// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

/* Package rpc provides access to the exported methods of an object across a network or other I/O connection. A server registers an object, making it visible as a service with the name of the type of the object. After registration, exported methods of the object will be accessible remotely. A server may register multiple objects (services) of different types but it is an error to register multiple objects of the same type. Only methods that satisfy these criteria will be made available for remote access; other methods will be ignored: - the method's type is exported. - the method is exported. - the method has two arguments, both exported (or builtin) types. - the method's second argument is a pointer. - the method has return type error. In effect, the method must look schematically like func (t *T) MethodName(argType T1, replyType *T2) error where T1 and T2 can be marshaled by encoding/gob. These requirements apply even if a different codec is used. (In the future, these requirements may soften for custom codecs.) The method's first argument represents the arguments provided by the caller; the second argument represents the result parameters to be returned to the caller. The method's return value, if non-nil, is passed back as a string that the client sees as if created by errors.New. If an error is returned, the reply parameter will not be sent back to the client. The server may handle requests on a single connection by calling ServeConn. More typically it will create a network listener and call Accept or, for an HTTP listener, HandleHTTP and http.Serve. A client wishing to use the service establishes a connection and then invokes NewClient on the connection. The convenience function Dial (DialHTTP) performs both steps for a raw network connection (an HTTP connection). The resulting Client object has two methods, Call and Go, that specify the service and method to call, a pointer containing the arguments, and a pointer to receive the result parameters. The Call method waits for the remote call to complete while the Go method launches the call asynchronously and signals completion using the Call structure's Done channel. Unless an explicit codec is set up, package encoding/gob is used to transport the data. Here is a simple example. A server wishes to export an object of type Arith: package server import "errors" type Args struct { A, B int } type Quotient struct { Quo, Rem int } type Arith int func (t *Arith) Multiply(args *Args, reply *int) error { *reply = args.A * args.B return nil } func (t *Arith) Divide(args *Args, quo *Quotient) error { if args.B == 0 { return errors.New("divide by zero") } quo.Quo = args.A / args.B quo.Rem = args.A % args.B return nil } The server calls (for HTTP service): arith := new(Arith) rpc.Register(arith) rpc.HandleHTTP() l, e := net.Listen("tcp", ":1234") if e != nil { log.Fatal("listen error:", e) } go http.Serve(l, nil) At this point, clients can see a service "Arith" with methods "Arith.Multiply" and "Arith.Divide". To invoke one, a client first dials the server: client, err := rpc.DialHTTP("tcp", serverAddress + ":1234") if err != nil { log.Fatal("dialing:", err) } Then it can make a remote call: // Synchronous call args := &server.Args{7,8} var reply int err = client.Call("Arith.Multiply", args, &reply) if err != nil { log.Fatal("arith error:", err) } fmt.Printf("Arith: %d*%d=%d", args.A, args.B, reply) or // Asynchronous call quotient := new(Quotient) divCall := client.Go("Arith.Divide", args, quotient, nil) replyCall := <-divCall.Done // will be equal to divCall // check errors, print, etc. A server implementation will often provide a simple, type-safe wrapper for the client. The net/rpc package is frozen and is not accepting new features. */
package rpc import ( ) const ( // Defaults used by HandleHTTP DefaultRPCPath = "/_goRPC_" DefaultDebugPath = "/debug/rpc" ) // Precompute the reflect type for error. Can't use error directly // because Typeof takes an empty interface value. This is annoying. var typeOfError = reflect.TypeOf((*error)(nil)).Elem() type methodType struct { sync.Mutex // protects counters method reflect.Method ArgType reflect.Type ReplyType reflect.Type numCalls uint } type service struct { name string // name of service rcvr reflect.Value // receiver of methods for the service typ reflect.Type // type of the receiver method map[string]*methodType // registered methods } // Request is a header written before every RPC call. It is used internally // but documented here as an aid to debugging, such as when analyzing // network traffic. type Request struct { ServiceMethod string // format: "Service.Method" Seq uint64 // sequence number chosen by client next *Request // for free list in Server } // Response is a header written before every RPC return. It is used internally // but documented here as an aid to debugging, such as when analyzing // network traffic. type Response struct { ServiceMethod string // echoes that of the Request Seq uint64 // echoes that of the request Error string // error, if any. next *Response // for free list in Server } // Server represents an RPC Server. type Server struct { serviceMap sync.Map // map[string]*service reqLock sync.Mutex // protects freeReq freeReq *Request respLock sync.Mutex // protects freeResp freeResp *Response } // NewServer returns a new Server. func () *Server { return &Server{} } // DefaultServer is the default instance of *Server. var DefaultServer = NewServer() // Is this type exported or a builtin? func isExportedOrBuiltinType( reflect.Type) bool { for .Kind() == reflect.Ptr { = .Elem() } // PkgPath will be non-empty even for an exported type, // so we need to check the type name as well. return token.IsExported(.Name()) || .PkgPath() == "" } // Register publishes in the server the set of methods of the // receiver value that satisfy the following conditions: // - exported method of exported type // - two arguments, both of exported type // - the second argument is a pointer // - one return value, of type error // It returns an error if the receiver is not an exported type or has // no suitable methods. It also logs the error using package log. // The client accesses each method using a string of the form "Type.Method", // where Type is the receiver's concrete type. func ( *Server) ( interface{}) error { return .register(, "", false) } // RegisterName is like Register but uses the provided name for the type // instead of the receiver's concrete type. func ( *Server) ( string, interface{}) error { return .register(, , true) } func ( *Server) ( interface{}, string, bool) error { := new(service) .typ = reflect.TypeOf() .rcvr = reflect.ValueOf() := reflect.Indirect(.rcvr).Type().Name() if { = } if == "" { := "rpc.Register: no service name for type " + .typ.String() log.Print() return errors.New() } if !token.IsExported() && ! { := "rpc.Register: type " + + " is not exported" log.Print() return errors.New() } .name = // Install the methods .method = suitableMethods(.typ, true) if len(.method) == 0 { := "" // To help the user, see if a pointer receiver would work. := suitableMethods(reflect.PtrTo(.typ), false) if len() != 0 { = "rpc.Register: type " + + " has no exported methods of suitable type (hint: pass a pointer to value of that type)" } else { = "rpc.Register: type " + + " has no exported methods of suitable type" } log.Print() return errors.New() } if , := .serviceMap.LoadOrStore(, ); { return errors.New("rpc: service already defined: " + ) } return nil } // suitableMethods returns suitable Rpc methods of typ, it will report // error using log if reportErr is true. func suitableMethods( reflect.Type, bool) map[string]*methodType { := make(map[string]*methodType) for := 0; < .NumMethod(); ++ { := .Method() := .Type := .Name // Method must be exported. if .PkgPath != "" { continue } // Method needs three ins: receiver, *args, *reply. if .NumIn() != 3 { if { log.Printf("rpc.Register: method %q has %d input parameters; needs exactly three\n", , .NumIn()) } continue } // First arg need not be a pointer. := .In(1) if !isExportedOrBuiltinType() { if { log.Printf("rpc.Register: argument type of method %q is not exported: %q\n", , ) } continue } // Second arg must be a pointer. := .In(2) if .Kind() != reflect.Ptr { if { log.Printf("rpc.Register: reply type of method %q is not a pointer: %q\n", , ) } continue } // Reply type must be exported. if !isExportedOrBuiltinType() { if { log.Printf("rpc.Register: reply type of method %q is not exported: %q\n", , ) } continue } // Method needs one out. if .NumOut() != 1 { if { log.Printf("rpc.Register: method %q has %d output parameters; needs exactly one\n", , .NumOut()) } continue } // The return type of the method must be error. if := .Out(0); != typeOfError { if { log.Printf("rpc.Register: return type of method %q is %q, must be error\n", , ) } continue } [] = &methodType{method: , ArgType: , ReplyType: } } return } // A value sent as a placeholder for the server's response value when the server // receives an invalid request. It is never decoded by the client since the Response // contains an error when it is used. var invalidRequest = struct{}{} func ( *Server) ( *sync.Mutex, *Request, interface{}, ServerCodec, string) { := .getResponse() // Encode the response header .ServiceMethod = .ServiceMethod if != "" { .Error = = invalidRequest } .Seq = .Seq .Lock() := .WriteResponse(, ) if debugLog && != nil { log.Println("rpc: writing response:", ) } .Unlock() .freeResponse() } func ( *methodType) () ( uint) { .Lock() = .numCalls .Unlock() return } func ( *service) ( *Server, *sync.Mutex, *sync.WaitGroup, *methodType, *Request, , reflect.Value, ServerCodec) { if != nil { defer .Done() } .Lock() .numCalls++ .Unlock() := .method.Func // Invoke the method, providing a new value for the reply. := .Call([]reflect.Value{.rcvr, , }) // The return value for the method is an error. := [0].Interface() := "" if != nil { = .(error).Error() } .sendResponse(, , .Interface(), , ) .freeRequest() } type gobServerCodec struct { rwc io.ReadWriteCloser dec *gob.Decoder enc *gob.Encoder encBuf *bufio.Writer closed bool } func ( *gobServerCodec) ( *Request) error { return .dec.Decode() } func ( *gobServerCodec) ( interface{}) error { return .dec.Decode() } func ( *gobServerCodec) ( *Response, interface{}) ( error) { if = .enc.Encode(); != nil { if .encBuf.Flush() == nil { // Gob couldn't encode the header. Should not happen, so if it does, // shut down the connection to signal that the connection is broken. log.Println("rpc: gob error encoding response:", ) .Close() } return } if = .enc.Encode(); != nil { if .encBuf.Flush() == nil { // Was a gob problem encoding the body but the header has been written. // Shut down the connection to signal that the connection is broken. log.Println("rpc: gob error encoding body:", ) .Close() } return } return .encBuf.Flush() } func ( *gobServerCodec) () error { if .closed { // Only call c.rwc.Close once; otherwise the semantics are undefined. return nil } .closed = true return .rwc.Close() } // ServeConn runs the server on a single connection. // ServeConn blocks, serving the connection until the client hangs up. // The caller typically invokes ServeConn in a go statement. // ServeConn uses the gob wire format (see package gob) on the // connection. To use an alternate codec, use ServeCodec. // See NewClient's comment for information about concurrent access. func ( *Server) ( io.ReadWriteCloser) { := bufio.NewWriter() := &gobServerCodec{ rwc: , dec: gob.NewDecoder(), enc: gob.NewEncoder(), encBuf: , } .ServeCodec() } // ServeCodec is like ServeConn but uses the specified codec to // decode requests and encode responses. func ( *Server) ( ServerCodec) { := new(sync.Mutex) := new(sync.WaitGroup) for { , , , , , , := .readRequest() if != nil { if debugLog && != io.EOF { log.Println("rpc:", ) } if ! { break } // send a response if we actually managed to read a header. if != nil { .sendResponse(, , invalidRequest, , .Error()) .freeRequest() } continue } .Add(1) go .call(, , , , , , , ) } // We've seen that there are no more requests. // Wait for responses to be sent before closing codec. .Wait() .Close() } // ServeRequest is like ServeCodec but synchronously serves a single request. // It does not close the codec upon completion. func ( *Server) ( ServerCodec) error { := new(sync.Mutex) , , , , , , := .readRequest() if != nil { if ! { return } // send a response if we actually managed to read a header. if != nil { .sendResponse(, , invalidRequest, , .Error()) .freeRequest() } return } .call(, , nil, , , , , ) return nil } func ( *Server) () *Request { .reqLock.Lock() := .freeReq if == nil { = new(Request) } else { .freeReq = .next * = Request{} } .reqLock.Unlock() return } func ( *Server) ( *Request) { .reqLock.Lock() .next = .freeReq .freeReq = .reqLock.Unlock() } func ( *Server) () *Response { .respLock.Lock() := .freeResp if == nil { = new(Response) } else { .freeResp = .next * = Response{} } .respLock.Unlock() return } func ( *Server) ( *Response) { .respLock.Lock() .next = .freeResp .freeResp = .respLock.Unlock() } func ( *Server) ( ServerCodec) ( *service, *methodType, *Request, , reflect.Value, bool, error) { , , , , = .readRequestHeader() if != nil { if ! { return } // discard body .ReadRequestBody(nil) return } // Decode the argument value. := false // if true, need to indirect before calling. if .ArgType.Kind() == reflect.Ptr { = reflect.New(.ArgType.Elem()) } else { = reflect.New(.ArgType) = true } // argv guaranteed to be a pointer now. if = .ReadRequestBody(.Interface()); != nil { return } if { = .Elem() } = reflect.New(.ReplyType.Elem()) switch .ReplyType.Elem().Kind() { case reflect.Map: .Elem().Set(reflect.MakeMap(.ReplyType.Elem())) case reflect.Slice: .Elem().Set(reflect.MakeSlice(.ReplyType.Elem(), 0, 0)) } return } func ( *Server) ( ServerCodec) ( *service, *methodType, *Request, bool, error) { // Grab the request header. = .getRequest() = .ReadRequestHeader() if != nil { = nil if == io.EOF || == io.ErrUnexpectedEOF { return } = errors.New("rpc: server cannot decode request: " + .Error()) return } // We read the header successfully. If we see an error now, // we can still recover and move on to the next request. = true := strings.LastIndex(.ServiceMethod, ".") if < 0 { = errors.New("rpc: service/method request ill-formed: " + .ServiceMethod) return } := .ServiceMethod[:] := .ServiceMethod[+1:] // Look up the request. , := .serviceMap.Load() if ! { = errors.New("rpc: can't find service " + .ServiceMethod) return } = .(*service) = .method[] if == nil { = errors.New("rpc: can't find method " + .ServiceMethod) } return } // Accept accepts connections on the listener and serves requests // for each incoming connection. Accept blocks until the listener // returns a non-nil error. The caller typically invokes Accept in a // go statement. func ( *Server) ( net.Listener) { for { , := .Accept() if != nil { log.Print("rpc.Serve: accept:", .Error()) return } go .ServeConn() } } // Register publishes the receiver's methods in the DefaultServer. func ( interface{}) error { return DefaultServer.Register() } // RegisterName is like Register but uses the provided name for the type // instead of the receiver's concrete type. func ( string, interface{}) error { return DefaultServer.RegisterName(, ) } // A ServerCodec implements reading of RPC requests and writing of // RPC responses for the server side of an RPC session. // The server calls ReadRequestHeader and ReadRequestBody in pairs // to read requests from the connection, and it calls WriteResponse to // write a response back. The server calls Close when finished with the // connection. ReadRequestBody may be called with a nil // argument to force the body of the request to be read and discarded. // See NewClient's comment for information about concurrent access. type ServerCodec interface { ReadRequestHeader(*Request) error ReadRequestBody(interface{}) error WriteResponse(*Response, interface{}) error // Close can be called multiple times and must be idempotent. Close() error } // ServeConn runs the DefaultServer on a single connection. // ServeConn blocks, serving the connection until the client hangs up. // The caller typically invokes ServeConn in a go statement. // ServeConn uses the gob wire format (see package gob) on the // connection. To use an alternate codec, use ServeCodec. // See NewClient's comment for information about concurrent access. func ( io.ReadWriteCloser) { DefaultServer.ServeConn() } // ServeCodec is like ServeConn but uses the specified codec to // decode requests and encode responses. func ( ServerCodec) { DefaultServer.ServeCodec() } // ServeRequest is like ServeCodec but synchronously serves a single request. // It does not close the codec upon completion. func ( ServerCodec) error { return DefaultServer.ServeRequest() } // Accept accepts connections on the listener and serves requests // to DefaultServer for each incoming connection. // Accept blocks; the caller typically invokes it in a go statement. func ( net.Listener) { DefaultServer.Accept() } // Can connect to RPC service using HTTP CONNECT to rpcPath. var connected = "200 Connected to Go RPC" // ServeHTTP implements an http.Handler that answers RPC requests. func ( *Server) ( http.ResponseWriter, *http.Request) { if .Method != "CONNECT" { .Header().Set("Content-Type", "text/plain; charset=utf-8") .WriteHeader(http.StatusMethodNotAllowed) io.WriteString(, "405 must CONNECT\n") return } , , := .(http.Hijacker).Hijack() if != nil { log.Print("rpc hijacking ", .RemoteAddr, ": ", .Error()) return } io.WriteString(, "HTTP/1.0 "+connected+"\n\n") .ServeConn() } // HandleHTTP registers an HTTP handler for RPC messages on rpcPath, // and a debugging handler on debugPath. // It is still necessary to invoke http.Serve(), typically in a go statement. func ( *Server) (, string) { http.Handle(, ) http.Handle(, debugHTTP{}) } // HandleHTTP registers an HTTP handler for RPC messages to DefaultServer // on DefaultRPCPath and a debugging handler on DefaultDebugPath. // It is still necessary to invoke http.Serve(), typically in a go statement. func () { DefaultServer.HandleHTTP(DefaultRPCPath, DefaultDebugPath) }