Golang原生rpc(rpc服务端源码解读)

创建rpc接口，需要几个条件

• 方法的类型是可输出的
• 方法的本身也是可输出的
• 方法必须有两个参数，必须是输出类型或者是内建类型
• 方法的第二个参数是指针类型
• 方法返回的类型为error

rpc服务原理分析

server端

• 服务注册
• 处理网络调用

服务注册 通过反射处理，将接口存入到map中，进行调用 注册服务两个方法

func Register (rcvr interface{}) error {}
func RegisterName (rcvr interface{} , name string) error {}
//指定注册的名称

注册方法的源代码解读 首先，无论是Register还是RegisterName底层代码都是调用register方法，进行服务注册。 server.go register方法解读

func (server *Server) register(rcvr interface{}, name string, useName bool) error {
	//创建一个service实例
	s := new(service)
	s.typ = reflect.TypeOf(rcvr)
	s.rcvr = reflect.ValueOf(rcvr)
	sname := reflect.Indirect(s.rcvr).Type().Name()
	//如果服务名为空，则使用默认的服务名
	if useName {
		sname = name
	}
	if sname == "" {
		s := "rpc.Register: no service name for type " + s.typ.String()
		log.Print(s)
		return errors.New(s)
	}
	//判断方法名是否暴漏的，如果方法名不是暴露的，则会导致调用不成功，所以返回false
	if !token.IsExported(sname) && !useName {
		s := "rpc.Register: type " + sname + " is not exported"
		log.Print(s)
		return errors.New(s)
	}
	s.name = sname

	// Install the methods
	//调用suitableMethods函数，进行返回接口，在suitableMethods中判断方法是否符合作为rpc接口的条件，如果符合，则进行添加到services中
	s.method = suitableMethods(s.typ, true)

	if len(s.method) == 0 {
		str := ""

		// To help the user, see if a pointer receiver would work.
		//如果方法绑定到结构体的地址上，使用reflect.TypeOf()是不会发现方法的，所以也要进行查找绑定到结构体地址上的方法
		method := suitableMethods(reflect.PtrTo(s.typ), false)
		if len(method) != 0 {
			str = "rpc.Register: type " + sname + " has no exported methods of suitable type (hint: pass a pointer to value of that type)"
		} else {
			str = "rpc.Register: type " + sname + " has no exported methods of suitable type"
		}
		log.Print(str)
		return errors.New(str)
	}
	//判断服务接口是否已经注册。
	if _, dup := server.serviceMap.LoadOrStore(sname, s); dup {
		return errors.New("rpc: service already defined: " + sname)
	}
	return nil
}

suitableMethod方法解读

func suitableMethods(typ reflect.Type, reportErr bool) map[string]*methodType {
	//创建一个方法的切片
	methods := make(map[string]*methodType)
	for m := 0; m < typ.NumMethod(); m++ {
		method := typ.Method(m)
		mtype := method.Type
		mname := method.Name
		// Method must be exported.
		if method.PkgPath != "" {
			continue
		}
		// Method needs three ins: receiver, *args, *reply.
		//如果传入的参数，不为三个，则会报错，这里为什么是三个？
		//golang方法体中默认传入结构体实例，所以request,*response,结构体实例一共三个参数
		if mtype.NumIn() != 3 {
			if reportErr {
				log.Printf("rpc.Register: method %q has %d input parameters; needs exactly three\n", mname, mtype.NumIn())
			}
			continue
		}
		// First arg need not be a pointer.
		argType := mtype.In(1)
		if !isExportedOrBuiltinType(argType) {
			if reportErr {
				log.Printf("rpc.Register: argument type of method %q is not exported: %q\n", mname, argType)
			}
			continue
		}
		// Second arg must be a pointer.
		//判断第二个参数是否为指针，如果不为指针，则返回false。
		replyType := mtype.In(2)
		if replyType.Kind() != reflect.Ptr {
			if reportErr {
				log.Printf("rpc.Register: reply type of method %q is not a pointer: %q\n", mname, replyType)
			}
			continue
		}
		// Reply type must be exported.
		if !isExportedOrBuiltinType(replyType) {
			if reportErr {
				log.Printf("rpc.Register: reply type of method %q is not exported: %q\n", mname, replyType)
			}
			continue
		}
		// Method needs one out.
		//返回结果是否为一个值，且为error
		if mtype.NumOut() != 1 {
			if reportErr {
				log.Printf("rpc.Register: method %q has %d output parameters; needs exactly one\n", mname, mtype.NumOut())
			}
			continue
		}
		// The return type of the method must be error.
		if returnType := mtype.Out(0); returnType != typeOfError {
			if reportErr {
				log.Printf("rpc.Register: return type of method %q is %q, must be error\n", mname, returnType)
			}
			continue
		}
		//将接口加入service
		methods[mname] = &methodType{method: method, ArgType: argType, ReplyType: replyType}
	}
	return methods
}

接收到请求后会不断的解析请求 解析请求的两个方法 readRequestHeader

func (server *Server) readRequestHeader(codec ServerCodec) (svc *service, mtype *methodType, req *Request, keepReading bool, err error) {
	// Grab the request header.
	//接收到请求，对请求进行编码
	req = server.getRequest()
	err = codec.ReadRequestHeader(req)
	if err != nil {
		req = nil
		if err == io.EOF || err == io.ErrUnexpectedEOF {
			return
		}
		err = errors.New("rpc: server cannot decode request: " + err.Error())
		return
	}

	// We read the header successfully. If we see an error now,
	// we can still recover and move on to the next request.
	keepReading = true
//编码后的请求，进行间隔，所以只要进行将.的左右两边的数据进行分割，就能解码
	dot := strings.LastIndex(req.ServiceMethod, ".")
	if dot < 0 {
		err = errors.New("rpc: service/method request ill-formed: " + req.ServiceMethod)
		return
	}
	serviceName := req.ServiceMethod[:dot]
	methodName := req.ServiceMethod[dot+1:]

	// Look up the request.
	svci, ok := server.serviceMap.Load(serviceName)
	if !ok {
		err = errors.New("rpc: can't find service " + req.ServiceMethod)
		return
	}
	svc = svci.(*service)
	//获取到注册服务时，注册的接口
	mtype = svc.method[methodName]
	if mtype == nil {
		err = errors.New("rpc: can't find method " + req.ServiceMethod)
	}
	return
}

readRequest方法

func (server *Server) readRequest(codec ServerCodec) (service *service, mtype *methodType, req *Request, argv, replyv reflect.Value, keepReading bool, err error) {
	service, mtype, req, keepReading, err = server.readRequestHeader(codec)
//调用上面的readRequestHeader方法，进行解码，并返返回接口数据
	if err != nil {
		if !keepReading {
			return
		}
		// discard body
		codec.ReadRequestBody(nil)
		return
	}

	// Decode the argument value.
	argIsValue := false // if true, need to indirect before calling.
	//判断传擦是否为指针，如果为指针，需要使用Elem()方法，进行指向结构体
	if mtype.ArgType.Kind() == reflect.Ptr {
		argv = reflect.New(mtype.ArgType.Elem())
	} else {
		argv = reflect.New(mtype.ArgType)
		argIsValue = true
	}
	// argv guaranteed to be a pointer now.
	if err = codec.ReadRequestBody(argv.Interface()); err != nil {
		return
	}
	if argIsValue {
		argv = argv.Elem()
	}

	replyv = reflect.New(mtype.ReplyType.Elem())

	switch mtype.ReplyType.Elem().Kind() {
	case reflect.Map:
		replyv.Elem().Set(reflect.MakeMap(mtype.ReplyType.Elem()))
	case reflect.Slice:
		replyv.Elem().Set(reflect.MakeSlice(mtype.ReplyType.Elem(), 0, 0))
	}
	return
}

call方法

func (s *service) call(server *Server, sending *sync.Mutex, wg *sync.WaitGroup, mtype *methodType, req *Request, argv, replyv reflect.Value, codec ServerCodec) {
	if wg != nil {
		defer wg.Done()
	}
	mtype.Lock()
	mtype.numCalls++
	mtype.Unlock()
	function := mtype.method.Func
	// Invoke the method, providing a new value for the reply.
	//调用call方法，并将参数转化为valueof型参数，
	returnValues := function.Call([]reflect.Value{s.rcvr, argv, replyv})
	// The return value for the method is an error.
	//将返回的error进行读取，转化为interface{}型
	errInter := returnValues[0].Interface()
	errmsg := ""
	if errInter != nil {
	//将error进行断言
		errmsg = errInter.(error).Error()
	}
	server.sendResponse(sending, req, replyv.Interface(), codec, errmsg)
	server.freeRequest(req)
}

注册的大概流程

• 根据反射，进行接口的获取
• 使用方法判断接口是否符合作为rpc接口的规范（有两个参数，第二个参数为指针，返回一个参数error）
• 如果不符合规范，将返回error，符合规范，将存入map，进行提供调用

接收请求的大概流程

• 首先，不断的接收数据流，并进行解码，解码之后为data.data，所以我们需要使用 . 作为分隔符，进行数据的截切和读取
• 将读取的数据在注册的map中进行查找，如果查找到，返回相关的service和其他数据
• 进行调用

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