -id=1 -v=4
I0126 13:46:59.753974 35080 leaderelection.go:243] attempting to acquire leader lease default/example...
I0126 13:47:00.660260 35080 leaderelection.go:253] successfully acquired lease default/example
I0126 13:47:00.660368 35080 main.go:75] Controller loop...
^CI0126 13:53:16.629114 35080 main.go:92] Received termination, signaling shutdown
I0126 13:53:17.057999 35080 main.go:135] leader lost: 1现在kill掉id=1进程，在等待lock释放之后（有个LeaseDuration时间），观察进程2和进程3的输出，看哪个进程成为新的leader。
id=2的进程输出：
......
I0126 13:53:11.208487 35096 leaderelection.go:346] lock is held by 1 and has not yet expired
I0126 13:53:11.208512 35096 leaderelection.go:248] failed to acquire lease default/example
I0126 13:53:18.189514 35096 leaderelection.go:253] successfully acquired lease default/example
I0126 13:53:18.189587 35096 main.go:75] Controller loop...这里可以看出来id=2的进程持有锁，并且运行的程序。
id=3的进程输出：
......
I0126 13:53:04.675216 35112 leaderelection.go:248] failed to acquire lease default/example
I0126 13:53:12.918706 35112 leaderelection.go:346] lock is held by 1 and has not yet expired
I0126 13:53:12.918736 35112 leaderelection.go:248] failed to acquire lease default/example
I0126 13:53:19.544314 35112 leaderelection.go:346] lock is held by 2 and has not yet expired
I0126 13:53:19.544372 35112 leaderelection.go:248] failed to acquire lease default/example
I0126 13:53:19.544387 35112 main.go:145] new leader elected: 2
I0126 13:53:26.346561 35112 leaderelection.go:346] lock is held by 2 and has not yet expired
I0126 13:53:26.346591 35112 leaderelection.go:248] failed to acquire lease default/example
......这里可以看出来id=2的进程持有锁，并且运行的程序，而id=3的进程表示无法获取到锁，在不断的进行尝试。
2.3 查看资源锁对象
[root@master1 ~]# kubectl get leases.coordination.k8s.io example -o yaml
apiVersion: coordination.k8s.io/v1
kind: Lease
metadata:
creationTimestamp: "2022-01-26T05:46:38Z"
managedFields:
.......
manager: main
operation: Update
time: "2022-01-26T06:05:43Z"
name: example
namespace: default
resourceVersion: "314956587"
selfLink: /apis/coordination.k8s.io/v1/namespaces/default/leases/example
uid: 5ce63489-c754-42b4-9e6c-a0a0a8442c3f
spec:
acquireTime: "2022-01-26T05:53:17.905076Z" //获得锁时间
holderIdentity: "2" //持有锁进程的标识
leaseDurationSeconds: 60 //lease租约时长
leaseTransitions: 1 //leader更换次数
renewTime: "2022-01-26T06:06:06.248393Z" //更新租约的时间锁已经被进程2获得, 此时如果进程1再启动的话, 也只能一直尝试获取锁。
3、leaderelection源码分析
leaderelection基本原理其实就是利用通过Kubernetes中lease、configmap 、endpoints资源实现一个分布式锁，获取到锁的进程成为leader，并且定期更新租约（renew）。其他进程也在不断的尝试进行抢占，抢占不到则继续等待下次循环。当leader节点挂掉之后，租约到期，其他节点就成为新的leader。
代码路径在client-go/tools/leaderelection下.逻辑结构如下图：
注意： 请注意client-go的版本，不同版本对应LeaderElection的逻辑架构图也略微有所不同。
3.1、Interface接口
Interface: 中定义了一系列方法, 包括增加、修改、获取一个LeaderElectionRecord, 说白了就是一个客户端, 而且每个客户端实例都要有自己分布式唯一的id。
// tools/leaderelection/resourcelock/interface.go
// 资源占有者的描述信息
type LeaderElectionRecord struct {
// 持有锁进程的标识 也就是leader的id
HolderIdentity string `json:"holderIdentity"`
// 一个租约多长时间
LeaseDurationSeconds int `json:"leaseDurationSeconds"`
// 获得leader的时间
AcquireTime metav1.Time `json:"acquireTime"`
// 续约的时间
RenewTime metav1.Time `json:"renewTime"`
// leader变更的次数
LeaderTransitions int `json:"leaderTransitions"`
}
type Interface interface {
// 返回当前资源LeaderElectionRecord
Get() (*LeaderElectionRecord, error)
// 创建一个资源LeaderElectionRecord
Create(ler LeaderElectionRecord) error
// 更新资源
Update(ler LeaderElectionRecord) error
// 记录事件
RecordEvent(string)
// 返回当前该应用的id
Identity() string
// 描述信息（namespace/name）
Describe() string
}Interface有四个实现类, 分别为EndpointLock, ConfigMapLock、LeaseLock和MultiLock（一般不用），分别可以操作Kubernetes中的endpoint, configmap和lease。这里以LeaseLock为例子说明。
// tools/leaderelection/resourcelock/leaselock.go
type LeaseLock struct {
// LeaseMeta should contain a Name and a Namespace of a
// LeaseMeta object that the LeaderElector will attempt to lead.
LeaseMeta metav1.ObjectMeta
// 访问api-server的客户端
Client coordinationv1client.LeasesGetter
// 该LeaseLock的分布式唯一身份id
LockConfig ResourceLockConfig
// 资源锁对应的lease资源对象
lease *coordinationv1.Lease
}
// tools/leaderelection/resourcelock/interface.go
type ResourceLockConfig struct {
// 分布式唯一id
Identity string
EventRecorder EventRecorder
}LeaseLock类型对应函数详解：Create, Update, Get方法都是利用client去访问kubernetes的api-server。
// tools/leaderelection/resourcelock/leaselock.go
// 通过访问apiserver获取当前资源锁对象ll.lease，并组织返回对应的LeaderElectionRecord对象和LeaderElectionRecord序列化值
// Get returns the election record from a Lease spec
func (ll *LeaseLock) Get(ctx context.Context) (*LeaderElectionRecord, []byte, error) {
var err error
// 获取资源锁对应的资源对象ll.lease
ll.lease, err = ll.Client.Leases(ll.LeaseMeta.Namespace).Get(ctx, ll.LeaseMeta.Name, metav1.GetOptions{})
if err != nil {
return nil, nil, err
}
// 利用lease资源对象spec生成对应LeaderElectionRecord资源对象
record := LeaseSpecToLeaderElectionRecord(&ll.lease.Spec)
// 序列化LeaderElectionRecord资源对象（byte[]）
recordByte, err := json.Marshal(*record)
if err != nil {
return nil, nil, err
}
return record, recordByte, nil
}
// 根据LeaderElectionRecord创建对应资源锁对象 ll.lease
// Create attempts to create a Lease
func (ll *LeaseLock) Create(ctx context.Context, ler LeaderElectionRecord) error {
var err error
ll.lease, err = ll.Client.Leases(ll.LeaseMeta.Namespace).Create(ctx, &coordinationv1.Lease{
ObjectMeta: metav1.ObjectMeta{
Name: ll.LeaseMeta.Name,
Namespace: ll.LeaseMeta.Namespace,
},
// 利用ElectionRecord资源对象生成对应lease资源对象spec
Spec: LeaderElectionRecordToLeaseSpec(&ler),
}, metav1.CreateOptions{})
return err
}
// Update will update an existing Lease spec.
func (ll *LeaseLock) Update(ctx context.Context, ler LeaderElectionRecord) error {
if ll.lease == nil {
return errors.New("lease not initialized, call get or create first")
}
// 利用ElectionRecord资源对象生成对应lease资源对象spec
ll.lease.Spec = LeaderElectionRecordToLeaseSpec(&ler)
lease, err := ll.Client.Leases(ll.LeaseMeta.Namespace).Update(ctx, ll.lease, metav1.UpdateOptions{})
if err != nil {
return err
}
ll.lease = lease
return nil
}
// RecordEvent in leader election while adding meta-data
func (ll *LeaseLock) RecordEvent(s string) {
if ll.LockConfig.EventRecorder == nil {
return
}
events := fmt.Sprintf("%v %v", ll.LockConfig.Identity, s)
ll.LockConfig.EventRecorder.Eventf(&coordinationv1.Lease{ObjectMeta: ll.lease.ObjectMeta}, corev1.EventTypeNormal, "LeaderElection", events)
}
// Describe is used to convert details on current resource lock
// into a string
func (ll *LeaseLock) Describe() string {
return fmt.Sprintf("%v/%v", ll.LeaseMeta.Namespace, ll.LeaseMeta.Name)
}
// Identity returns the Identity of the lock
func (ll *LeaseLock) Identity() string {
return ll.LockConfig.Identity
}
// 利用lease资源对象spec生成对应LeaderElectionRecord资源对象
func LeaseSpecToLeaderElectionRecord(spec *coordinationv1.LeaseSpec) *LeaderElectionRecord {
var r LeaderElectionRecord
if spec.HolderIdentity != nil {
r.HolderIdentity = *spec.HolderIdentity
}
if spec.LeaseDurationSeconds != nil {
r.LeaseDurationSeconds = int(*spec.LeaseDurationSeconds)
}
if spec.LeaseTransitions != nil {
r.LeaderTransitions = int(*spec.LeaseTransitions)
}
if spec.AcquireTime != nil {
r.AcquireTime = metav1.Time{spec.AcquireTime.Time}
}
if spec.RenewTime != nil {
r.RenewTime = metav1.Time{spec.RenewTime.Time}
}
return &r
}
// 利用ElectionRecord资源对象生成对应lease资源对象spec
func LeaderElectionRecordToLeaseSpec(ler *LeaderElectionRecord) coordinationv1.LeaseSpec {
leaseDurationSeconds := int32(ler.LeaseDurationSeconds)
leaseTransitions := int32(ler.LeaderTransitions)
return coordinationv1.LeaseSpec{
HolderIdentity: &ler.HolderIdentity,
LeaseDurationSeconds: &leaseDurationSeconds,
AcquireTime: &metav1.MicroTime{ler.AcquireTime.Time},
RenewTime: &metav1.MicroTime{ler.RenewTime.Time},
LeaseTransitions: &leaseTransitions,
}
}3.2 LeaderElector
LeaderElectionConfig:
定义了一些竞争资源的参数，用于保存当前应用的一些配置，包括资源锁、持有锁的时间等，LeaderElectionConfig.lock 支持保存在以下三种资源中：
configmapendpointlease
包中还提供了一个 multilock ，即可以进行选择两种，当其中一种保存失败时，选择第二种。
//client-go/tools/leaderelection/leaderelection.go
type LeaderElectionConfig struct {
// Lock 的类型
Lock rl.Interface
//持有锁的时间
LeaseDuration time.Duration
//在更新租约的超时时间
RenewDeadline time.Duration
//竞争获取锁的时间
RetryPeriod time.Duration
//需要用户配置的状态变化时执行的函数，支持三种：
//1、OnStartedLeading 启动是执行的业务代码
//2、OnStoppedLeading leader停止执行的方法
//3、OnNewLeader 当产生新的leader后执行的方法
Callbacks LeaderCallbacks
//进行监控检查
// WatchDog is the associated health checker
// WatchDog may be null if its not needed/configured.
WatchDog *HealthzAdaptor
//leader退出时，是否执行release方法
ReleaseOnCancel bool
// Name is the name of the resource lock for debugging
Name string
}LeaderElector：
是一个竞争资源的实体。
//client-go/tools/leaderelection/leaderelection.go// LeaderElector is a leader election client.
type LeaderElector struct {
// 用于保存当前应用的一些配置
config LeaderElectionConfig
// 通过apiserver远程获取的资源锁对象 (不一定自己是leader) 所有想竞争此资源的应用获取的是同一份
// internal bookkeeping
observedRecord rl.LeaderElectionRecord
//资源锁对象spec，用于和远程获取的资源锁对象值比较
observedRawRecord []byte
// 获取的时间
observedTime time.Time
// used to implement OnNewLeader(), may lag slightly from the
// value observedRecord.HolderIdentity if the transition has
// not yet been reported.
reportedLeader string
// clock is wrapper around time to allow for less flaky testing
clock clock.Clock
metrics leaderMetricsAdapter
}这里着重要关注以下几个属性:
config: 该LeaderElectionConfig对象配置了当前应用的客户端, 以及此客户端的唯一id等等。
observedRecord: 该LeaderElectionRecord就是保存着从api-server中获得的leader的信息。
observedTime: 获得的时间。
很明显判断当前进程是不是leader只需要判断config中的id和observedRecord中的id是不是一致即可.
func (le *LeaderElector) GetLeader() string {
return le.observedRecord.HolderIdentity
}
// IsLeader returns true if the last observed leader was this client else returns false.
func (le *LeaderElector) IsLeader() bool {
return le.observedRecord.HolderIdentity == le.config.Lock.Identity()
}3.3 LeaderElector运行逻辑
3.3.1 run
func (le *LeaderElector) Run(ctx context.Context) {
defer func() {
runtime.HandleCrash()
le.config.Callbacks.OnStoppedLeading()
}()
// 如果获取成功 那就是ctx signalled done
// 不然即使失败, 该client也会一直去尝试获得leader位置
if !le.acquire(ctx) {
return // ctx signalled done
}
// 如果获得leadership 以goroutine和回调的形式启动用户自己的逻辑方法OnStartedLeading
ctx, cancel := context.WithCancel(ctx)
defer cancel()
go le.config.Callbacks.OnStartedLeading(ctx)
// 一直去续约 这里也是一个循环操作
// 如果失去了leadership 该方法才会返回
// 该方法返回 整个Run方法就返回了
le.renew(ctx)
}1. 该client(也就是le这个实例)首先会调用acquire方法一直尝试去竞争leadership（如果竞争失败, 继续竞争, 不会进入2. 竞争成功, 进入2）。
2. 异步启动用户自己的逻辑程序(OnStartedLeading)（进入3）。
3. 通过调用renew方法续约自己的leadership. 续约成功, 继续续约，续约失败, 整个Run就结束了。
3.3.2 acquire
//检查是否需要广播新产生的leader
func (le *LeaderElector) maybeReportTransition() {
// 如果没有变化 则不需要更新
if le.observedRecord.HolderIdentity == le.reportedLeader {
return
}
// 更新reportedLeader为最新的leader的id
le.reportedLeader = le.observedRecord.HolderIdentity
if le.config.Callbacks.OnNewLeader != nil {
// 调用当前应用的回调函数OnNewLeader报告新的leader产生
go le.config.Callbacks.OnNewLeader(le.reportedLeader)
}
}
// 一旦获得leadership 立马返回true，那就是ctx signalled done
// 失败的话，该client会一直去尝试获得leader位置
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)
wait.JitterUntil(func() {
// 尝试获得或者更新资源
succeeded = le.tryAcquireOrRenew()
// 有可能会产生新的leader
// 所以调用maybeReportTransition检查是否需要广播新产生的leader
le.maybeReportTransition()
if !succeeded {
// 如果获得leadership失败 则返回后继续竞争
klog.V(4).Infof("failed to acquire lease %v", desc)
return
}
// 自己成为leader
// 可以调用cancel方法退出JitterUntil进而从acquire中返回
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
}acquire的作用如下:
1. 一旦获得leadership，立马返回true，否则会隔RetryPeriod时间尝试一次。
这里的逻辑比较简单, 主要的逻辑是在tryAcquireOrRenew方法中。
3.3.3 renew and release
// RenewDeadline=15s RetryPeriod=5s
// 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()
// 每隔RetryPeriod会调用 除非cancel()方法被调用才会退出
wait.Until(func() {
timeoutCtx, timeoutCancel := context.WithTimeout(ctx, le.config.RenewDeadline)
defer timeoutCancel()
// 每隔5s调用该方法直到该方法返回true为止
// 如果超时了也会退出该方法 并且err中有错误信息
err := wait.PollImmediateUntil(le.config.RetryPeriod, func() (bool, error) {
return le.tryAcquireOrRenew(timeoutCtx), nil
}, timeoutCtx.Done())
// 有可能会产生新的leader 如果有会广播新产生的leader
le.maybeReportTransition()
desc := le.config.Lock.Describe()
if err == nil {
// 如果err == nil, 表明上面PollImmediateUntil中返回true了 续约成功 依然处于leader位置
// 返回后 继续运行wait.Until的逻辑
klog.V(4).Infof("successfully renewed lease %v", desc)
return
}
// err != nil 表明超时了 试的总时间超过了RenewDeadline 失去了leader位置 续约失败
// 调用cancel方法退出wait.Until
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()
}
}
// leader续约cancel()的时候释放资源锁对象holderIdentity字段的值
// release attempts to release the leader lease if we have acquired it.
func (le *LeaderElector) release() bool {
if !le.IsLeader() {
return true
}
now := metav1.Now()
leaderElectionRecord := rl.LeaderElectionRecord{
LeaderTransitions: le.observedRecord.LeaderTransitions,
LeaseDurationSeconds: 1,
RenewTime: now,
AcquireTime: now,
}
if err := le.config.Lock.Update(context.TODO(), leaderElectionRecord); err != nil {
klog.Errorf("Failed to release lock: %v", err)
return false
}
le.observedRecord = leaderElectionRecord
le.observedTime = le.clock.Now()
return true
}可以看到该client的base条件是它自己是当前的leader, 然后来续约操作。
这里来说一下RenewDeadline和RetryPeriod的作用。
每隔RetryPeriod时间会通过tryAcquireOrRenew续约, 如果续约失败, 还会进行再次尝试. 一直到尝试的总时间超过RenewDeadline后该client就会失去leadership。
3.3.4 tryAcquireOrRenew
// 竞争或者更新leadership
// 成功返回true 失败返回false
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
// client通过apiserver获得ElectionRecord和ElectionRecord序列化值
oldLeaderElectionRecord, oldLeaderElectionRawRecord, err := le.config.Lock.Get(ctx)
if err != nil {
if !errors.IsNotFound(err) {
// 失败直接退出
klog.Errorf("error retrieving resource lock %v: %v", le.config.Lock.Describe(), err)
return false
}
// 因为没有获取到, 因此创建一个新的进去
if err = le.config.Lock.Create(ctx, leaderElectionRecord); err != nil {
klog.Errorf("error initially creating leader election record: %v", err)
return false
}
// 然后设置observedRecord为刚刚加入进去的leaderElectionRecord
le.observedRecord = leaderElectionRecord
le.observedTime = le.clock.Now()
return true
}
// 2. Record obtained, check the Identity & Time
// 从远端获取到record(资源)成功存到oldLeaderElectionRecord
// 如果oldLeaderElectionRecord与observedRecord不相同 更新observedRecord
// 因为observedRecord代表是从远端存在Record
// 需要注意的是每个client都在竞争leadership, 而leader一直在续约, leader会更新它的RenewTime字段
// 所以一旦leader续约成功 每个non-leader候选者都需要更新其observedTime和observedRecord
if !bytes.Equal(le.observedRawRecord, oldLeaderElectionRawRecord) {
le.observedRecord = *oldLeaderElectionRecord
le.observedRawRecord = oldLeaderElectionRawRecord
le.observedTime = le.clock.Now()
}
// 如果leader已经被占有并且不是当前自己这个应用, 而且时间还没有到期
// 那就直接返回false, 因为已经无法抢占 时间没有过期
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.
if le.IsLeader() {
// 如果当前服务就是以前的占有者
leaderElectionRecord.AcquireTime = oldLeaderElectionRecord.AcquireTime
leaderElectionRecord.LeaderTransitions = oldLeaderElectionRecord.LeaderTransitions
} else {
// 如果当前服务不是以前的占有者 LeaderTransitions加1
leaderElectionRecord.LeaderTransitions = oldLeaderElectionRecord.LeaderTransitions + 1
}
// update the lock itself
// 当前client占有该资源 成为leader
if err = le.config.Lock.Update(ctx, leaderElectionRecord); err != nil {
klog.Errorf("Failed to update lock: %v", err)
return false
}
le.observedRecord = leaderElectionRecord
le.observedTime = le.clock.Now()
return true
}这里需要注意的是当前client不是leader的时候, 如何去判断一个leader是否已经expired了?
通过le.observedTime.Add(le.config.LeaseDuration).After(now.Time)；
- le.observedTime: 代表的是获得leader(截止当前时间为止的最后一次renew)对象的时间；
- le.config.LeaseDuration: 当前进程获得leadership需要的等待时间；
- le.observedTime.Add(le.config.LeaseDuration): 就是自己(当前进程)被允许获得leadership的时间。
如果le.observedTime.Add(le.config.LeaseDuration).before(now.Time)为true的话, 就表明leader过期了。白话文的意思就是从leader上次续约完, 已经超过le.config.LeaseDuration的时间没有续约了, 所以被认为该leader过期了，这时候non-leader就可以抢占leader了。
4、总结
leaderelection 主要是利用了k8s API操作的原子性实现了一个分布式锁，在不断的竞争中进行选举。选中为leader的进行才会执行具体的业务代码，这在k8s中非常的常见，而且我们很方便的利用这个包完成组件的编写，从而实现组件的高可用，比如部署为一个多副本的Deployment，当leader的pod退出后会重新启动，可能锁就被其他pod获取继续执行。
当应用在k8s上部署时，使用k8s的资源锁，可方便的实现高可用，但需要注意：
- 推荐使用lease或
configmap作为资源锁，原因是某些组件（如kube-proxy）会去监听endpoints来更新节点iptables规则，当有大量资源锁时，势必会对性能有影响。
