go任务调度6（etcd租约机制/自动过期）

对于实现分布式乐观锁非常重要。如果锁了，突然宕机了，锁是需要自动释放的。所以这锁在etcd里是需要生命期的。
过期演示：

package main

import (
    "context"
    "fmt"
    "go.etcd.io/etcd/clientv3"
    "time"
)

func main() {
    var (
        config  clientv3.Config
        client  *clientv3.Client
        err     error
        lease clientv3.Lease
        leaseGrantResp *clientv3.LeaseGrantResponse
        leaseId clientv3.LeaseID
        putResp *clientv3.PutResponse
        kv clientv3.KV
        getResp *clientv3.GetResponse
    )

    //客户端配置
    config = clientv3.Config{
        Endpoints:   []string{"0.0.0.0:2379"}, //集群列表
        DialTimeout: 5 * time.Second,
    }

    //建立客户端
    if client, err = clientv3.New(config); err != nil {
        fmt.Println(err)
        return
    }

    //申请一个lease(租约)
    lease = clientv3.NewLease(client)

    //申请一个5秒的租约
    if leaseGrantResp, err = lease.Grant(context.TODO(), 5); err != nil {
        fmt.Println(err)
        return
    }

    //拿到租约的id
    leaseId = leaseGrantResp.ID

    //获得kv api子集
    kv = clientv3.NewKV(client)

    //put一个kv，让它与租约关联起来，从而实现10秒后自动过期
    if putResp, err = kv.Put(context.TODO(), "/cron/lock/job1", "", clientv3.WithLease(leaseId)); err != nil {
        fmt.Println(err)
        return
    }

    fmt.Println("写入成功:", putResp.Header.Revision)

    //定时看key过期没
    for {
        if getResp, err = kv.Get(context.TODO(), "/cron/lock/job1"); err != nil {
            fmt.Println(err)
            return
        }
        if getResp.Count == 0 {
            fmt.Println("kv过期了")
            break
        }
        fmt.Println("还没过期:", getResp.Kvs)
        time.Sleep(time.Second)
    }
}

[root@bogon etcd]# go run demo6.go
写入成功: 27
还没过期: [key:"/cron/lock/job1" create_revision:27 mod_revision:27 version:1 lease:7587837741646622005 ]
还没过期: [key:"/cron/lock/job1" create_revision:27 mod_revision:27 version:1 lease:7587837741646622005 ]
还没过期: [key:"/cron/lock/job1" create_revision:27 mod_revision:27 version:1 lease:7587837741646622005 ]
还没过期: [key:"/cron/lock/job1" create_revision:27 mod_revision:27 version:1 lease:7587837741646622005 ]
还没过期: [key:"/cron/lock/job1" create_revision:27 mod_revision:27 version:1 lease:7587837741646622005 ]
还没过期: [key:"/cron/lock/job1" create_revision:27 mod_revision:27 version:1 lease:7587837741646622005 ]
kv过期了
[root@bogon etcd]#

申请一把分布式锁的时候，是谁抢到了key就是抢到了锁，如果不主动释放这锁，按道理讲不应该让租约过期，租约过期主要是为了程序宕掉之后，锁自动释放，防止程序异常退出。如果程序抢到了这个锁，我们希望锁一直不失效，知道我们主动释放它：

package main

import (
    "context"
    "fmt"
    "go.etcd.io/etcd/clientv3"
    "time"
)

func main() {
    var (
        config  clientv3.Config
        client  *clientv3.Client
        err     error
        lease clientv3.Lease
        leaseGrantResp *clientv3.LeaseGrantResponse
        leaseId clientv3.LeaseID
        putResp *clientv3.PutResponse
        kv clientv3.KV
        getResp *clientv3.GetResponse
        keepResp *clientv3.LeaseKeepAliveResponse
        keepRespChan <-chan *clientv3.LeaseKeepAliveResponse //只读channel
    )

    //客户端配置
    config = clientv3.Config{
        Endpoints:   []string{"0.0.0.0:2379"}, //集群列表
        DialTimeout: 5 * time.Second,
    }

    //建立客户端
    if client, err = clientv3.New(config); err != nil {
        fmt.Println(err)
        return
    }

    //申请一个lease(租约)
    lease = clientv3.NewLease(client)

    //申请一个5秒的租约
    if leaseGrantResp, err = lease.Grant(context.TODO(), 5); err != nil {
        fmt.Println(err)
        return
    }

    //拿到租约的id
    leaseId = leaseGrantResp.ID

    //(自动续租)当我们申请了租约之后，我们就可以启动一个续租
    if keepRespChan, err = lease.KeepAlive(context.TODO(), leaseId); err != nil {
        fmt.Println(err)
        return
    }

    //处理续租应答的协程
    go func() {
        for {
            select {
            case keepResp = <-keepRespChan:
                if keepRespChan == nil {
                    fmt.Println("租约已经失效")
                    goto END
                } else { //每秒会续租一次，所以就会受到一次应答
                    fmt.Println("收到自动续租应答:", keepResp.ID)
                }
            }
        }
        END:
    }()

    //获得kv api子集
    kv = clientv3.NewKV(client)

    //put一个kv，让它与租约关联起来，从而实现10秒后自动过期
    if putResp, err = kv.Put(context.TODO(), "/cron/lock/job1", "", clientv3.WithLease(leaseId)); err != nil {
        fmt.Println(err)
        return
    }

    fmt.Println("写入成功:", putResp.Header.Revision)

    //定时看key过期没
    for {
        if getResp, err = kv.Get(context.TODO(), "/cron/lock/job1"); err != nil {
            fmt.Println(err)
            return
        }
        if getResp.Count == 0 {
            fmt.Println("kv过期了")
            break
        }
        fmt.Println("还没过期:", getResp.Kvs)
        time.Sleep(time.Second)
    }
}

[root@bogon etcd]# go run demo7.go
写入成功: 30
收到自动续租应答: 7587837741646622039
还没过期: [key:"/cron/lock/job1" create_revision:29 mod_revision:30 version:2 lease:7587837741646622039 ]
还没过期: [key:"/cron/lock/job1" create_revision:29 mod_revision:30 version:2 lease:7587837741646622039 ]
收到自动续租应答: 7587837741646622039
还没过期: [key:"/cron/lock/job1" create_revision:29 mod_revision:30 version:2 lease:7587837741646622039 ]
还没过期: [key:"/cron/lock/job1" create_revision:29 mod_revision:30 version:2 lease:7587837741646622039 ]
还没过期: [key:"/cron/lock/job1" create_revision:29 mod_revision:30 version:2 lease:7587837741646622039 ]
收到自动续租应答: 7587837741646622039
还没过期: [key:"/cron/lock/job1" create_revision:29 mod_revision:30 version:2 lease:7587837741646622039 ]
还没过期: [key:"/cron/lock/job1" create_revision:29 mod_revision:30 version:2 lease:7587837741646622039 ]
收到自动续租应答: 7587837741646622039
......