client-go: k8s选主

快速上手

下面这个代码就是一个选主的大概逻辑

go 复制代码

package main
 
import (
    "context"
    "flag"
    "fmt"
    _ "net/http/pprof"
    "os"
    "path/filepath"
    "time"
 
    "golang.org/x/exp/rand"
    v1 "k8s.io/api/core/v1"
    metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
    "k8s.io/apimachinery/pkg/util/uuid"
    "k8s.io/client-go/kubernetes"
    "k8s.io/client-go/kubernetes/scheme"
    corev1 "k8s.io/client-go/kubernetes/typed/core/v1"
    "k8s.io/client-go/tools/clientcmd"
    "k8s.io/client-go/tools/leaderelection"
    "k8s.io/client-go/tools/leaderelection/resourcelock"
    "k8s.io/client-go/tools/record"
    "k8s.io/client-go/util/homedir"
)
 
func main() {
    ctx := context.Background()
 
    var kubeconfig *string
    if home := homedir.HomeDir(); home != "" {
        kubeconfig = flag.String("kubeconfig", filepath.Join(home, ".kube", "config"), "")
    }
 
    config, err := clientcmd.BuildConfigFromFlags("", *kubeconfig)
    if err != nil {
        panic(err)
    }
 
    clientset, err := kubernetes.NewForConfig(config)
    if err != nil {
        panic(err)
    }
 
    broadcaster := record.NewBroadcaster()
    broadcaster.StartRecordingToSink(&corev1.EventSinkImpl{
        Interface: clientset.CoreV1().Events("default"),
    })
 
    eventRecorder := broadcaster.NewRecorder(scheme.Scheme,
        v1.EventSource{Component: "hello-word"})
 
    createIdentity := func() string {
        hostname, err := os.Hostname()
        if err != nil {
            hostname = fmt.Sprintf("rand%d", rand.Intn(10000))
        }
        return fmt.Sprintf("%s_%s", hostname, string(uuid.NewUUID()))
    }
 
    lock := &resourcelock.LeaseLock{
        LeaseMeta: metav1.ObjectMeta{
            Namespace: "default",
            Name:      "hello-world",
        },
        Client: clientset.CoordinationV1(),
        LockConfig: resourcelock.ResourceLockConfig{
            Identity:      createIdentity(),
            EventRecorder: eventRecorder,
        },
    }
 
    leaderelection.RunOrDie(ctx, leaderelection.LeaderElectionConfig{
        Lock:          lock,
        LeaseDuration: 5 * time.Second,
        RenewDeadline: 4 * time.Second,
        RetryPeriod:   2 * time.Second,
        Callbacks: leaderelection.LeaderCallbacks{
            OnStartedLeading: func(ctx context.Context) {
                fmt.Println("start leading")
            },
            OnStoppedLeading: func() {
                fmt.Println("stop leading")
            },
            OnNewLeader: func(identity string) {
                fmt.Printf("new leader: %s\n", identity)
            },
        },
        Coordinated: false,
    })
}

我们同时启动多个终端来运行这个程序，并且杀掉主节点来模拟节点挂掉，观察是否会重新进行选举出新的master

从图中可以看到，第一个程序选为主节点之后，第二三个程序自动成为slave，我们 kill 掉第一个程序之后，第二个程序抢到了锁成为了 master

租约 Lease

k8s 内置很多种资源，其中 lease 也是k8s的一种资源，顾名思义表达的是租户对某种资源的

占有的信息表示

bash 复制代码

➜  ~ k get lease -A                                                                 
NAMESPACE         NAME                      HOLDER                                                      AGE
default           hello-world               VM-221-245-tencentos_bada2219-3a27-4b19-8b80-23fc05604391   6d7h
kube-node-lease   vm-221-245-tencentos      vm-221-245-tencentos                                        36d
kube-system       kube-controller-manager   VM-221-245-tencentos_8f5a4f85-ca0c-4b5f-ac81-8b0ff5ff2e49   36d
kube-system       kube-scheduler            VM-221-245-tencentos_4fab96c4-156b-4a77-b862-87224be44cb2   36d

比如我们查看一个 k8s 的 node 的 lease

bash 复制代码

➜  ~ k get lease vm-221-245-tencentos -n kube-node-lease -oyaml                   
apiVersion: coordination.k8s.io/v1
kind: Lease                                    # 资源的种类
metadata:
  creationTimestamp: "2024-09-27T12:24:07Z"    # 这个资源的创建时间戳
  name: vm-221-245-tencentos                   # 名称
  namespace: kube-node-lease                   # 命名空间
  ownerReferences:
  - apiVersion: v1
    kind: Node
    name: vm-221-245-tencentos                 # 这个资源的占有名称
    uid: 5df61ad6-cc1a-4669-8b0e-d48a5b0ffb91
  resourceVersion: "3811080"
  uid: 411c6d4e-5afb-4eba-a1a0-8a56d00b75db
spec:
  holderIdentity: vm-221-245-tencentos
  leaseDurationSeconds: 40                    # 租约的时常40s
  renewTime: "2024-11-03T01:35:05.171458Z"    # 租约的更新时间

而实际上 leader 选举中的资源 lock 其实就是一种 lease，表明 master 主节点持有对某个资源

的唯一性

查看 github.com/kubernetes/... 的源文件

可以看到 leaderElection 的目录结构，主要分为 resourcelock 和 leaderelection 的主文件，

文件内容不是很多

bash 复制代码

➜  leaderelection git:(master) tree                                                                                                  jinchaozhu@VM-221-245-tencentos leaderelection %
.
├── healthzadaptor.go
├── healthzadaptor_test.go
├── leaderelection.go
├── leaderelection_test.go
├── leasecandidate.go
├── leasecandidate_test.go
├── metrics.go
├── OWNERS
└── resourcelock
    ├── interface.go
    ├── leaselock.go
    └── multilock.go

数据结构

存储 Lease 相关的信息

go 复制代码

// LeaderElectionRecord is the record that is stored in the leader election annotation.
// This information should be used for observational purposes only and could be replaced
// with a random string (e.g. UUID) with only slight modification of this code.
// TODO(mikedanese): this should potentially be versioned
type LeaderElectionRecord struct {
    // leader的标识
    HolderIdentity       string                      `json:"holderIdentity"`
    
    // 选举间隔
    LeaseDurationSeconds int                         `json:"leaseDurationSeconds"`
    
    // 选举成为leader的时间
    AcquireTime          metav1.Time                 `json:"acquireTime"`
    
    // 续任时间
    RenewTime            metav1.Time                 `json:"renewTime"`
    
    // leader位置的转让次数
    LeaderTransitions    int                         `json:"leaderTransitions"`
    
    // 选举策略
    Strategy             v1.CoordinatedLeaseStrategy `json:"strategy"`
    PreferredHolder      string                      `json:"preferredHolder"`
}

Elector 相关的配置文件

go 复制代码

type LeaderElectionConfig struct {
    // 锁，用来保证时序竞态
    Lock rl.Interface
 
    // 非leader候选者尝试获取leadership的间隔时间
    // Core clients default this value to 15 seconds.
    LeaseDuration time.Duration
    
    // leade 放弃leadership角色之前的确认时间
    // Core clients default this value to 10 seconds.
    RenewDeadline time.Duration
    
    // 候选者应该获取leader角色的重试时间
    // Core clients default this value to 2 seconds.
    RetryPeriod time.Duration
 
    
    // 回掉函数
    // 比如开始leader选举触发什么、成为leader触发什么、放弃leader触发什么
    Callbacks LeaderCallbacks
 
    // WatchDog is the associated health checker
    // WatchDog may be null if it's not needed/configured.
    WatchDog *HealthzAdaptor
 
    // ReleaseOnCancel should be set true if the lock should be released
    // when the run context is cancelled. If you set this to true, you must
    // ensure all code guarded by this lease has successfully completed
    // prior to cancelling the context, or you may have two processes
    // simultaneously acting on the critical path.
    ReleaseOnCancel bool
 
    // Name is the name of the resource lock for debugging
    Name string
 
    // Coordinated will use the Coordinated Leader Election feature
    // WARNING: Coordinated leader election is ALPHA.
    Coordinated bool
}

主要逻辑

选举的逻辑大概如下：

刚开始实例启动的时候，各个实例都是一个 LeaderElector 的角色，最先开始选举的就成

为 leader；成为 leader 之后便会维护一个 LeaseLock 供每个 LeaderElector 进行访问查询

其余的 LeaderElector 进入候选状态，hang 住监控 leader 的状态，必要时异常会再次参与选举
Leader 获取到 Leadership 之后会持续性的刷新自己的 leader 状态

go 复制代码

func (le *LeaderElector) Run(ctx context.Context) {
    defer runtime.HandleCrash()
    defer le.config.Callbacks.OnStoppedLeading()  // StoppedLeading 函数每个节点都会执行
 
    // 未获得leadership的节点这里就会返回
    // acquire 就是各个节点来争抢 leadership
    if !le.acquire(ctx) {
        return // ctx signalled done
    }
    ctx, cancel := context.WithCancel(ctx)
    defer cancel()
    
    // 这里只有获取到leadership的角色的节点才会执行 StartedLeading
    go le.config.Callbacks.OnStartedLeading(ctx)
    
    // 获取到 leadership 之后不停的刷新当前的状态信息
    le.renew(ctx)
}
 
 
func (le *LeaderElector) acquire(ctx context.Context) bool {
    ...
    klog.Infof("attempting to acquire leader lease %v...", desc)
    wait.JitterUntil(func() {
        if !le.config.Coordinated {
            succeeded = le.tryAcquireOrRenew(ctx)    // 尝试竞争
        } else {
            succeeded = le.tryCoordinatedRenew(ctx)
        }
        ....
        klog.Infof("successfully acquired lease %v", desc)
        cancel()
        .....
    }, le.config.RetryPeriod, JitterFactor, true, ctx.Done())
    return succeeded
}

核心逻辑 tryAcquireOrRenew

go 复制代码

// tryAcquireOrRenew tries to acquire a leader lease if it is not already acquired,
// else it tries to renew the lease if it has already been acquired. Returns true
// on success else returns false.
func (le *LeaderElector) tryAcquireOrRenew(ctx context.Context) bool {
    now := metav1.NewTime(le.clock.Now())
    leaderElectionRecord := rl.LeaderElectionRecord{
        // 这里 identity 为当前竞选者的标识
        HolderIdentity:       le.config.Lock.Identity(),    
        LeaseDurationSeconds: int(le.config.LeaseDuration / time.Second),
        RenewTime:            now,
        AcquireTime:          now,
    }
 
    // 1.判断是否是Leader，如果是Leader并且Lease有效，则进行 Lock 的信息更新
    if le.IsLeader() && le.isLeaseValid(now.Time) {
        oldObservedRecord := le.getObservedRecord()
        leaderElectionRecord.AcquireTime = oldObservedRecord.AcquireTime
        leaderElectionRecord.LeaderTransitions = oldObservedRecord.LeaderTransitions
 
        err := le.config.Lock.Update(ctx, leaderElectionRecord)
        ........
    }
 
    // 2.不是Leader,则进行锁的获取
    oldLeaderElectionRecord, oldLeaderElectionRawRecord, err := le.config.Lock.Get(ctx)
    if err != nil {
        ........
    }
 
    // 3.对比检查是否 Elctection 的 Record 信息
    // 需要更新则刷新本地的缓存信息
    if !bytes.Equal(le.observedRawRecord, oldLeaderElectionRawRecord) {
        le.setObservedRecord(oldLeaderElectionRecord)
 
        le.observedRawRecord = oldLeaderElectionRawRecord
    }
    if len(oldLeaderElectionRecord.HolderIdentity) > 0 && le.isLeaseValid(now.Time) && !le.IsLeader() {
        klog.V(4).Infof("lock is held by %v and has not yet expired", oldLeaderElectionRecord.HolderIdentity)
        return false
    }
 
    // 4. 按照是否Leader判断是否进行更新 ElectionRecord
    if le.IsLeader() {
        leaderElectionRecord.AcquireTime = oldLeaderElectionRecord.AcquireTime
        leaderElectionRecord.LeaderTransitions = oldLeaderElectionRecord.LeaderTransitions
        le.metrics.slowpathExercised(le.config.Name)
    } else {
        leaderElectionRecord.LeaderTransitions = oldLeaderElectionRecord.LeaderTransitions + 1
    }
 
    // 5.更新锁本身的信息
    if err = le.config.Lock.Update(ctx, leaderElectionRecord); err != nil {
        klog.Errorf("Failed to update lock: %v", err)
        return false
    }
       
    // 更新锁成功则说明当前节点持有锁：抢锁成功/续期成功
    le.setObservedRecord(&leaderElectionRecord)
    return true
}

上面的 Lock 其实是自实现的一种 LeaseLock

go 复制代码

// Interface offers a common interface for locking on arbitrary
// resources used in leader election.  The Interface is used
// to hide the details on specific implementations in order to allow
// them to change over time.  This interface is strictly for use
// by the leaderelection code.
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
 
    .....
}
 
 
// Get returns the election record from a Lease spec
func (ll *LeaseLock) Get(ctx context.Context) (*LeaderElectionRecord, []byte, error) {
    lease, err := ll.Client.Leases(ll.LeaseMeta.Namespace).Get(ctx, ll.LeaseMeta.Name, metav1.GetOptions{})
    if err != nil {
        return nil, nil, err
    }
    ll.lease = lease
    record := LeaseSpecToLeaderElectionRecord(&ll.lease.Spec)
    recordByte, err := json.Marshal(*record)
    if err != nil {
        return nil, nil, err
    }
    return record, recordByte, nil
}
 
// 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,
        },
        Spec: LeaderElectionRecordToLeaseSpec(&ler),
    }, metav1.CreateOptions{})
    return err
}

怎么判断当前leader是持有租约的呢？

go 复制代码

func (le *LeaderElector) isLeaseValid(now time.Time) bool {
    return le.observedTime.Add(time.Second * time.Duration(le.getObservedRecord().LeaseDurationSeconds)).After(now)
}

其实就是判断上次观察到的时间与当前之间的差是否在 LeaseDurationSeconds 的范围内，在

范围内就代表是有效的

那这个选主的主到底是怎么判断的呢？

我们查看下面的判断逻辑

go 复制代码

// IsLeader returns true if the last observed leader was this client else returns false.
func (le *LeaderElector) IsLeader() bool {
    return le.getObservedRecord().HolderIdentity == le.config.Lock.Identity()
}

其实就是拿当前的 ElectionRecord 和每个实例启动时的配置文件里面的 Identity 来进行比较

判断是否一致即可

而这个 ObservedRecord 的信息是从 k8s 里面进行获取的，这就保证了唯一性

Identity 是每个实例启动的唯一标识，这个字段千万不能重复，否则选举一定失败，报错如下

bash 复制代码

E1103 15:44:21.019391 3024650 leaderelection.go:429] Failed to update lock optimitically: Operation cannot be fulfilled on leases.coordination.k8s.io "hello-world": the object has been modified; please apply your changes to the latest version and try again, falling back to slow path