func (a *DaprRuntime) initRuntime(opts *runtimeOpts) error {
  ......
  // Start HTTP Server
	err = a.startHTTPServer(a.runtimeConfig.HTTPPort, a.runtimeConfig.PublicPort, a.runtimeConfig.ProfilePort, a.runtimeConfig.AllowedOrigins, pipeline)
	if err != nil {
		log.Fatalf("failed to start HTTP server: %s", err)
	}
  ......
}

func (a *DaprRuntime) startHTTPServer(......) error {
	a.daprHTTPAPI = http.NewAPI(......)

	server := http.NewServer(a.daprHTTPAPI, ......)
  if err := server.StartNonBlocking(); err != nil {		// StartNonBlocking 启动 fasthttp server
		return err
	}
}

// StartNonBlocking starts a new server in a goroutine.
func (s *server) StartNonBlocking() error {
  	......
  	for _, apiListenAddress := range s.config.APIListenAddresses {
			l, err := net.Listen("tcp", fmt.Sprintf("%s:%v", apiListenAddress, s.config.Port))
      listeners = append(listeners, l)
		}
  
  	for _, listener := range listeners {
		// customServer is created in a loop because each instance
		// has a handle on the underlying listener.
		customServer := &fasthttp.Server{
			Handler:            handler,
			MaxRequestBodySize: s.config.MaxRequestBodySize * 1024 * 1024,
			ReadBufferSize:     s.config.ReadBufferSize * 1024,
			StreamRequestBody:  s.config.StreamRequestBody,
		}
		s.servers = append(s.servers, customServer)

		go func(l net.Listener) {
			if err := customServer.Serve(l); err != nil {
				log.Fatal(err)
			}
		}(listener)
	}
}

func NewAPI(
  appID string,
	appChannel channel.AppChannel,
	directMessaging messaging.DirectMessaging,
  ......
  	shutdown func()) API {
  
  	api := &api{
		appChannel:               appChannel,
		directMessaging:          directMessaging,
		......
	}
  
  	// 附加 DirectMessaging 的 HTTP 端点
  	api.endpoints = append(api.endpoints, api.constructDirectMessagingEndpoints()...)
}

func (a *api) constructDirectMessagingEndpoints() []Endpoint {
	return []Endpoint{
		{
			Methods:           []string{router.MethodWild},
			Route:             "invoke/{id}/method/{method:*}",
			Alias:             "{method:*}",
			Version:           apiVersionV1,
			KeepParamUnescape: true,
			Handler:           a.onDirectMessage,
		},
	}
}

func (a *DaprRuntime) initRuntime(opts *runtimeOpts) error {
    // Create and start internal and external gRPC servers
	grpcAPI := a.getGRPCAPI()
    
	err = a.startGRPCAPIServer(grpcAPI, a.runtimeConfig.APIGRPCPort)
    ......
}

func (a *DaprRuntime) startGRPCAPIServer(api grpc.API, port int) error {
	serverConf := a.getNewServerConfig(a.runtimeConfig.APIListenAddresses, port)
	server := grpc.NewAPIServer(api, serverConf, a.globalConfig.Spec.TracingSpec, a.globalConfig.Spec.MetricSpec, a.globalConfig.Spec.APISpec, a.proxy)
    if err := server.StartNonBlocking(); err != nil {
		return err
	}
	......
}

// NewAPIServer returns a new user facing gRPC API server.
func NewAPIServer(api API, config ServerConfig, ......) Server {
	return &server{
		api:         api,
		config:      config,
		kind:        apiServer, // const apiServer = "apiServer"
		......
	}
}

func (s *server) StartNonBlocking() error {
		if s.kind == internalServer {
			internalv1pb.RegisterServiceInvocationServer(server, s.api)
		} else if s.kind == apiServer {
            runtimev1pb.RegisterDaprServer(server, s.api)		// 注意：s.api (即 gRPC api 实现) 被传递进去
		}
		......
}

func RegisterDaprServer(s grpc.ServiceRegistrar, srv DaprServer) {
	s.RegisterService(&Dapr_ServiceDesc, srv)					// srv 即 gRPC api 实现
}

var Dapr_ServiceDesc = grpc.ServiceDesc{
	ServiceName: "dapr.proto.runtime.v1.Dapr",
	HandlerType: (*DaprServer)(nil),
	Methods: []grpc.MethodDesc{
		{
			MethodName: "InvokeService",				# 注册方法名
			Handler:    _Dapr_InvokeService_Handler,	# 关联实现的 Handler
		},
        ......
        },
	},
	Metadata: "dapr/proto/runtime/v1/dapr.proto",
}

func _Dapr_InvokeService_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
	in := new(InvokeServiceRequest)
	if err := dec(in); err != nil {
		return nil, err
	}
	if interceptor == nil {
		return srv.(DaprServer).InvokeService(ctx, in)
	}
	info := &grpc.UnaryServerInfo{
		Server:     srv,
		FullMethod: "/dapr.proto.runtime.v1.Dapr/InvokeService",
	}
	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
		return srv.(DaprServer).InvokeService(ctx, req.(*InvokeServiceRequest))		// 这里调用的 srv 即 gRPC api 实现
	}
	return interceptor(ctx, in, info, handler)
}

func (a *DaprRuntime) initRuntime(opts *runtimeOpts) error {
	err = a.startGRPCInternalServer(grpcAPI, a.runtimeConfig.InternalGRPCPort)
	if err != nil {
		log.Fatalf("failed to start internal gRPC server: %s", err)
	}
	log.Infof("internal gRPC server is running on port %v", a.runtimeConfig.InternalGRPCPort)
    ......
}

func (a *DaprRuntime) startGRPCInternalServer(api grpc.API, port int) error {
	serverConf := a.getNewServerConfig([]string{""}, port)
	server := grpc.NewInternalServer(api, serverConf, a.globalConfig.Spec.TracingSpec, a.globalConfig.Spec.MetricSpec, a.authenticator, a.proxy)
	if err := server.StartNonBlocking(); err != nil {
		return err
	}
	a.apiClosers = append(a.apiClosers, server)

	return nil
}

func FromFlags() (*DaprRuntime, error) {	
	var daprInternalGRPC int
	if *daprInternalGRPCPort != "" {
		daprInternalGRPC, err = strconv.Atoi(*daprInternalGRPCPort)
		if err != nil {
			return nil, errors.Wrap(err, "error parsing dapr-internal-grpc-port")
		}
	} else {
		daprInternalGRPC, err = grpc.GetFreePort()
		if err != nil {
			return nil, errors.Wrap(err, "failed to get free port for internal grpc server")
		}
	}
    ......
}

grpcAPI := a.getGRPCAPI()

err = a.startGRPCAPIServer(grpcAPI, a.runtimeConfig.APIGRPCPort)
err = a.startGRPCInternalServer(grpcAPI, a.runtimeConfig.InternalGRPCPort)

func (s *server) StartNonBlocking() error {
		if s.kind == internalServer {
			internalv1pb.RegisterServiceInvocationServer(server, s.api)		// 注意：s.api (即 gRPC api 实现) 被传递进去
		} else if s.kind == apiServer {
			runtimev1pb.RegisterDaprServer(server, s.api)
		}
		......
}

func RegisterServiceInvocationServer(s grpc.ServiceRegistrar, srv ServiceInvocationServer) {
	s.RegisterService(&ServiceInvocation_ServiceDesc, srv)  					// srv 即 gRPC api 实现
}

var ServiceInvocation_ServiceDesc = grpc.ServiceDesc{
	ServiceName: "dapr.proto.internals.v1.ServiceInvocation",
	HandlerType: (*ServiceInvocationServer)(nil),
	Methods: []grpc.MethodDesc{
		{
			MethodName: "CallActor",
			Handler:    _ServiceInvocation_CallActor_Handler,
		},
		{
			MethodName: "CallLocal",
			Handler:    _ServiceInvocation_CallLocal_Handler,
		},
	},
	Streams:  []grpc.StreamDesc{},
	Metadata: "dapr/proto/internals/v1/service_invocation.proto",
}

func _ServiceInvocation_CallLocal_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
	in := new(InternalInvokeRequest)
	if err := dec(in); err != nil {
		return nil, err
	}
	if interceptor == nil {
		return srv.(ServiceInvocationServer).CallLocal(ctx, in)
	}
	info := &grpc.UnaryServerInfo{
		Server:     srv,
		FullMethod: "/dapr.proto.internals.v1.ServiceInvocation/CallLocal",
	}
	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
        // 这里调用的 srv 即 gRPC api 实现
		return srv.(ServiceInvocationServer).CallLocal(ctx, req.(*InternalInvokeRequest))  
	}
	return interceptor(ctx, in, info, handler)
}

DaprClient client = (new DaprClientBuilder()).build();
byte[] response = client.invokeMethod(SERVICE_APP_ID, "say", message, HttpExtension.POST, null,
            byte[].class).block();

  @Override
  public <T> Mono<T> invokeMethod(String appId, String methodName,......) {
    return methodInvocationOverrideClient.invokeMethod(appId, methodName, request, httpExtension, metadata, clazz);
  }
  
  public <T> Mono<T> invokeMethod(InvokeMethodRequest invokeMethodRequest, TypeRef<T> type) {
    try {
      final String appId = invokeMethodRequest.getAppId();
      final String method = invokeMethodRequest.getMethod();
      ......
      Mono<DaprHttp.Response> response = Mono.subscriberContext().flatMap(
          context -> this.client.invokeApi(httpMethod, pathSegments,
              httpExtension.getQueryParams(), serializedRequestBody, headers, context)
      );
  }

  private CompletableFuture<Response> doInvokeApi(String method,
                               String[] pathSegments,
                               Map<String, List<String>> urlParameters,
                               byte[] content, Map<String, String> headers,
                               Context context) {
      ......
      Request.Builder requestBuilder = new Request.Builder()
        .url(urlBuilder.build())
        .addHeader(HEADER_DAPR_REQUEST_ID, requestId);
      
    CompletableFuture<Response> future = new CompletableFuture<>();
    this.httpClient.newCall(request).enqueue(new ResponseFutureCallback(future));
    return future;
  }

client, err := dapr.NewClient()
content := &dapr.DataContent{
		ContentType: "text/plain",
		Data:        []byte("hellow"),
	}
// invoke a method named "app-2" on another dapr enabled service named "method-1"
resp, err := client.InvokeMethodWithContent(ctx, "app-2", "method-1", "post", content)

// Client is the interface for Dapr client implementation.
type Client interface {
    	// InvokeMethod invokes service without raw data
	InvokeMethod(ctx context.Context, appID, methodName, verb string) (out []byte, err error)

	// InvokeMethodWithContent invokes service with content
	InvokeMethodWithContent(ctx context.Context, appID, methodName, verb string, content *DataContent) (out []byte, err error)

	// InvokeMethodWithCustomContent invokes app with custom content (struct + content type).
	InvokeMethodWithCustomContent(ctx context.Context, appID, methodName, verb string, contentType string, content interface{}) (out []byte, err error)
    ......
}

func (c *GRPCClient) invokeServiceWithRequest(ctx context.Context, req *pb.InvokeServiceRequest) (out []byte, err error) {
	resp, err := c.protoClient.InvokeService(c.withAuthToken(ctx), req)
	......
}

func (c *daprClient) InvokeService(ctx context.Context, in *InvokeServiceRequest, opts ...grpc.CallOption) (*v1.InvokeResponse, error) {
	out := new(v1.InvokeResponse)
	err := c.cc.Invoke(ctx, "/dapr.proto.runtime.v1.Dapr/InvokeService", in, out, opts...)
	......
}

func (a *api) constructDirectMessagingEndpoints() []Endpoint {
	return []Endpoint{
		{
			Methods:           []string{router.MethodWild},
			Route:             "invoke/{id}/method/{method:*}",
			Alias:             "{method:*}",
			Version:           apiVersionV1,
			KeepParamUnescape: true,
			Handler:           a.onDirectMessage,
		},
	}
}

func (a *api) onDirectMessage(reqCtx *fasthttp.RequestCtx) {
	......
  req := invokev1.NewInvokeMethodRequest(...)
	resp, err := a.directMessaging.Invoke(reqCtx, targetID, req)
	......
}

func (a *api) InvokeService(ctx context.Context, in *runtimev1pb.InvokeServiceRequest) (*commonv1pb.InvokeResponse, error) {
	......
	resp, err := a.directMessaging.Invoke(ctx, in.Id, req)
	......
	return resp.Message(), respError
}

// DirectMessaging is the API interface for invoking a remote app.
type DirectMessaging interface {
	Invoke(ctx context.Context, targetAppID string, req *invokev1.InvokeMethodRequest) (*invokev1.InvokeMethodResponse, error)
}

func (d *directMessaging) Invoke(ctx context.Context, targetAppID string, req *invokev1.InvokeMethodRequest) (*invokev1.InvokeMethodResponse, error) {
	app, err := d.getRemoteApp(targetAppID)

	if app.id == d.appID && app.namespace == d.namespace {
		return d.invokeLocal(ctx, req)   // 如果调用的 appid 就是自己的 appid，这个场景好奇怪。忽略这里的代码先
	}
	return d.invokeWithRetry(ctx, retry.DefaultLinearRetryCount, retry.DefaultLinearBackoffInterval, app, d.invokeRemote, req)
}

func (d *directMessaging) invokeRemote(ctx context.Context, appID, namespace, appAddress string, req *invokev1.InvokeMethodRequest) (*invokev1.InvokeMethodResponse, error) {
    // 建立连接
	conn, err := d.connectionCreatorFn(context.TODO(), appAddress, appID, namespace, false, false, false)
    // 构建 gRPC stub 作为 client
	clientV1 := internalv1pb.NewServiceInvocationClient(conn)
    // 调用 gRPC 的 CallLocal 方法发出远程调用请求到另外一个 Dapr runtime
	resp, err := clientV1.CallLocal(ctx, req.Proto(), opts...)
    // 处理应答
	return invokev1.InternalInvokeResponse(resp)
}

func (c *serviceInvocationClient) CallLocal(ctx context.Context, in *InternalInvokeRequest, opts ...grpc.CallOption) (*InternalInvokeResponse, error) {
	out := new(InternalInvokeResponse)
	err := c.cc.Invoke(ctx, "/dapr.proto.internals.v1.ServiceInvocation/CallLocal", in, out, opts...)
	if err != nil {
		return nil, err
	}
	return out, nil
}

func _ServiceInvocation_CallLocal_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
	......
	if interceptor == nil {
		return srv.(ServiceInvocationServer).CallLocal(ctx, in)
	}
	info := &grpc.UnaryServerInfo{
		Server:     srv,
		FullMethod: "/dapr.proto.internals.v1.ServiceInvocation/CallLocal",
	}
	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
        // 这里调用的 srv 即 gRPC api 实现
		return srv.(ServiceInvocationServer).CallLocal(ctx, req.(*InternalInvokeRequest))  
	}
	return interceptor(ctx, in, info, handler)
}

func (a *api) CallLocal(ctx context.Context, in *internalv1pb.InternalInvokeRequest) (*internalv1pb.InternalInvokeResponse, error) {
	// 1. 构造请求
	req, err := invokev1.InternalInvokeRequest(in)
  if a.accessControlList != nil {
		......
	}
	// 2. 通过 appChannel 向应用发出请求
	resp, err := a.appChannel.InvokeMethod(ctx, req)
    // 3. 处理应答
	return resp.Proto(), err
}

// InternalInvokeRequest creates InvokeMethodRequest object from InternalInvokeRequest pb object.
func InternalInvokeRequest(pb *internalv1pb.InternalInvokeRequest) (*InvokeMethodRequest, error) {
	req := &InvokeMethodRequest{r: pb}
	if pb.Message == nil {
		return nil, errors.New("Message field is nil")
	}

	return req, nil
}

	if a.accessControlList != nil {
		// An access control policy has been specified for the app. Apply the policies.
		operation := req.Message().Method
		var httpVerb commonv1pb.HTTPExtension_Verb
		// Get the http verb in case the application protocol is http
		if a.appProtocol == config.HTTPProtocol && req.Metadata() != nil && len(req.Metadata()) > 0 {
			httpExt := req.Message().GetHttpExtension()
			if httpExt != nil {
				httpVerb = httpExt.GetVerb()
			}
		}
		callAllowed, errMsg := acl.ApplyAccessControlPolicies(ctx, operation, httpVerb, a.appProtocol, a.accessControlList)

		if !callAllowed {
			return nil, status.Errorf(codes.PermissionDenied, errMsg)
		}
	}

func (a *api) CallLocal(ctx context.Context, in *internalv1pb.InternalInvokeRequest) (*internalv1pb.InternalInvokeResponse, error) {
	......
	resp, err := a.appChannel.InvokeMethod(ctx, req)
  ......
}

func (a *DaprRuntime) initRuntime(opts *runtimeOpts) error {
    ......
    a.blockUntilAppIsReady()

	err = a.createAppChannel()
	a.daprHTTPAPI.SetAppChannel(a.appChannel)
	grpcAPI.SetAppChannel(a.appChannel)
    ......
}

func (a *DaprRuntime) createAppChannel() error {
    // 为了建立 app channel，必须配置有 app port
	if a.runtimeConfig.ApplicationPort > 0 {
		var channelCreatorFn func(port, maxConcurrency int, spec config.TracingSpec, sslEnabled bool, maxRequestBodySize int, readBufferSize int) (channel.AppChannel, error)

		switch a.runtimeConfig.ApplicationProtocol {
		case GRPCProtocol:
			channelCreatorFn = a.grpc.CreateLocalChannel
		case HTTPProtocol:
			channelCreatorFn = http_channel.CreateLocalChannel
		default:
      // 只支持 HTTP 和 gRPC
			return errors.Errorf("cannot create app channel for protocol %s", string(a.runtimeConfig.ApplicationProtocol))
		}

		ch, err := channelCreatorFn(a.runtimeConfig.ApplicationPort, a.runtimeConfig.MaxConcurrency, a.globalConfig.Spec.TracingSpec, a.runtimeConfig.AppSSL, a.runtimeConfig.MaxRequestBodySize, a.runtimeConfig.ReadBufferSize)
		a.appChannel = ch
	} else {
		log.Warn("app channel is not initialized. did you make sure to configure an app-port?")
	}

	return nil
}

func FromFlags() (*DaprRuntime, error) {
    ......
    appPort := flag.String("app-port", "", "The port the application is listening on")
	appProtocol := flag.String("app-protocol", string(HTTPProtocol), "Protocol for the application: grpc or http")	
	appMaxConcurrency := flag.Int("app-max-concurrency", -1, "Controls the concurrency level when forwarding requests to user code")

func (h *Channel) InvokeMethod(ctx context.Context, req *invokev1.InvokeMethodRequest) (*invokev1.InvokeMethodResponse, error) {
  ......
	switch req.APIVersion() {
	case internalv1pb.APIVersion_V1:
		rsp, err = h.invokeMethodV1(ctx, req)
  ......
	return rsp, err
}

func (h *Channel) invokeMethodV1(ctx context.Context, req *invokev1.InvokeMethodRequest) (*invokev1.InvokeMethodResponse, error) {
  // 1. 构建HTTP请求
	channelReq := h.constructRequest(ctx, req)
  // 2. 发送请求到应用
	err := h.client.DoTimeout(channelReq, resp, channel.DefaultChannelRequestTimeout)
  // 3. 处理返回的应答
	rsp := h.parseChannelResponse(req, resp, err)
	return rsp, nil
}

func (h *Channel) constructRequest(ctx context.Context, req *invokev1.InvokeMethodRequest) *fasthttp.Request {
	var channelReq = fasthttp.AcquireRequest()

	// Construct app channel URI: VERB http://localhost:3000/method?query1=value1
	uri := fmt.Sprintf("%s/%s", h.baseAddress, req.Message().GetMethod())
	channelReq.SetRequestURI(uri)
	channelReq.URI().SetQueryString(req.EncodeHTTPQueryString())
	channelReq.Header.SetMethod(req.Message().HttpExtension.Verb.String())

	// Recover headers
	invokev1.InternalMetadataToHTTPHeader(ctx, req.Metadata(), channelReq.Header.Set)

  ......
}

// CreateLocalChannel creates a new gRPC AppChannel.
func (g *Manager) CreateLocalChannel(port, maxConcurrency int, spec config.TracingSpec, sslEnabled bool, maxRequestBodySize int, readBufferSize int) (channel.AppChannel, error) {
  // IP地址写死了 127.0.0.1
	conn, err := g.GetGRPCConnection(context.TODO(), fmt.Sprintf("127.0.0.1:%v", port), "", "", true, false, sslEnabled)
  ......
	g.AppClient = conn
	ch := grpc_channel.CreateLocalChannel(port, maxConcurrency, conn, spec, maxRequestBodySize, readBufferSize)
	return ch, nil
}

func (g *Channel) InvokeMethod(ctx context.Context, req *invokev1.InvokeMethodRequest) (*invokev1.InvokeMethodResponse, error) {
  ......
	switch req.APIVersion() {
	case internalv1pb.APIVersion_V1:
		rsp, err = g.invokeMethodV1(ctx, req)
  ......
	return rsp, err
}

func (g *Channel) invokeMethodV1(ctx context.Context, req *invokev1.InvokeMethodRequest) (*invokev1.InvokeMethodResponse, error) {
  // 1. 创建 AppCallback 的 grpc client
	clientV1 := runtimev1pb.NewAppCallbackClient(g.client)
  // 2. 调用 AppCallback 的 OnInvoke() 方法
	resp, err := clientV1.OnInvoke(ctx, req.Message(), grpc.Header(&header), grpc.Trailer(&trailer))
  // 3. 处理返回的应答
	return rsp.WithMessage(resp), nil
}

// AppCallbackServer is the server API for AppCallback service.
type AppCallbackServer interface {
	// Invokes service method with InvokeRequest.
	OnInvoke(context.Context, *v1.InvokeRequest) (*v1.InvokeResponse, error)
}

service AppCallback {
  // Invokes service method with InvokeRequest.
  rpc OnInvoke (common.v1.InvokeRequest) returns (common.v1.InvokeResponse) {}
  ......
}

func (s *Server) OnInvoke(ctx context.Context, in *cpb.InvokeRequest) (*cpb.InvokeResponse, error) {
	if fn, ok := s.invokeHandlers[in.Method]; ok {
		e := &cc.InvocationEvent{}
		ct, er := fn(ctx, e)
		return &cpb.InvokeResponse{......}, nil
	}
	return nil, fmt.Errorf("method not implemented: %s", in.Method)
}

// Server is the gRPC service implementation for Dapr.
type Server struct {
	invokeHandlers  map[string]common.ServiceInvocationHandler
}
type  ServiceInvocationHandler func(ctx context.Context, in *InvocationEvent) (out *Content, err error)

func (s *Server) AddServiceInvocationHandler(method string, fn func(ctx context.Context, in *cc.InvocationEvent) (our *cc.Content, err error)) error {
	s.invokeHandlers[method] = fn
	return nil
}

func main() {
	// create a Dapr service server
	s, err := daprd.NewService(":50001")

	// add a service to service invocation handler
	if err := s.AddServiceInvocationHandler("echo", echoHandler); err != nil {
		log.Fatalf("error adding invocation handler: %v", err)
	}

	// start the server
	if err := s.Start(); err != nil {
		log.Fatalf("server error: %v", err)
	}
}

// Resolver是命名解析器的接口。
type Resolver interface {
	// Init initializes name resolver.
	Init(metadata Metadata) error
	// ResolveID resolves name to address.
	ResolveID(req ResolveRequest) (string, error)
}

// ResolveRequest 表示服务发现解析器请求。
type ResolveRequest struct {
	ID        string
	Namespace string
	Port      int
	Data      map[string]string
}

func (d *directMessaging) getRemoteApp(appID string) (remoteApp, error) {
  // 从appID中获取id和namespace
  // appID 可能是类似 "appID.namespace" 的格式
	id, namespace, err := d.requestAppIDAndNamespace(appID)
	if err != nil {
		return remoteApp{}, err
	}

  // 执行 resolver 的解析
	request := nr.ResolveRequest{ID: id, Namespace: namespace, Port: d.grpcPort}
	address, err := d.resolver.ResolveID(request)
	if err != nil {
		return remoteApp{}, err
	}

  // 返回 remoteApp 的地址
	return remoteApp{
		namespace: namespace,
		id:        id,
		address:   address,
	}, nil
}

// Invoke takes a message requests and invokes an app, either local or remote.
func (d *directMessaging) Invoke(ctx context.Context, targetAppID string, req *invokev1.InvokeMethodRequest) (*invokev1.InvokeMethodResponse, error) {
	app, err := d.getRemoteApp(targetAppID)
	if err != nil {
		return nil, err
	}

  // 如果目标应用的 id 和 namespace 都和 directMessaging 的一致，则执行 invokeLocal()
	if app.id == d.appID && app.namespace == d.namespace {
		return d.invokeLocal(ctx, req)
	}
  
  // 这是在带有重试机制的情况下调用 invokeRemote
	return d.invokeWithRetry(ctx, retry.DefaultLinearRetryCount, retry.DefaultLinearBackoffInterval, app, d.invokeRemote, req)
}

func (d *directMessaging) invokeWithRetry(
	ctx context.Context,
	numRetries int,
	backoffInterval time.Duration,
	app remoteApp,
	fn func(ctx context.Context, appID, namespace, appAddress string, req *invokev1.InvokeMethodRequest) (*invokev1.InvokeMethodResponse, error),
	req *invokev1.InvokeMethodRequest,
) (*invokev1.InvokeMethodResponse, error) {
  
}

func (d *directMessaging) invokeRemote(ctx context.Context, appID, namespace, appAddress string, req *invokev1.InvokeMethodRequest) (*invokev1.InvokeMethodResponse, error) {
  // 
	conn, teardown, err := d.connectionCreatorFn(context.TODO(), appAddress, appID, namespace, false, false, false)
	defer teardown()
	if err != nil {
		return nil, err
	}

	ctx = d.setContextSpan(ctx)

	d.addForwardedHeadersToMetadata(req)
	d.addDestinationAppIDHeaderToMetadata(appID, req)

	clientV1 := internalv1pb.NewServiceInvocationClient(conn)

	var opts []grpc.CallOption
	opts = append(opts, grpc.MaxCallRecvMsgSize(d.maxRequestBodySize*1024*1024), grpc.MaxCallSendMsgSize(d.maxRequestBodySize*1024*1024))

	resp, err := clientV1.CallLocal(ctx, req.Proto(), opts...)
	if err != nil {
		return nil, err
	}

	return invokev1.InternalInvokeResponse(resp)
}

// ResolveID 通过 mDNS 将名称解析为地址。
func (m *Resolver) ResolveID(req nameresolution.ResolveRequest) (string, error) {
	m.browseOne(ctx, req.ID, published)

	select {
	case addr := <-sub.AddrChan:
		return addr, nil
	case err := <-sub.ErrChan:
		return "", err
	case <-time.After(subscriberTimeout):
		return "", fmt.Errorf("timeout waiting for address for app id %s", req.ID)
	}
}

func (m *Resolver) browseOne(ctx context.Context, appID string, published chan struct{}) {
  err := m.browse(browseCtx, appID, onFirst)
}

func (m *Resolver) browseOne(ctx context.Context, appID string, published chan struct{}) {
  // 启动一个 goroutine 异步执行
	go func() {
		var addr string

		browseCtx, cancel := context.WithCancel(ctx)
		defer cancel()

    // 准备回调函数，收到第一个地址之后就取消 browse，所以这个函数名为 browseOne
		onFirst := func(ip string) {
			addr = ip
			cancel() // cancel to stop browsing.
		}

		m.logger.Debugf("Browsing for first mDNS address for app id %s", appID)

    // 执行 browse
		err := m.browse(browseCtx, appID, onFirst)
		// 忽略错误处理
    ......
    
		m.pubAddrToSubs(appID, addr)

		published <- struct{}{} // signal that all subscribers have been notified.
	}()
}

// browse 将对所提供的 App ID 进行无阻塞的 mdns 网络浏览
func (m *Resolver) browse(ctx context.Context, appID string, onEach func(ip string)) error {
  ......
}

  import "github.com/grandcat/zeroconf"

	resolver, err := zeroconf.NewResolver(nil)
  if err != nil {
		return fmt.Errorf("failed to initialize resolver: %w", err)
	}
  ......

	entries := make(chan *zeroconf.ServiceEntry)	
  if err = resolver.Browse(ctx, appID, "local.", entries); err != nil {
		return fmt.Errorf("failed to browse: %w", err)
	}

	// handle each service entry returned from the mDNS browse.
	go func(results <-chan *zeroconf.ServiceEntry) {
		for {
			select {
			case entry := <-results:
				if entry == nil {
					break
				}
        // 调用 handleEntry 方法来处理每个返回的 service entry
				handleEntry(entry)
			case <-ctx.Done():
        // 如果所有 service entry 都处理完成了，或者是出错（取消或者超时）
        // 此时需要推出 browse，但在退出之前需要检查一下是否有已经收到但还没有处理的结果
				for len(results) > 0 {
					handleEntry(<-results)
				}

				if errors.Is(ctx.Err(), context.Canceled) {
					m.logger.Debugf("mDNS browse for app id %s canceled.", appID)
				} else if errors.Is(ctx.Err(), context.DeadlineExceeded) {
					m.logger.Debugf("mDNS browse for app id %s timed out.", appID)
				}

				return // stop listening for results.
			}
		}
	}(entries)

	handleEntry := func(entry *zeroconf.ServiceEntry) {
		for _, text := range entry.Text {
      // 检查appID看是否是自己要查找的app
			if text != appID {
				m.logger.Debugf("mDNS response doesn't match app id %s, skipping.", appID)
				break
			}

			m.logger.Debugf("mDNS response for app id %s received.", appID)

      // 检查是否有 IPv4 或者 ipv6 地址
			hasIPv4Address := len(entry.AddrIPv4) > 0
			hasIPv6Address := len(entry.AddrIPv6) > 0

			if !hasIPv4Address && !hasIPv6Address {
				m.logger.Debugf("mDNS response for app id %s doesn't contain any IPv4 or IPv6 addresses, skipping.", appID)
				break
			}

			var addr string
			port := entry.Port
      // 目前只支持取第一个地址
			// TODO: we currently only use the first IPv4 and IPv6 address.
			// We should understand the cases in which additional addresses
			// are returned and whether we need to support them.
      // 加入到缓存中，缓存后面细看
			if hasIPv4Address {
				addr = fmt.Sprintf("%s:%d", entry.AddrIPv4[0].String(), port)
				m.addAppAddressIPv4(appID, addr)
			}
			if hasIPv6Address {
				addr = fmt.Sprintf("%s:%d", entry.AddrIPv6[0].String(), port)
				m.addAppAddressIPv6(appID, addr)
			}

      // 开始回调，就是前面说的拿到第一个地址就取消 browse
			if onEach != nil {
				onEach(addr) // invoke callback.
			}
		}
	}

// addAppAddressIPv4 adds an IPv4 address to the
// cache for the provided app id.
func (m *Resolver) addAppAddressIPv4(appID string, addr string) {
	m.ipv4Mu.Lock()
	defer m.ipv4Mu.Unlock()

	m.logger.Debugf("Adding IPv4 address %s for app id %s cache entry.", addr, appID)
	if _, ok := m.appAddressesIPv4[appID]; !ok {
		var addrList addressList
		m.appAddressesIPv4[appID] = &addrList
	}
	m.appAddressesIPv4[appID].add(addr)
}

func (m *Resolver) ResolveID(req nameresolution.ResolveRequest) (string, error) {
	// check for cached IPv4 addresses for this app id first.
	if addr := m.nextIPv4Address(req.ID); addr != nil {
		return *addr, nil
	}

	// check for cached IPv6 addresses for this app id second.
	if addr := m.nextIPv6Address(req.ID); addr != nil {
		return *addr, nil
	}
  ......
}

// nextIPv4Address returns the next IPv4 address for
// the provided app id from the cache.
func (m *Resolver) nextIPv4Address(appID string) *string {
	m.ipv4Mu.RLock()
	defer m.ipv4Mu.RUnlock()
	addrList, exists := m.appAddressesIPv4[appID]
	if exists {
		addr := addrList.next()
		if addr != nil {
			m.logger.Debugf("found mDNS IPv4 address in cache: %s", *addr)

			return addr
		}
	}

	return nil
}

// addressList represents a set of addresses along with
// data used to control and access said addresses.
type addressList struct {
	addresses []address
	counter   int
	mu        sync.RWMutex
}

// max integer value supported on this architecture.
const maxInt = int(^uint(0) >> 1)

// next 从列表中获取下一个地址，考虑到当前的循环实现。除了尽力而为的线性迭代，对选择没有任何保证。
func (a *addressList) next() *string {
  // 获取读锁
	a.mu.RLock()
	defer a.mu.RUnlock()

	if len(a.addresses) == 0 {
		return nil
	}
  // 如果 counter 达到 maxInt，就从头再来
	if a.counter == maxInt {
		a.counter = 0
	}
  // 用地址数量 对 counter 求余，去余数所对应的地址，然后counter递增
  // 相当于一个最简单常见的 轮询 算法
	index := a.counter % len(a.addresses)
	addr := a.addresses[index]
	a.counter++

	return &addr.ip
}

// ResolveID resolves name to address via mDNS.
func (m *Resolver) ResolveID(req nameresolution.ResolveRequest) (string, error) {

	sub := NewSubscriber()

	// add the sub to the pool of subs for this app id.
	m.subMu.Lock()
	appIDSubs, exists := m.subs[req.ID]
	if !exists {
		// WARN: must set appIDSubs variable for use below.
		appIDSubs = NewSubscriberPool(sub)
		m.subs[req.ID] = appIDSubs
	} else {
		appIDSubs.Add(sub)
	}
	m.subMu.Unlock()

	// only one subscriber per pool will perform the first browse for the
	// requested app id. The rest will subscribe for an address or error.
	var once *sync.Once
	var published chan struct{}
	ctx, cancel := context.WithTimeout(context.Background(), browseOneTimeout)
	defer cancel()
	appIDSubs.Once.Do(func() {
		published = make(chan struct{})
		m.browseOne(ctx, req.ID, published)

		// once will only be set for the first browser.
		once = new(sync.Once)
	})
	......
}

// ResolveID resolves name to address in Kubernetes.
func (k *resolver) ResolveID(req nameresolution.ResolveRequest) (string, error) {
	// Dapr requires this formatting for Kubernetes services
	return fmt.Sprintf("%s-dapr.%s.svc.%s:%d", req.ID, req.Namespace, k.clusterDomain, req.Port), nil
}