rpc

package rpc

import "net/rpc"

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, err := net.Listen("tcp", ":1234")
if err != nil {
	log.Fatal("listen error:", err)
}
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)

// 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.

Index

Constants
Variables
func Accept(lis net.Listener)
func HandleHTTP()
func Register(rcvr any) error
func RegisterName(name string, rcvr any) error
func ServeCodec(codec ServerCodec)
func ServeConn(conn io.ReadWriteCloser)
func ServeRequest(codec ServerCodec) error
type Call
type Client
type ClientCodec
type Request
type Response
type Server
type ServerCodec
type ServerError
- func (e ServerError) Error() string

Constants

const (
	// Defaults used by HandleHTTP
	DefaultRPCPath   = "/_goRPC_"
	DefaultDebugPath = "/debug/rpc"
)

Variables

var DefaultServer = NewServer()

DefaultServer is the default instance of *Server.

var ErrShutdown = errors.New("connection is shut down")

Functions

func Accept

func Accept(lis net.Listener)

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 HandleHTTP

func HandleHTTP()

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 Register

func Register(rcvr any) error

func RegisterName

func RegisterName(name string, rcvr any) error

RegisterName is like Register but uses the provided name for the type instead of the receiver's concrete type.

func ServeCodec

func ServeCodec(codec ServerCodec)

ServeCodec is like ServeConn but uses the specified codec to decode requests and encode responses.

func ServeConn

func ServeConn(conn io.ReadWriteCloser)

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 ServeRequest

func ServeRequest(codec ServerCodec) error

ServeRequest is like ServeCodec but synchronously serves a single request. It does not close the codec upon completion.

Types

type Call

type Call struct {
	ServiceMethod string     // The name of the service and method to call.
	Args          any        // The argument to the function (*struct).
	Reply         any        // The reply from the function (*struct).
	Error         error      // After completion, the error status.
	Done          chan *Call // Receives *Call when Go is complete.
}

Call represents an active RPC.

type Client

type Client struct {
	// contains filtered or unexported fields
}

Client represents an RPC Client. There may be multiple outstanding Calls associated with a single Client, and a Client may be used by multiple goroutines simultaneously.

func Dial

func Dial(network, address string) (*Client, error)

Dial connects to an RPC server at the specified network address.

func DialHTTP

func DialHTTP(network, address string) (*Client, error)

DialHTTP connects to an HTTP RPC server at the specified network address listening on the default HTTP RPC path.

func DialHTTPPath

func DialHTTPPath(network, address, path string) (*Client, error)

DialHTTPPath connects to an HTTP RPC server at the specified network address and path.

func NewClient

func NewClient(conn io.ReadWriteCloser) *Client

NewClient returns a new Client to handle requests to the set of services at the other end of the connection. It adds a buffer to the write side of the connection so the header and payload are sent as a unit.

The read and write halves of the connection are serialized independently, so no interlocking is required. However each half may be accessed concurrently so the implementation of conn should protect against concurrent reads or concurrent writes.

func NewClientWithCodec

func NewClientWithCodec(codec ClientCodec) *Client

NewClientWithCodec is like NewClient but uses the specified codec to encode requests and decode responses.

func (*Client) Call

func (client *Client) Call(serviceMethod string, args any, reply any) error

Call invokes the named function, waits for it to complete, and returns its error status.

func (*Client) Close

func (client *Client) Close() error

Close calls the underlying codec's Close method. If the connection is already shutting down, ErrShutdown is returned.

func (*Client) Go

func (client *Client) Go(serviceMethod string, args any, reply any, done chan *Call) *Call

Go invokes the function asynchronously. It returns the Call structure representing the invocation. The done channel will signal when the call is complete by returning the same Call object. If done is nil, Go will allocate a new channel. If non-nil, done must be buffered or Go will deliberately crash.

type ClientCodec

type ClientCodec interface {
	WriteRequest(*Request, any) error
	ReadResponseHeader(*Response) error
	ReadResponseBody(any) error

	Close() error
}

A ClientCodec implements writing of RPC requests and reading of RPC responses for the client side of an RPC session. The client calls [ClientCodec.WriteRequest] to write a request to the connection and calls [ClientCodec.ReadResponseHeader] and [ClientCodec.ReadResponseBody] in pairs to read responses. The client calls [ClientCodec.Close] when finished with the connection. ReadResponseBody may be called with a nil argument to force the body of the response to be read and then discarded. See NewClient's comment for information about concurrent access.

type Request

type Request struct {
	ServiceMethod string // format: "Service.Method"
	Seq           uint64 // sequence number chosen by client
	// contains filtered or unexported fields
}

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 Response

type Response struct {
	ServiceMethod string // echoes that of the Request
	Seq           uint64 // echoes that of the request
	Error         string // error, if any.
	// contains filtered or unexported fields
}

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 Server

type Server struct {
	// contains filtered or unexported fields
}

Server represents an RPC Server.

func NewServer

func NewServer() *Server

NewServer returns a new Server.

func (*Server) Accept

func (server *Server) Accept(lis net.Listener)

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) HandleHTTP

func (server *Server) HandleHTTP(rpcPath, debugPath string)

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) Register

func (server *Server) Register(rcvr any) error

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) RegisterName

func (server *Server) RegisterName(name string, rcvr any) error

RegisterName is like Register but uses the provided name for the type instead of the receiver's concrete type.

func (*Server) ServeCodec

func (server *Server) ServeCodec(codec ServerCodec)

ServeCodec is like ServeConn but uses the specified codec to decode requests and encode responses.

func (*Server) ServeConn

func (server *Server) ServeConn(conn io.ReadWriteCloser)

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) ServeHTTP

func (server *Server) ServeHTTP(w http.ResponseWriter, req *http.Request)

ServeHTTP implements an http.Handler that answers RPC requests.

func (*Server) ServeRequest

func (server *Server) ServeRequest(codec ServerCodec) error

ServeRequest is like ServeCodec but synchronously serves a single request. It does not close the codec upon completion.

type ServerCodec

type ServerCodec interface {
	ReadRequestHeader(*Request) error
	ReadRequestBody(any) error
	WriteResponse(*Response, any) error

	// Close can be called multiple times and must be idempotent.
	Close() error
}

A ServerCodec implements reading of RPC requests and writing of RPC responses for the server side of an RPC session. The server calls [ServerCodec.ReadRequestHeader] and [ServerCodec.ReadRequestBody] in pairs to read requests from the connection, and it calls [ServerCodec.WriteResponse] to write a response back. The server calls [ServerCodec.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 ServerError

type ServerError string

ServerError represents an error that has been returned from the remote side of the RPC connection.

func (ServerError) Error

func (e ServerError) Error() string

Directories

jsonrpc

Package jsonrpc implements a JSON-RPC 1.0 ClientCodec and ServerCodec for the rpc package.