1 // Copyright 2009 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
6 Package rpc provides access to the exported methods of an object across a
7 network or other I/O connection. A server registers an object, making it visible
8 as a service with the name of the type of the object. After registration, exported
9 methods of the object will be accessible remotely. A server may register multiple
10 objects (services) of different types but it is an error to register multiple
11 objects of the same type.
13 Only methods that satisfy these criteria will be made available for remote access;
14 other methods will be ignored:
16 - the method is exported.
17 - the method has two arguments, both exported (or builtin) types.
18 - the method's second argument is a pointer.
19 - the method has return type error.
21 In effect, the method must look schematically like
23 func (t *T) MethodName(argType T1, replyType *T2) error
25 where T, T1 and T2 can be marshaled by encoding/gob.
26 These requirements apply even if a different codec is used.
27 (In the future, these requirements may soften for custom codecs.)
29 The method's first argument represents the arguments provided by the caller; the
30 second argument represents the result parameters to be returned to the caller.
31 The method's return value, if non-nil, is passed back as a string that the client
32 sees as if created by errors.New. If an error is returned, the reply parameter
33 will not be sent back to the client.
35 The server may handle requests on a single connection by calling ServeConn. More
36 typically it will create a network listener and call Accept or, for an HTTP
37 listener, HandleHTTP and http.Serve.
39 A client wishing to use the service establishes a connection and then invokes
40 NewClient on the connection. The convenience function Dial (DialHTTP) performs
41 both steps for a raw network connection (an HTTP connection). The resulting
42 Client object has two methods, Call and Go, that specify the service and method to
43 call, a pointer containing the arguments, and a pointer to receive the result
46 The Call method waits for the remote call to complete while the Go method
47 launches the call asynchronously and signals completion using the Call
48 structure's Done channel.
50 Unless an explicit codec is set up, package encoding/gob is used to
53 Here is a simple example. A server wishes to export an object of type Arith:
61 type Quotient struct {
67 func (t *Arith) Multiply(args *Args, reply *int) error {
68 *reply = args.A * args.B
72 func (t *Arith) Divide(args *Args, quo *Quotient) error {
74 return errors.New("divide by zero")
76 quo.Quo = args.A / args.B
77 quo.Rem = args.A % args.B
81 The server calls (for HTTP service):
86 l, e := net.Listen("tcp", ":1234")
88 log.Fatal("listen error:", e)
92 At this point, clients can see a service "Arith" with methods "Arith.Multiply" and
93 "Arith.Divide". To invoke one, a client first dials the server:
95 client, err := rpc.DialHTTP("tcp", serverAddress + ":1234")
97 log.Fatal("dialing:", err)
100 Then it can make a remote call:
103 args := &server.Args{7,8}
105 err = client.Call("Arith.Multiply", args, &reply)
107 log.Fatal("arith error:", err)
109 fmt.Printf("Arith: %d*%d=%d", args.A, args.B, reply)
114 quotient := new(Quotient)
115 divCall := client.Go("Arith.Divide", args, quotient, nil)
116 replyCall := <-divCall.Done // will be equal to divCall
117 // check errors, print, etc.
119 A server implementation will often provide a simple, type-safe wrapper for the
140 // Defaults used by HandleHTTP
141 DefaultRPCPath = "/_goRPC_"
142 DefaultDebugPath = "/debug/rpc"
145 // Precompute the reflect type for error. Can't use error directly
146 // because Typeof takes an empty interface value. This is annoying.
147 var typeOfError = reflect.TypeOf((*error)(nil)).Elem()
149 type methodType struct {
150 sync.Mutex // protects counters
151 method reflect.Method
153 ReplyType reflect.Type
157 type service struct {
158 name string // name of service
159 rcvr reflect.Value // receiver of methods for the service
160 typ reflect.Type // type of the receiver
161 method map[string]*methodType // registered methods
164 // Request is a header written before every RPC call. It is used internally
165 // but documented here as an aid to debugging, such as when analyzing
167 type Request struct {
168 ServiceMethod string // format: "Service.Method"
169 Seq uint64 // sequence number chosen by client
170 next *Request // for free list in Server
173 // Response is a header written before every RPC return. It is used internally
174 // but documented here as an aid to debugging, such as when analyzing
176 type Response struct {
177 ServiceMethod string // echoes that of the Request
178 Seq uint64 // echoes that of the request
179 Error string // error, if any.
180 next *Response // for free list in Server
183 // Server represents an RPC Server.
185 mu sync.RWMutex // protects the serviceMap
186 serviceMap map[string]*service
187 reqLock sync.Mutex // protects freeReq
189 respLock sync.Mutex // protects freeResp
193 // NewServer returns a new Server.
194 func NewServer() *Server {
195 return &Server{serviceMap: make(map[string]*service)}
198 // DefaultServer is the default instance of *Server.
199 var DefaultServer = NewServer()
201 // Is this an exported - upper case - name?
202 func isExported(name string) bool {
203 rune, _ := utf8.DecodeRuneInString(name)
204 return unicode.IsUpper(rune)
207 // Is this type exported or a builtin?
208 func isExportedOrBuiltinType(t reflect.Type) bool {
209 for t.Kind() == reflect.Ptr {
212 // PkgPath will be non-empty even for an exported type,
213 // so we need to check the type name as well.
214 return isExported(t.Name()) || t.PkgPath() == ""
217 // Register publishes in the server the set of methods of the
218 // receiver value that satisfy the following conditions:
220 // - two arguments, both pointers to exported structs
221 // - one return value, of type error
222 // It returns an error if the receiver is not an exported type or has
223 // no methods or unsuitable methods. It also logs the error using package log.
224 // The client accesses each method using a string of the form "Type.Method",
225 // where Type is the receiver's concrete type.
226 func (server *Server) Register(rcvr interface{}) error {
227 return server.register(rcvr, "", false)
230 // RegisterName is like Register but uses the provided name for the type
231 // instead of the receiver's concrete type.
232 func (server *Server) RegisterName(name string, rcvr interface{}) error {
233 return server.register(rcvr, name, true)
236 func (server *Server) register(rcvr interface{}, name string, useName bool) error {
238 defer server.mu.Unlock()
239 if server.serviceMap == nil {
240 server.serviceMap = make(map[string]*service)
243 s.typ = reflect.TypeOf(rcvr)
244 s.rcvr = reflect.ValueOf(rcvr)
245 sname := reflect.Indirect(s.rcvr).Type().Name()
250 log.Fatal("rpc: no service name for type", s.typ.String())
252 if !isExported(sname) && !useName {
253 s := "rpc Register: type " + sname + " is not exported"
257 if _, present := server.serviceMap[sname]; present {
258 return errors.New("rpc: service already defined: " + sname)
261 s.method = make(map[string]*methodType)
263 // Install the methods
264 s.method = suitableMethods(s.typ, true)
266 if len(s.method) == 0 {
268 // To help the user, see if a pointer receiver would work.
269 method := suitableMethods(reflect.PtrTo(s.typ), false)
270 if len(method) != 0 {
271 str = "rpc.Register: type " + sname + " has no exported methods of suitable type (hint: pass a pointer to value of that type)"
273 str = "rpc.Register: type " + sname + " has no exported methods of suitable type"
276 return errors.New(str)
278 server.serviceMap[s.name] = s
282 // suitableMethods returns suitable Rpc methods of typ, it will report
283 // error using log if reportErr is true.
284 func suitableMethods(typ reflect.Type, reportErr bool) map[string]*methodType {
285 methods := make(map[string]*methodType)
286 for m := 0; m < typ.NumMethod(); m++ {
287 method := typ.Method(m)
290 // Method must be exported.
291 if method.PkgPath != "" {
294 // Method needs three ins: receiver, *args, *reply.
295 if mtype.NumIn() != 3 {
297 log.Println("method", mname, "has wrong number of ins:", mtype.NumIn())
301 // First arg need not be a pointer.
302 argType := mtype.In(1)
303 if !isExportedOrBuiltinType(argType) {
305 log.Println(mname, "argument type not exported:", argType)
309 // Second arg must be a pointer.
310 replyType := mtype.In(2)
311 if replyType.Kind() != reflect.Ptr {
313 log.Println("method", mname, "reply type not a pointer:", replyType)
317 // Reply type must be exported.
318 if !isExportedOrBuiltinType(replyType) {
320 log.Println("method", mname, "reply type not exported:", replyType)
324 // Method needs one out.
325 if mtype.NumOut() != 1 {
327 log.Println("method", mname, "has wrong number of outs:", mtype.NumOut())
331 // The return type of the method must be error.
332 if returnType := mtype.Out(0); returnType != typeOfError {
334 log.Println("method", mname, "returns", returnType.String(), "not error")
338 methods[mname] = &methodType{method: method, ArgType: argType, ReplyType: replyType}
343 // A value sent as a placeholder for the server's response value when the server
344 // receives an invalid request. It is never decoded by the client since the Response
345 // contains an error when it is used.
346 var invalidRequest = struct{}{}
348 func (server *Server) sendResponse(sending *sync.Mutex, req *Request, reply interface{}, codec ServerCodec, errmsg string) {
349 resp := server.getResponse()
350 // Encode the response header
351 resp.ServiceMethod = req.ServiceMethod
354 reply = invalidRequest
358 err := codec.WriteResponse(resp, reply)
360 log.Println("rpc: writing response:", err)
363 server.freeResponse(resp)
366 func (m *methodType) NumCalls() (n uint) {
373 func (s *service) call(server *Server, sending *sync.Mutex, mtype *methodType, req *Request, argv, replyv reflect.Value, codec ServerCodec) {
377 function := mtype.method.Func
378 // Invoke the method, providing a new value for the reply.
379 returnValues := function.Call([]reflect.Value{s.rcvr, argv, replyv})
380 // The return value for the method is an error.
381 errInter := returnValues[0].Interface()
384 errmsg = errInter.(error).Error()
386 server.sendResponse(sending, req, replyv.Interface(), codec, errmsg)
387 server.freeRequest(req)
390 type gobServerCodec struct {
391 rwc io.ReadWriteCloser
397 func (c *gobServerCodec) ReadRequestHeader(r *Request) error {
398 return c.dec.Decode(r)
401 func (c *gobServerCodec) ReadRequestBody(body interface{}) error {
402 return c.dec.Decode(body)
405 func (c *gobServerCodec) WriteResponse(r *Response, body interface{}) (err error) {
406 if err = c.enc.Encode(r); err != nil {
409 if err = c.enc.Encode(body); err != nil {
412 return c.encBuf.Flush()
415 func (c *gobServerCodec) Close() error {
419 // ServeConn runs the server on a single connection.
420 // ServeConn blocks, serving the connection until the client hangs up.
421 // The caller typically invokes ServeConn in a go statement.
422 // ServeConn uses the gob wire format (see package gob) on the
423 // connection. To use an alternate codec, use ServeCodec.
424 func (server *Server) ServeConn(conn io.ReadWriteCloser) {
425 buf := bufio.NewWriter(conn)
426 srv := &gobServerCodec{conn, gob.NewDecoder(conn), gob.NewEncoder(buf), buf}
427 server.ServeCodec(srv)
430 // ServeCodec is like ServeConn but uses the specified codec to
431 // decode requests and encode responses.
432 func (server *Server) ServeCodec(codec ServerCodec) {
433 sending := new(sync.Mutex)
435 service, mtype, req, argv, replyv, keepReading, err := server.readRequest(codec)
438 log.Println("rpc:", err)
443 // send a response if we actually managed to read a header.
445 server.sendResponse(sending, req, invalidRequest, codec, err.Error())
446 server.freeRequest(req)
450 go service.call(server, sending, mtype, req, argv, replyv, codec)
455 // ServeRequest is like ServeCodec but synchronously serves a single request.
456 // It does not close the codec upon completion.
457 func (server *Server) ServeRequest(codec ServerCodec) error {
458 sending := new(sync.Mutex)
459 service, mtype, req, argv, replyv, keepReading, err := server.readRequest(codec)
464 // send a response if we actually managed to read a header.
466 server.sendResponse(sending, req, invalidRequest, codec, err.Error())
467 server.freeRequest(req)
471 service.call(server, sending, mtype, req, argv, replyv, codec)
475 func (server *Server) getRequest() *Request {
476 server.reqLock.Lock()
477 req := server.freeReq
481 server.freeReq = req.next
484 server.reqLock.Unlock()
488 func (server *Server) freeRequest(req *Request) {
489 server.reqLock.Lock()
490 req.next = server.freeReq
492 server.reqLock.Unlock()
495 func (server *Server) getResponse() *Response {
496 server.respLock.Lock()
497 resp := server.freeResp
501 server.freeResp = resp.next
504 server.respLock.Unlock()
508 func (server *Server) freeResponse(resp *Response) {
509 server.respLock.Lock()
510 resp.next = server.freeResp
511 server.freeResp = resp
512 server.respLock.Unlock()
515 func (server *Server) readRequest(codec ServerCodec) (service *service, mtype *methodType, req *Request, argv, replyv reflect.Value, keepReading bool, err error) {
516 service, mtype, req, keepReading, err = server.readRequestHeader(codec)
522 codec.ReadRequestBody(nil)
526 // Decode the argument value.
527 argIsValue := false // if true, need to indirect before calling.
528 if mtype.ArgType.Kind() == reflect.Ptr {
529 argv = reflect.New(mtype.ArgType.Elem())
531 argv = reflect.New(mtype.ArgType)
534 // argv guaranteed to be a pointer now.
535 if err = codec.ReadRequestBody(argv.Interface()); err != nil {
542 replyv = reflect.New(mtype.ReplyType.Elem())
546 func (server *Server) readRequestHeader(codec ServerCodec) (service *service, mtype *methodType, req *Request, keepReading bool, err error) {
547 // Grab the request header.
548 req = server.getRequest()
549 err = codec.ReadRequestHeader(req)
552 if err == io.EOF || err == io.ErrUnexpectedEOF {
555 err = errors.New("rpc: server cannot decode request: " + err.Error())
559 // We read the header successfully. If we see an error now,
560 // we can still recover and move on to the next request.
563 serviceMethod := strings.Split(req.ServiceMethod, ".")
564 if len(serviceMethod) != 2 {
565 err = errors.New("rpc: service/method request ill-formed: " + req.ServiceMethod)
568 // Look up the request.
570 service = server.serviceMap[serviceMethod[0]]
573 err = errors.New("rpc: can't find service " + req.ServiceMethod)
576 mtype = service.method[serviceMethod[1]]
578 err = errors.New("rpc: can't find method " + req.ServiceMethod)
583 // Accept accepts connections on the listener and serves requests
584 // for each incoming connection. Accept blocks; the caller typically
585 // invokes it in a go statement.
586 func (server *Server) Accept(lis net.Listener) {
588 conn, err := lis.Accept()
590 log.Fatal("rpc.Serve: accept:", err.Error()) // TODO(r): exit?
592 go server.ServeConn(conn)
596 // Register publishes the receiver's methods in the DefaultServer.
597 func Register(rcvr interface{}) error { return DefaultServer.Register(rcvr) }
599 // RegisterName is like Register but uses the provided name for the type
600 // instead of the receiver's concrete type.
601 func RegisterName(name string, rcvr interface{}) error {
602 return DefaultServer.RegisterName(name, rcvr)
605 // A ServerCodec implements reading of RPC requests and writing of
606 // RPC responses for the server side of an RPC session.
607 // The server calls ReadRequestHeader and ReadRequestBody in pairs
608 // to read requests from the connection, and it calls WriteResponse to
609 // write a response back. The server calls Close when finished with the
610 // connection. ReadRequestBody may be called with a nil
611 // argument to force the body of the request to be read and discarded.
612 type ServerCodec interface {
613 ReadRequestHeader(*Request) error
614 ReadRequestBody(interface{}) error
615 WriteResponse(*Response, interface{}) error
620 // ServeConn runs the DefaultServer on a single connection.
621 // ServeConn blocks, serving the connection until the client hangs up.
622 // The caller typically invokes ServeConn in a go statement.
623 // ServeConn uses the gob wire format (see package gob) on the
624 // connection. To use an alternate codec, use ServeCodec.
625 func ServeConn(conn io.ReadWriteCloser) {
626 DefaultServer.ServeConn(conn)
629 // ServeCodec is like ServeConn but uses the specified codec to
630 // decode requests and encode responses.
631 func ServeCodec(codec ServerCodec) {
632 DefaultServer.ServeCodec(codec)
635 // ServeRequest is like ServeCodec but synchronously serves a single request.
636 // It does not close the codec upon completion.
637 func ServeRequest(codec ServerCodec) error {
638 return DefaultServer.ServeRequest(codec)
641 // Accept accepts connections on the listener and serves requests
642 // to DefaultServer for each incoming connection.
643 // Accept blocks; the caller typically invokes it in a go statement.
644 func Accept(lis net.Listener) { DefaultServer.Accept(lis) }
646 // Can connect to RPC service using HTTP CONNECT to rpcPath.
647 var connected = "200 Connected to Go RPC"
649 // ServeHTTP implements an http.Handler that answers RPC requests.
650 func (server *Server) ServeHTTP(w http.ResponseWriter, req *http.Request) {
651 if req.Method != "CONNECT" {
652 w.Header().Set("Content-Type", "text/plain; charset=utf-8")
653 w.WriteHeader(http.StatusMethodNotAllowed)
654 io.WriteString(w, "405 must CONNECT\n")
657 conn, _, err := w.(http.Hijacker).Hijack()
659 log.Print("rpc hijacking ", req.RemoteAddr, ": ", err.Error())
662 io.WriteString(conn, "HTTP/1.0 "+connected+"\n\n")
663 server.ServeConn(conn)
666 // HandleHTTP registers an HTTP handler for RPC messages on rpcPath,
667 // and a debugging handler on debugPath.
668 // It is still necessary to invoke http.Serve(), typically in a go statement.
669 func (server *Server) HandleHTTP(rpcPath, debugPath string) {
670 http.Handle(rpcPath, server)
671 http.Handle(debugPath, debugHTTP{server})
674 // HandleHTTP registers an HTTP handler for RPC messages to DefaultServer
675 // on DefaultRPCPath and a debugging handler on DefaultDebugPath.
676 // It is still necessary to invoke http.Serve(), typically in a go statement.
678 DefaultServer.HandleHTTP(DefaultRPCPath, DefaultDebugPath)