RPC explained by writing simple RPC framework in 300 lines of pure Golang.

type User struct {
	Name string
	Age int
}

var userDB = map[int]User{
	1: User{"Ankur", 85},
	9: User{"Anand", 25},
	8: User{"Ankur Anand", 27},
}


func QueryUser(id int) (User, error) {
	if u, ok := userDB[id]; ok {
		return u, nil
	}

	return User{}, fmt.Errorf("id %d not in user db", id)
}


func main() {
	u , err := QueryUser(8)
	if err != nil {
		fmt.Println(err)
		return
	}

	fmt.Printf("name: %s, age: %d \n", u.Name, u.Age)
}

// RPCdata transmission format
type RPCdata struct {
	Name string        // name of the function
	Args []interface{} // request's or response's body expect error.
	Err  string        // Error any executing remote server
}

// be sent over the network.
func Encode(data RPCdata) ([]byte, error) {
	var buf bytes.Buffer
	encoder := gob.NewEncoder(&buf)
	if err := encoder.Encode(data); err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}

// Decode the binary data into the Go struct
func Decode(b []byte) (RPCdata, error) {
	buf := bytes.NewBuffer(b)
	decoder := gob.NewDecoder(buf)
	var data RPCdata
	if err := decoder.Decode(&data); err != nil {
		return Data{}, err
	}
	return data, nil
}

// Transport will use TLV protocol
type Transport struct {
	conn net.Conn // Conn is a generic stream-oriented network connection.
}

// NewTransport creates a Transport
func NewTransport(conn net.Conn) *Transport {
	return &Transport{conn}
}

// Send TLV data over the network
func (t *Transport) Send(data []byte) error {
	// we will need 4 more byte then the len of data
	// as TLV header is 4bytes and in this header
	// we will encode how much byte of data
	// we are sending for this request.
	buf := make([]byte, 4+len(data))
	binary.BigEndian.PutUint32(buf[:4], uint32(len(data)))
	copy(buf[4:], data)
	_, err := t.conn.Write(buf)
	if err != nil {
		return err
	}
	return nil
}

// Read TLV sent over the wire
func (t *Transport) Read() ([]byte, error) {
	header := make([]byte, 4)
	_, err := io.ReadFull(t.conn, header)
	if err != nil {
		return nil, err
	}
	dataLen := binary.BigEndian.Uint32(header)
	data := make([]byte, dataLen)
	_, err = io.ReadFull(t.conn, data)
	if err != nil {
		return nil, err
	}
	return data, nil
}

// RPCServer ...
type RPCServer struct {
	addr string
	funcs map[string] reflect.Value
}

// Register the name of the function and its entries
func (s *RPCServer) Register(fnName string, fFunc interface{}) {
	if _,ok := s.funcs[fnName]; ok {
		return
	}

	s.funcs[fnName] = reflect.ValueOf(fFunc)
}

// Execute the given function if present
func (s *RPCServer) Execute(req RPCdata) RPCdata {
	// get method by name
	f, ok := s.funcs[req.Name]
	if !ok {
		// since method is not present
		e := fmt.Sprintf("func %s not Registered", req.Name)
		log.Println(e)
		return RPCdata{Name: req.Name, Args: nil, Err: e}
	}

	log.Printf("func %s is called\n", req.Name)
	// unpackage request arguments
	inArgs := make([]reflect.Value, len(req.Args))
	for i := range req.Args {
		inArgs[i] = reflect.ValueOf(req.Args[i])
	}

	// invoke requested method
	out := f.Call(inArgs)
	// now since we have followed the function signature style where last argument will be an error
	// so we will pack the response arguments expect error.
	resArgs := make([]interface{}, len(out) - 1)
	for i := 0; i < len(out) - 1; i ++ {
		// Interface returns the constant value stored in v as an interface{}.
		resArgs[i] = out[i].Interface()
	}

	// pack error argument
	var er string
	if e, ok := out[len(out) - 1].Interface().(error); ok {
		// convert the error into error string value
		er = e.Error()
	}
	return RPCdata{Name: req.Name, Args: resArgs, Err: er}
}

func (c *Client) callRPC(rpcName string, fPtr interface{}) {
	container := reflect.ValueOf(fPtr).Elem()
	f := func(req []reflect.Value) []reflect.Value {
		cReqTransport := NewTransport(c.conn)
		errorHandler := func(err error) []reflect.Value {
			outArgs := make([]reflect.Value, container.Type().NumOut())
			for i := 0; i < len(outArgs)-1; i++ {
				outArgs[i] = reflect.Zero(container.Type().Out(i))
			}
			outArgs[len(outArgs)-1] = reflect.ValueOf(&err).Elem()
			return outArgs
		}

		// Process input parameters
		inArgs := make([]interface{}, 0, len(req))
		for _, arg := range req {
			inArgs = append(inArgs, arg.Interface())
		}

		// ReqRPC
		reqRPC := RPCdata{Name: rpcName, Args: inArgs}
		b, err := Encode(reqRPC)
		if err != nil {
			panic(err)
		}
		err = cReqTransport.Send(b)
		if err != nil {
			return errorHandler(err)
		}
		// receive response from server
		rsp, err := cReqTransport.Read()
		if err != nil { // local network error or decode error
			return errorHandler(err)
		}
		rspDecode, _ := Decode(rsp)
		if rspDecode.Err != "" { // remote server error
			return errorHandler(errors.New(rspDecode.Err))
		}

		if len(rspDecode.Args) == 0 {
			rspDecode.Args = make([]interface{}, container.Type().NumOut())
		}
		// unpackage response arguments
		numOut := container.Type().NumOut()
		outArgs := make([]reflect.Value, numOut)
		for i := 0; i < numOut; i++ {
			if i != numOut-1 { // unpackage arguments (except error)
				if rspDecode.Args[i] == nil { // if argument is nil (gob will ignore "Zero" in transmission), set "Zero" value
					outArgs[i] = reflect.Zero(container.Type().Out(i))
				} else {
					outArgs[i] = reflect.ValueOf(rspDecode.Args[i])
				}
			} else { // unpackage error argument
				outArgs[i] = reflect.Zero(container.Type().Out(i))
			}
		}

		return outArgs
	}
	container.Set(reflect.MakeFunc(container.Type(), f))
}

package main

import (
	"encoding/gob"
	"fmt"
	"net"
)

type User struct {
	Name string
	Age  int
}

var userDB = map[int]User{
	1: User{"Ankur", 85},
	9: User{"Anand", 25},
	8: User{"Ankur Anand", 27},
}

func QueryUser(id int) (User, error) {
	if u, ok := userDB[id]; ok {
		return u, nil
	}

	return User{}, fmt.Errorf("id %d not in user db", id)
}

func main() {
	// new Type needs to be registered
	gob.Register(User{})
	addr := "localhost:3212"
	srv := NewServer(addr)

	// start server
	srv.Register("QueryUser", QueryUser)
	go srv.Run()

	// wait for server to start.
	time.Sleep(1 * time.Second)

	// start client
	conn, err := net.Dial("tcp", addr)
	if err != nil {
		panic(err)
	}
	cli := NewClient(conn)

	var Query func(int) (User, error)
	cli.callRPC("QueryUser", &Query)

	u, err := Query(1)
	if err != nil {
		panic(err)
	}
	fmt.Println(u)

	u2, err := Query(8)
	if err != nil {
		panic(err)
	}
	fmt.Println(u2)
}

2019/07/23 20:26:18 func QueryUser is called
{Ankur 85}
2019/07/23 20:26:18 func QueryUser is called
{Ankur Anand 27}