conn, err := grpc.Dial(
  "",
  grpc.WithInsecure(),
  // 负载均衡，使用 consul 作服务发现
  grpc.WithBalancer(grpc.RoundRobin(grpclb.NewConsulResolver(
    "127.0.0.1:8500", "grpc.health.v1.add",
  ))),
)

func NewConsulResolver(address string, service string) naming.Resolver {
  return &consulResolver{
    address: address,
    service: service,
  }
}
type consulResolver struct {
  address string
  service string
}
func (r *consulResolver) Resolve(target string) (naming.Watcher, error) {
  config := api.DefaultConfig()
  config.Address = r.address
  client, err := api.NewClient(config)
  if err != nil {
    return nil, err
  }
  return &consulWatcher{
    client: client,
    service: r.service,
    addrs:  map[string]struct{}{},
  }, nil
}
type consulWatcher struct {
  client  *api.Client
  service  string
  addrs   map[string]struct{}
  lastIndex uint64
}
func (w *consulWatcher) Next() ([]*naming.Update, error) {
  for {
    services, metainfo, err := w.client.Health().Service(w.service, "", true, &api.QueryOptions{
      WaitIndex: w.lastIndex, // 同步点，这个调用将一直阻塞，直到有新的更新
    })
    if err != nil {
      logrus.Warn("error retrieving instances from Consul: %v", err)
    }
    w.lastIndex = metainfo.LastIndex
    addrs := map[string]struct{}{}
    for _, service := range services {
      addrs[net.JoinHostPort(service.Service.Address, strconv.Itoa(service.Service.Port))] = struct{}{}
    }
    var updates []*naming.Update
    for addr := range w.addrs {
      if _, ok := addrs[addr]; !ok {
        updates = append(updates, &naming.Update{Op: naming.Delete, Addr: addr})
      }
    }
    for addr := range addrs {
      if _, ok := w.addrs[addr]; !ok {
        updates = append(updates, &naming.Update{Op: naming.Add, Addr: addr})
      }
    }
    if len(updates) != 0 {
      w.addrs = addrs
      return updates, nil
    }
  }
}
func (w *consulWatcher) Close() {
  // nothing to do
}