//在clientconn.go的DialContext函数中
go func() {var addrs []Addressif cc.dopts.balancer == nil {// Connect to target directly if balancer is nil.//如果没有设置负载均衡器，则直接连接addrs = append(addrs, Address{Addr: target})} else {var credsClone credentials.TransportCredentialsif creds != nil {credsClone = creds.Clone()}config := BalancerConfig{DialCreds: credsClone,}//启动一个负载均衡器,start函数会启动一个watch监听地址的变化，而Notify返回一个通道，在每次服务器地址变化后的最新地址信息.if err := cc.dopts.balancer.Start(target, config); err != nil {waitC <- errreturn}ch := cc.dopts.balancer.Notify()if ch == nil {// There is no name resolver installed.addrs = append(addrs, Address{Addr: target})} else {addrs, ok = <-chif !ok || len(addrs) == 0 {waitC <- errNoAddrreturn}}}//对每一个地址进行连接, 因为这是客户端启动时，所以需要对所有地址操作.for _, a := range addrs {if err := cc.resetAddrConn(a, false, nil); err != nil {waitC <- errreturn}}close(waitC)
}()
..............
//省略一些代码
if ok {//这里开启一个监听goroutine，主要是监听服务器地址变化并对新的地址建立连接，对老的地址关闭连接go cc.lbWatcher()
}

type Balancer interface {//启动一个负载均衡，内部会启动一个名称服务器的watcher，不断监听地址的变化Start(target string, config BalancerConfig) errorUp(addr Address) (down func(error))//得到下一次连接的地址Get(ctx context.Context, opts BalancerGetOptions) (addr Address, put func(), err error)//从服务器得到更新的地址,这个地址是更新后的所有地址Notify() <-chan []Address//关闭负载均衡器Close() error
}

type Update struct {// Op indicates the operation of the update.Op Operation// Addr is the updated address. It is empty string if there is no address update.Addr string// Metadata is the updated metadata. It is nil if there is no metadata update.// Metadata is not required for a custom naming implementation.Metadata interface{}
}
// Resolver creates a Watcher for a target to track its resolution changes.
type Resolver interface {// Resolve creates a Watcher for target.//通过一个名字得到一个watcher，监听服务器地址变化。Resolve(target string) (Watcher, error)
}
// Watcher watches for the updates on the specified target.
type Watcher interface {// Next blocks until an update or error happens. It may return one or more// updates. The first call should get the full set of the results. It should// return an error if and only if Watcher cannot recover.// 得到下次更新的地址Next() ([]*Update, error)// Close closes the Watcher.Close()
}