etcd的使用

一、安装配置

1、服务端

2、客户端

二、etcd的基础知识、原理分析

三、etcd的API使用

1.1、连接

想要访问etcd，我们必须实例化一个client：

    cli,err := clientv3.New(clientv3.Config{
        Endpoints:[]string{"localhost:2379"},
        DialTimeout: 5 * time.Second,
    })

这里需要传入的两个参数：

• Endpoints：etcd的多个节点服务地址，因为我是单点本机测试，所以只传1个。
• DialTimeout：创建client的首次连接超时，这里传了5秒，如果5秒都没有连接成功就会返回err；值得注意的是，一旦client创建成功，我们就不用再关心后续底层连接的状态了，client内部会重连。

接着我们来看一下client的定义：

type Client struct {
    Cluster
    KV
    Lease
    Watcher
    Auth
    Maintenance

    // Username is a user name for authentication.
    Username string
    // Password is a password for authentication.
    Password string
}

注意，这里显示的都是可导出的模块结构字段，代表了客户端能够使用的几大核心模块，其具体功能介绍如下：

• Cluster：向集群里增加etcd服务端节点之类，属于管理员操作。
• KV：我们主要使用的功能，即操作K-V。
• Lease：租约相关操作，比如申请一个TTL=10秒的租约。
• Watcher：观察订阅，从而监听最新的数据变化。
• Auth：管理etcd的用户和权限，属于管理员操作。
• Maintenance：维护etcd，比如主动迁移etcd的leader节点，属于管理员操作。

下面我们来分别具体介绍一下这几大核心模块：

1.2、k-v存取

1.2.1. kv对象的实例获取

kv  := clientev3.NewKV(client)

我们来看一下这个kv对象具体长什么样子：

type KV interface {
    // Put puts a key-value pair into etcd.
    // Note that key,value can be plain bytes array and string is
    // an immutable representation of that bytes array.
    // To get a string of bytes, do string([]byte{0x10, 0x20}).
    Put(ctx context.Context, key, val string, opts ...OpOption) (*PutResponse, error)

    // Get retrieves keys.
    // By default, Get will return the value for "key", if any.
    // When passed WithRange(end), Get will return the keys in the range [key, end).
    // When passed WithFromKey(), Get returns keys greater than or equal to key.
    // When passed WithRev(rev) with rev > 0, Get retrieves keys at the given revision;
    // if the required revision is compacted, the request will fail with ErrCompacted .
    // When passed WithLimit(limit), the number of returned keys is bounded by limit.
    // When passed WithSort(), the keys will be sorted.
    Get(ctx context.Context, key string, opts ...OpOption) (*GetResponse, error)

    // Delete deletes a key, or optionally using WithRange(end), [key, end).
    Delete(ctx context.Context, key string, opts ...OpOption) (*DeleteResponse, error)

    // Compact compacts etcd KV history before the given rev.
    Compact(ctx context.Context, rev int64, opts ...CompactOption) (*CompactResponse, error)

    // Do applies a single Op on KV without a transaction.
    // Do is useful when creating arbitrary operations to be issued at a
    // later time; the user can range over the operations, calling Do to
    // execute them. Get/Put/Delete, on the other hand, are best suited
    // for when the operation should be issued at the time of declaration.
    Do(ctx context.Context, op Op) (OpResponse, error)

    // Txn creates a transaction.
    Txn(ctx context.Context) Txn
}

从KV对象的定义我们可知，它就是一个接口对象，包含几个主要的kv操作方法

1.2.2. k-v 存储 put
案例:

putResp, err := kv.Put(context.TODO(),"/data-dir/example", "hello-world!")

put方法的原型如下：

Put(ctx context.Context, key, val string, opts ...OpOption) (*PutResponse, error)

参数介绍：
ctx: Context包对象，是用来跟踪上下文的，列如超时控制
key: 存储对象的key
val: 存储对象的value
opts: 　可变参数，额外选项

1.2.3 k-v查询 get
现在可以对存储的数据进行取值了：

getResp, err := kv.Get(context.TODO(), "/data-dir/example")

get方法原型如下：

// By default, Get will return the value for "key", if any.
// When passed WithRange(end), Get will return the keys in the range [key, end).
// When passed WithFromKey(), Get returns keys greater than or equal to key.
// When passed WithRev(rev) with rev > 0, Get retrieves keys at the given revision;
// if the required revision is compacted, the request will fail with ErrCompacted .
// When passed WithLimit(limit), the number of returned keys is bounded by limit.
// When passed WithSort(), the keys will be sorted.
Get(ctx context.Context, key string, opts ...OpOption) (*GetResponse, error)

从以上数据的存储和取值，我们知道put 返回PutResponse, get返回GetResponse，注意：不同的KV操作对应不同的response结构，定义如下：

type (
    CompactResponse pb.CompactionResponse
    PutResponse     pb.PutResponse
    GetResponse     pb.RangeResponse
    DeleteResponse  pb.DeleteRangeResponse
    TxnResponse     pb.TxnResponse
)

我们分别来看一看PutResponse和GetResponse映射的RangeResponse结构的定义：

type PutResponse struct {
    Header *ResponseHeader `protobuf:"bytes,1,opt,name=header" json:"header,omitempty"`
    // if prev_kv is set in the request, the previous key-value pair will be returned.
    PrevKv *mvccpb.KeyValue `protobuf:"bytes,2,opt,name=prev_kv,json=prevKv" json:"prev_kv,omitempty"`
}
//Header里保存的主要是本次更新的revision信息


type RangeResponse struct {
    Header *ResponseHeader `protobuf:"bytes,1,opt,name=header" json:"header,omitempty"`
    // kvs is the list of key-value pairs matched by the range request.
    // kvs is empty when count is requested.
    Kvs []*mvccpb.KeyValue `protobuf:"bytes,2,rep,name=kvs" json:"kvs,omitempty"`
    // more indicates if there are more keys to return in the requested range.
    More bool `protobuf:"varint,3,opt,name=more,proto3" json:"more,omitempty"`
    // count is set to the number of keys within the range when requested.
    Count int64 `protobuf:"varint,4,opt,name=count,proto3" json:"count,omitempty"`
}

Kvs字段，保存了本次Get查询到的所有k-v对,我们继续看一下mvccpb.KeyValue对象长什么样子：

type KeyValue struct {
    // key is the key in bytes. An empty key is not allowed.
    Key []byte `protobuf:"bytes,1,opt,name=key,proto3" json:"key,omitempty"`
    // create_revision is the revision of last creation on this key.
    CreateRevision int64 `protobuf:"varint,2,opt,name=create_revision,json=createRevision,proto3" json:"create_revision,omitempty"`
    // mod_revision is the revision of last modification on this key.
    ModRevision int64 `protobuf:"varint,3,opt,name=mod_revision,json=modRevision,proto3" json:"mod_revision,omitempty"`
    // version is the version of the key. A deletion resets
    // the version to zero and any modification of the key
    // increases its version.
    Version int64 `protobuf:"varint,4,opt,name=version,proto3" json:"version,omitempty"`
    // value is the value held by the key, in bytes.
    Value []byte `protobuf:"bytes,5,opt,name=value,proto3" json:"value,omitempty"`
    // lease is the ID of the lease that attached to key.
    // When the attached lease expires, the key will be deleted.
    // If lease is 0, then no lease is attached to the key.
    Lease int64 `protobuf:"varint,6,opt,name=lease,proto3" json:"lease,omitempty"`
}

至于RangeResponse.More和Count，当我们使用withLimit()选项进行Get时会发挥作用，相当于翻页查询。
接下来，我们通过一个特别的Get选项，获取/data-dir/example目录下的所有孩子：

rangeResp, err := kv.Get(context.TODO(), "/data-dir/example", clientv3.WithPrefix())

WithPrefix()是指查找以/data-dir/example为前缀的所有key，因此可以模拟出查找子目录的效果。

我们知道etcd是一个有序的k-v存储，因此/data-dir/example为前缀的key总是顺序排列在一起。

withPrefix实际上会转化为范围查询，它根据前缀/data-dir/example生成了一个key range，[“/test/”, “/test0”)，为什么呢？因为比/大的字符是’0’，所以以/test0作为范围的末尾，就可以扫描到所有的/test/打头的key了。

1.3、租约lease

我们先来获取一个lease对象：

lease := clientv3.NewLease(client)

接着我们来看一下lease对象长什么样子：

type Lease interface {
    // Grant creates a new lease.
    Grant(ctx context.Context, ttl int64) (*LeaseGrantResponse, error)
 
    // Revoke revokes the given lease.
    Revoke(ctx context.Context, id LeaseID) (*LeaseRevokeResponse, error)
 
    // TimeToLive retrieves the lease information of the given lease ID.
    TimeToLive(ctx context.Context, id LeaseID, opts ...LeaseOption) (*LeaseTimeToLiveResponse, error)
 
    // Leases retrieves all leases.
    Leases(ctx context.Context) (*LeaseLeasesResponse, error)
 
    // KeepAlive keeps the given lease alive forever.
    KeepAlive(ctx context.Context, id LeaseID) (<-chan *LeaseKeepAliveResponse, error)
 
    // KeepAliveOnce renews the lease once. In most of the cases, KeepAlive
    // should be used instead of KeepAliveOnce.
    KeepAliveOnce(ctx context.Context, id LeaseID) (*LeaseKeepAliveResponse, error)
 
    // Close releases all resources Lease keeps for efficient communication
    // with the etcd server.
    Close() error
}

Lease提供了几个功能：

• Grant：分配一个租约。
• Revoke：释放一个租约。
• TimeToLive：获取剩余TTL时间。
• Leases：列举所有etcd中的租约。
• KeepAlive：自动定时的续约某个租约。
• KeepAliveOnce：为某个租约续约一次。
• Close：貌似是关闭当前客户端建立的所有租约。

要想实现key自动过期，首先得创建一个租约，它有10秒的TTL：

grantResp, err := lease.Grant(context.TODO(), 10)

grantResp中主要使用到了ID，也就是租约ID：

// LeaseGrantResponse wraps the protobuf message LeaseGrantResponse.
type LeaseGrantResponse struct {
    *pb.ResponseHeader
    ID    LeaseID
    TTL   int64
    Error string
}

接下来，我们用这个租约来Put一个会自动过期的Key：

kv.Put(context.TODO(), "/example/expireme", "lease-go", clientv3.WithLease(grantResp.ID))

这里特别需要注意，有一种情况是在Put之前Lease已经过期了，那么这个Put操作会返回error，此时你需要重新分配Lease。

当我们实现服务注册时，需要主动给Lease进行续约，这需要调用KeepAlive/KeepAliveOnce，你可以在一个循环中定时的调用：

keepResp, err := lease.KeepAliveOnce(context.TODO(), grantResp.ID)
// sleep一会...

keepResp结构如下：

// LeaseKeepAliveResponse wraps the protobuf message LeaseKeepAliveResponse.
type LeaseKeepAliveResponse struct {
    *pb.ResponseHeader
    ID  LeaseID
    TTL int64
}

KeepAlive和Put一样，如果在执行之前Lease就已经过期了，那么需要重新分配Lease。Etcd并没有提供API来实现原子的Put with Lease。

1.4 Op

Op字面意思就是”操作”，Get和Put都属于Op，只是为了简化用户开发而开放的特殊API。

// Do is useful when creating arbitrary operations to be issued at a
// later time; the user can range over the operations, calling Do to
// execute them. Get/Put/Delete, on the other hand, are best suited
// for when the operation should be issued at the time of declaration.
Do(ctx context.Context, op Op) (OpResponse, error)

其参数Op是一个抽象的操作，可以是Put/Get/Delete…；而OpResponse是一个抽象的结果，可以是PutResponse/GetResponse…

可以通过一些函数来分配Op：

func OpDelete(key string, opts …OpOption) Op
func OpGet(key string, opts …OpOption) Op
func OpPut(key, val string, opts …OpOption) Op
func OpTxn(cmps []Cmp, thenOps []Op, elseOps []Op) Op

其实和直接调用KV.Put，KV.GET没什么区别。请看如下案例：

op1 := clientv3.OpPut("/hi", "hello", clientv3.WithPrevKV())
opResp, err := kv.Do(context.TODO(), op1)

这里设置一个key=/hi，value=hello，希望结果中返回覆盖之前的value。

把这个op交给Do方法执行，返回的opResp结构如下：

type OpResponse struct {
    put *PutResponse
    get *GetResponse
    del *DeleteResponse
    txn *TxnResponse
}

1.5、事务Tnx

etcd中事务是原子执行的，只支持if … then … else …这种表达，能实现一些有意思的场景。
首先，我们需要开启一个事务，这是通过KV对象的方法实现的：

txn := kv.Txn(context.TODO())

我写了如下的测试代码，Then和Else还比较好理解，If是比较陌生的。

txnResp, err := txn.If(clientv3.Compare(clientv3.Value("/hi"), "=", "hello")).
        Then(clientv3.OpGet("/hi")).
        Else(clientv3.OpGet("/test/", clientv3.WithPrefix())).
        Commit()

我们先看下Txn支持的方法：

type Txn interface {
    // If takes a list of comparison. If all comparisons passed in succeed,
    // the operations passed into Then() will be executed. Or the operations
    // passed into Else() will be executed.
    If(cs ...Cmp) Txn

    // Then takes a list of operations. The Ops list will be executed, if the
    // comparisons passed in If() succeed.
    Then(ops ...Op) Txn

    // Else takes a list of operations. The Ops list will be executed, if the
    // comparisons passed in If() fail.
    Else(ops ...Op) Txn

    // Commit tries to commit the transaction.
    Commit() (*TxnResponse, error)
}

我们来看一下value方法：

func Value(key string) Cmp {
    return Cmp{Key: []byte(key), Target: pb.Compare_VALUE}
}

这个Value(“/hi”)返回的Cmp表达了：”/hi这个key对应的value”。

接下来，利用Compare函数来继续为”主语”增加描述，形成了一个完整条件语句，即”/hi这个key对应的value”必须等于”hello”。

Compare函数实际上是对Value返回的Cmp对象进一步修饰，增加了”=”与”hello”两个描述信息：

func Compare(cmp Cmp, result string, v interface{}) Cmp {
    var r pb.Compare_CompareResult

    switch result {
    case "=":
        r = pb.Compare_EQUAL
    case "!=":
        r = pb.Compare_NOT_EQUAL
    case ">":
        r = pb.Compare_GREATER
    case "<":
        r = pb.Compare_LESS
    default:
        panic("Unknown result op")
    }

    cmp.Result = r
    switch cmp.Target {
    case pb.Compare_VALUE:
        val, ok := v.(string)
        if !ok {
            panic("bad compare value")
        }
        cmp.TargetUnion = &pb.Compare_Value{Value: []byte(val)}
    case pb.Compare_VERSION:
        cmp.TargetUnion = &pb.Compare_Version{Version: mustInt64(v)}
    case pb.Compare_CREATE:
        cmp.TargetUnion = &pb.Compare_CreateRevision{CreateRevision: mustInt64(v)}
    case pb.Compare_MOD:
        cmp.TargetUnion = &pb.Compare_ModRevision{ModRevision: mustInt64(v)}
    case pb.Compare_LEASE:
        cmp.TargetUnion = &pb.Compare_Lease{Lease: mustInt64orLeaseID(v)}
    default:
        panic("Unknown compare type")
    }
    return cmp
}

Cmp可以用于描述”key=xxx的yyy属性，必须=、!=、<、>，kkk值”，比如：

• key=xxx的value，必须!=，hello。
• key=xxx的create版本号，必须=，11233。
• key=xxx的lease id，必须=，12319231231238。

经过Compare函数修饰的Cmp对象，内部包含了完整的条件信息，传递给If函数即可。

类似于Value的函数用于指定yyy属性，有这么几个方法：

• func CreateRevision(key string) Cmp：key=xxx的创建版本必须满足…
• func LeaseValue(key string) Cmp：key=xxx的Lease ID必须满足…
• func ModRevision(key string) Cmp：key=xxx的最后修改版本必须满足…
• func Value(key string) Cmp：key=xxx的创建值必须满足…
• func Version(key string) Cmp：key=xxx的累计更新次数必须满足…

最后Commit提交整个Txn事务，我们需要判断txnResp获知If条件是否成立：

if txnResp.Succeeded { // If = true
        fmt.Println("~~~", txnResp.Responses[0].GetResponseRange().Kvs)
} else { // If =false
        fmt.Println("!!!", txnResp.Responses[0].GetResponseRange().Kvs)
}

Succeed=true表示If条件成立，接下来我们需要获取Then或者Else中的OpResponse列表（因为可以传多个Op），可以看一下txnResp的结构：

type TxnResponse struct {
    Header *ResponseHeader `protobuf:"bytes,1,opt,name=header" json:"header,omitempty"`
    // succeeded is set to true if the compare evaluated to true or false otherwise.
    Succeeded bool `protobuf:"varint,2,opt,name=succeeded,proto3" json:"succeeded,omitempty"`
    // responses is a list of responses corresponding to the results from applying
    // success if succeeded is true or failure if succeeded is false.
    Responses []*ResponseOp `protobuf:"bytes,3,rep,name=responses" json:"responses,omitempty"`
}

1.6、监听watch

在分析watch之前，我们先来罗列以下watch的整个流程：

• a、我们先往etcd写入一对Ｋ－Ｖ

• b、我们使用watch监听这对Ｋ－Ｖ，如果一切正常, 这时候请求会被阻塞住.

• c、现在我们修改这一对Ｋ－Ｖ

• d、阻塞的那个请求返回watch到的结果

    {
      "action":"set",
      "node":{ 
          "key":"/name",
          "value":"神蛋使者1号",
          "modifiedIndex":25,
         "createdIndex":25
      },
       "prevNode": {
         "key":"/name",
         "value":"神蛋使者",
         "modifiedIndex":24,
         "createdIndex":24
       }
    }
  

1.6.1. watch接口定义

type Watcher interface {
    // Watch watches on a key or prefix. The watched events will be returned
    // through the returned channel.
    // If the watch is slow or the required rev is compacted, the watch request
    // might be canceled from the server-side and the chan will be closed.
    // 'opts' can be: 'WithRev' and/or 'WithPrefix'.
    Watch(ctx context.Context, key string, opts ...OpOption) WatchChan

    // Close closes the watcher and cancels all watch requests.
    Close() error
}

该接口定义了两个方法, Watch 和 Close

Watch 方法返回一个WatchChan 类似的变量, WatchChan是一个channel, 定义如下:

type WatchChan <-chan WatchResponse

该通道传递WatchResponse类型

type WatchResponse struct {
    Header pb.ResponseHeader
    Events []*Event

    // CompactRevision is the minimum revision the watcher may receive.
    CompactRevision int64

    // Canceled is used to indicate watch failure.
    // If the watch failed and the stream was about to close, before the channel is closed,
    // the channel sends a final response that has Canceled set to true with a non-nil Err().
    Canceled bool

    // Created is used to indicate the creation of the watcher.
    Created bool

    closeErr error
}

其中Event类型是一个gRPC生成的消息对象

type Event struct {
    // type is the kind of event. If type is a PUT, it indicates
    // new data has been stored to the key. If type is a DELETE,
    // it indicates the key was deleted.
    Type Event_EventType `protobuf:"varint,1,opt,name=type,proto3,enum=mvccpb.Event_EventType" json:"type,omitempty"`
    // kv holds the KeyValue for the event.
    // A PUT event contains current kv pair.
    // A PUT event with kv.Version=1 indicates the creation of a key.
    // A DELETE/EXPIRE event contains the deleted key with
    // its modification revision set to the revision of deletion.
    Kv *KeyValue `protobuf:"bytes,2,opt,name=kv" json:"kv,omitempty"`
    // prev_kv holds the key-value pair before the event happens.
    PrevKv *KeyValue `protobuf:"bytes,3,opt,name=prev_kv,json=prevKv" json:"prev_kv,omitempty"`
}

接下来看实现了Watcher接口的watcher类型

// watcher implements the Watcher interface
type watcher struct {
    remote pb.WatchClient

    // mu protects the grpc streams map
    mu sync.RWMutex

    // streams holds all the active grpc streams keyed by ctx value.
    streams map[string]*watchGrpcStream
}

watcher结构很简单, 只有3个字段. remote抽象了发起watch请求的客户端, streams是一个map, 这个map映射了交互的数据流.还有一个保护并发环境下数据流读写安全的读写锁.

streams所属的watchGrpcStream类型抽象了所有交互的数据, 它的结构定义如下

type watchGrpcStream struct {
    owner  *watcher
    remote pb.WatchClient

    // ctx controls internal remote.Watch requests
    ctx context.Context
    // ctxKey is the key used when looking up this stream's context
    ctxKey string
    cancel context.CancelFunc

    // substreams holds all active watchers on this grpc stream
    substreams map[int64]*watcherStream
    // resuming holds all resuming watchers on this grpc stream
    resuming []*watcherStream

    // reqc sends a watch request from Watch() to the main goroutine
    reqc chan *watchRequest
    // respc receives data from the watch client
    respc chan *pb.WatchResponse
    // donec closes to broadcast shutdown
    donec chan struct{}
    // errc transmits errors from grpc Recv to the watch stream reconn logic
    errc chan error
    // closingc gets the watcherStream of closing watchers
    closingc chan *watcherStream
    // wg is Done when all substream goroutines have exited
    wg sync.WaitGroup

    // resumec closes to signal that all substreams should begin resuming
    resumec chan struct{}
    // closeErr is the error that closed the watch stream
    closeErr error
}

比较有意思的是, watchGrpcStream也包含了一个watcher类型的owner字段, watcher和watchGrpcStream可以互相引用到对方.同时又定义了watcher类型中已经定义过的remote,而且还不是指针类型, 这点不大明白作用是啥.

还有几个字段值得关注, 一个是substreams, 看下它的定义和注释:

// substreams holds all active watchers on this grpc stream
substreams map[int64]*watcherStream

再看看watcherStream类型的定义:

// watcherStream represents a registered watcher
type watcherStream struct {
    // initReq is the request that initiated this request
    initReq watchRequest

    // outc publishes watch responses to subscriber
    outc chan WatchResponse
    // recvc buffers watch responses before publishing
    recvc chan *WatchResponse
    // donec closes when the watcherStream goroutine stops.
    donec chan struct{}
    // closing is set to true when stream should be scheduled to shutdown.
    closing bool
    // id is the registered watch id on the grpc stream
    id int64

    // buf holds all events received from etcd but not yet consumed by the client
    buf []*WatchResponse
}

画个图整理下他们之间的关系:

1244770-8d56f4f0d90de613.png

接下来轮到watcher是如何watch方法的了:

// Watch posts a watch request to run() and waits for a new watcher channel
func (w *watcher) Watch(ctx context.Context, key string, opts ...OpOption) WatchChan {
    // 应用配置
    ow := opWatch(key, opts...)

    var filters []pb.WatchCreateRequest_FilterType
    if ow.filterPut {
    filters = append(filters, pb.WatchCreateRequest_NOPUT)
    }
    if ow.filterDelete {
    filters = append(filters, pb.WatchCreateRequest_NODELETE)
    }

    // 根据传入的参数构造watch请求
    wr := &watchRequest{
    ctx:            ctx,
    createdNotify:  ow.createdNotify,
    key:            string(ow.key),
    end:            string(ow.end),
    rev:            ow.rev,
    progressNotify: ow.progressNotify,
    filters:        filters,
    prevKV:         ow.prevKV,
    retc:           make(chan chan WatchResponse, 1),
    }

    ok := false
    // 将请求上下文格式化为字符串
    ctxKey := fmt.Sprintf("%v", ctx)

    // find or allocate appropriate grpc watch stream
    // 接下来配置对应的输出流, 注意得加锁
    w.mu.Lock()

    // 如果stream为空, 返回一个已经关闭的channel.
    // 这种情况应该是防止streams为空的情况
    if w.streams == nil {
    // closed
    w.mu.Unlock()
    ch := make(chan WatchResponse)
    close(ch)
    return ch
    }

    // 注意这里, 前面我们提到streams是一个map,该map的key是请求上下文
    // 如果该请求对应的流为空,则新建
    wgs := w.streams[ctxKey]
    if wgs == nil {
    wgs = w.newWatcherGrpcStream(ctx)
    w.streams[ctxKey] = wgs
    }
    donec := wgs.donec
    reqc := wgs.reqc
    w.mu.Unlock()

    // couldn't create channel; return closed channel
    // couldn't create channel; return closed channel
    // 这里要设置为缓冲的原因可能与下面的两个
    // closeCh <- WatchResponse{closeErr: wgs.closeErr}
    // 语句有关,这里不理解
    closeCh := make(chan WatchResponse, 1)

    // submit request
    select {
    // 发送上面构造好的watch请求给对应的流
    case reqc <- wr:
    ok = true
    // 请求断开(这里应该囊括了客户端请求断开的所有情况)
    case <-wr.ctx.Done():
    // watch完成
    // 这里应该是处理非正常完成的情况
    // 注意下面的重试逻辑
    case <-donec:
    if wgs.closeErr != nil {
        // 如果不是空上下文导致流被丢弃的情况
        // 则不应该重试
        closeCh <- WatchResponse{closeErr: wgs.closeErr}
        break
    }
    // retry; may have dropped stream from no ctxs
    return w.Watch(ctx, key, opts...)
    }

    // receive channel
    // 如果是初始请求顺利发送才会执行这里
    if ok {
    select {
    case ret := <-wr.retc:
        return ret
    case <-ctx.Done():
    case <-donec:
        if wgs.closeErr != nil {
            closeCh <- WatchResponse{closeErr: wgs.closeErr}
            break
        }
        // retry; may have dropped stream from no ctxs
        return w.Watch(ctx, key, opts...)
    }
    }

    close(closeCh)
    return closeCh
}

还有Watcher接口的另一个方法Close:

func (w *watcher) Close() (err error) {
    // 在锁内先将streams字段置为空
    // 在锁外再将一个个流都关闭
    // 这样做的意义在于不管哪个流关闭失败了
    // 都能先保证streams与这些流的关系被切断
    w.mu.Lock()
    streams := w.streams
    w.streams = nil
    w.mu.Unlock()
    for _, wgs := range streams {
    if werr := wgs.Close(); werr != nil {
        err = werr
    }
    }
    // etcd竟然也只是返回一个error
    // 虽然上面的for循环可能产生多个error
    return err
}