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Published: Nov 25, 2019 License: GPL-3.0 Imports: 36 Imported by: 26



Package rpc provides access to the exported methods of an object across a network or other I/O connection. After creating a server instance objects can be registered, making it visible from the outside. Exported methods that follow specific conventions can be called remotely. It also has support for the publish/subscribe pattern.

Methods that satisfy the following criteria are made available for remote access:

  • object must be exported
  • method must be exported
  • method returns 0, 1 (response or error) or 2 (response and error) values
  • method argument(s) must be exported or builtin types
  • method returned value(s) must be exported or builtin types

An example method:

func (s *CalcService) Add(a, b int) (int, error)

When the returned error isn't nil the returned integer is ignored and the error is send back to the client. Otherwise the returned integer is send back to the client.

Optional arguments are supported by accepting pointer values as arguments. E.g. if we want to do the addition in an optional finite field we can accept a mod argument as pointer value.

func (s *CalService) Add(a, b int, mod *int) (int, error)

This RPC method can be called with 2 integers and a null value as third argument. In that case the mod argument will be nil. Or it can be called with 3 integers, in that case mod will be pointing to the given third argument. Since the optional argument is the last argument the RPC package will also accept 2 integers as arguments. It will pass the mod argument as nil to the RPC method.

The server offers the ServeCodec method which accepts a ServerCodec instance. It will read requests from the codec, process the request and sends the response back to the client using the codec. The server can execute requests concurrently. Responses can be sent back to the client out of order.

An example server which uses the JSON codec:

 type CalculatorService struct {}

 func (s *CalculatorService) Add(a, b int) int {
	return a + b

 func (s *CalculatorService Div(a, b int) (int, error) {
	if b == 0 {
		return 0, errors.New("divide by zero")
	return a/b, nil

 calculator := new(CalculatorService)
 server := NewServer()
 server.RegisterName("calculator", calculator")

 l, _ := net.ListenUnix("unix", &net.UnixAddr{Net: "unix", Name: "/tmp/calculator.sock"})
 for {
	c, _ := l.AcceptUnix()
	codec := v2.NewJSONCodec(c)
	go server.ServeCodec(codec)

The package also supports the publish subscribe pattern through the use of subscriptions. A method that is considered eligible for notifications must satisfy the following criteria:

  • object must be exported
  • method must be exported
  • first method argument type must be context.Context
  • method argument(s) must be exported or builtin types
  • method must return the tuple Subscription, error

An example method:

func (s *BlockChainService) NewBlocks(ctx context.Context) (Subscription, error) {

Subscriptions are deleted when:

  • the user sends an unsubscribe request
  • the connection which was used to create the subscription is closed. This can be initiated by the client and server. The server will close the connection on a write error or when the queue of buffered notifications gets too big.




View Source
const (
	PendingBlockNumber  = BlockNumber(-2)
	LatestBlockNumber   = BlockNumber(-1)
	EarliestBlockNumber = BlockNumber(0)
View Source
const MetadataApi = "rpc"


View Source
var (
	ErrClientQuit                = errors.New("client is closed")
	ErrNoResult                  = errors.New("no result in JSON-RPC response")
	ErrSubscriptionQueueOverflow = errors.New("subscription queue overflow")
View Source
var (
	// ErrNotificationsUnsupported is returned when the connection doesn't support notifications
	ErrNotificationsUnsupported = errors.New("notifications not supported")
	// ErrNotificationNotFound is returned when the notification for the given id is not found
	ErrSubscriptionNotFound = errors.New("subscription not found")


func NewHTTPServer deprecated

func NewHTTPServer(cors []string, vhosts []string, srv *Server) *http.Server

NewHTTPServer creates a new HTTP RPC server around an API provider.

Deprecated: Server implements http.Handler

func NewWSServer deprecated

func NewWSServer(allowedOrigins []string, srv *Server) *http.Server

NewWSServer creates a new websocket RPC server around an API provider.

Deprecated: use Server.WebsocketHandler


type API

type API struct {
	Namespace string      // namespace under which the rpc methods of Service are exposed
	Version   string      // api version for DApp's
	Service   interface{} // receiver instance which holds the methods
	Public    bool        // indication if the methods must be considered safe for public use

API describes the set of methods offered over the RPC interface

type BatchElem

type BatchElem struct {
	Method string
	Args   []interface{}
	// The result is unmarshaled into this field. Result must be set to a
	// non-nil pointer value of the desired type, otherwise the response will be
	// discarded.
	Result interface{}
	// Error is set if the server returns an error for this request, or if
	// unmarshaling into Result fails. It is not set for I/O errors.
	Error error

BatchElem is an element in a batch request.

type BlockNumber

type BlockNumber int64

func (BlockNumber) Int64

func (bn BlockNumber) Int64() int64

func (*BlockNumber) UnmarshalJSON

func (bn *BlockNumber) UnmarshalJSON(data []byte) error

UnmarshalJSON parses the given JSON fragment into a BlockNumber. It supports: - "latest", "earliest" or "pending" as string arguments - the block number Returned errors: - an invalid block number error when the given argument isn't a known strings - an out of range error when the given block number is either too little or too large

type Client

type Client struct {
	// contains filtered or unexported fields

Client represents a connection to an RPC server.

func Dial

func Dial(rawurl string) (*Client, error)

Dial creates a new client for the given URL.

The currently supported URL schemes are "http", "https", "ws" and "wss". If rawurl is a file name with no URL scheme, a local socket connection is established using UNIX domain sockets on supported platforms and named pipes on Windows. If you want to configure transport options, use DialHTTP, DialWebsocket or DialIPC instead.

For websocket connections, the origin is set to the local host name.

The client reconnects automatically if the connection is lost.

func DialContext

func DialContext(ctx context.Context, rawurl string) (*Client, error)

DialContext creates a new RPC client, just like Dial.

The context is used to cancel or time out the initial connection establishment. It does not affect subsequent interactions with the client.

func DialHTTP

func DialHTTP(endpoint string) (*Client, error)

DialHTTP creates a new RPC client that connects to an RPC server over HTTP.

func DialHTTPWithClient

func DialHTTPWithClient(endpoint string, client *http.Client) (*Client, error)

DialHTTPWithClient creates a new RPC client that connects to an RPC server over HTTP using the provided HTTP Client.

func DialIPC

func DialIPC(ctx context.Context, endpoint string) (*Client, error)

DialIPC create a new IPC client that connects to the given endpoint. On Unix it assumes the endpoint is the full path to a unix socket, and Windows the endpoint is an identifier for a named pipe.

The context is used for the initial connection establishment. It does not affect subsequent interactions with the client.

func DialInProc

func DialInProc(handler *Server) *Client

DialInProc attaches an in-process connection to the given RPC server.

func DialStdIO

func DialStdIO(ctx context.Context) (*Client, error)

func DialWebsocket

func DialWebsocket(ctx context.Context, endpoint, origin string) (*Client, error)

DialWebsocket creates a new RPC client that communicates with a JSON-RPC server that is listening on the given endpoint.

The context is used for the initial connection establishment. It does not affect subsequent interactions with the client.

func (*Client) BatchCall

func (c *Client) BatchCall(b []BatchElem) error

BatchCall sends all given requests as a single batch and waits for the server to return a response for all of them.

In contrast to Call, BatchCall only returns I/O errors. Any error specific to a request is reported through the Error field of the corresponding BatchElem.

Note that batch calls may not be executed atomically on the server side.

func (*Client) BatchCallContext

func (c *Client) BatchCallContext(ctx context.Context, b []BatchElem) error

BatchCall sends all given requests as a single batch and waits for the server to return a response for all of them. The wait duration is bounded by the context's deadline.

In contrast to CallContext, BatchCallContext only returns errors that have occurred while sending the request. Any error specific to a request is reported through the Error field of the corresponding BatchElem.

Note that batch calls may not be executed atomically on the server side.

func (*Client) Call

func (c *Client) Call(result interface{}, method string, args ...interface{}) error

Call performs a JSON-RPC call with the given arguments and unmarshals into result if no error occurred.

The result must be a pointer so that package json can unmarshal into it. You can also pass nil, in which case the result is ignored.

func (*Client) CallContext

func (c *Client) CallContext(ctx context.Context, result interface{}, method string, args ...interface{}) error

CallContext performs a JSON-RPC call with the given arguments. If the context is canceled before the call has successfully returned, CallContext returns immediately.

The result must be a pointer so that package json can unmarshal into it. You can also pass nil, in which case the result is ignored.

func (*Client) Close

func (c *Client) Close()

Close closes the client, aborting any in-flight requests.

func (*Client) EthSubscribe

func (c *Client) EthSubscribe(ctx context.Context, channel interface{}, args ...interface{}) (*ClientSubscription, error)

EthSubscribe registers a subscripion under the "eth" namespace.

func (*Client) ShhSubscribe

func (c *Client) ShhSubscribe(ctx context.Context, channel interface{}, args ...interface{}) (*ClientSubscription, error)

ShhSubscribe registers a subscripion under the "shh" namespace.

func (*Client) Subscribe

func (c *Client) Subscribe(ctx context.Context, namespace string, channel interface{}, args ...interface{}) (*ClientSubscription, error)

Subscribe calls the "<namespace>_subscribe" method with the given arguments, registering a subscription. Server notifications for the subscription are sent to the given channel. The element type of the channel must match the expected type of content returned by the subscription.

The context argument cancels the RPC request that sets up the subscription but has no effect on the subscription after Subscribe has returned.

Slow subscribers will be dropped eventually. Client buffers up to 8000 notifications before considering the subscriber dead. The subscription Err channel will receive ErrSubscriptionQueueOverflow. Use a sufficiently large buffer on the channel or ensure that the channel usually has at least one reader to prevent this issue.

func (*Client) SupportedModules

func (c *Client) SupportedModules() (map[string]string, error)

SupportedModules calls the rpc_modules method, retrieving the list of APIs that are available on the server.

type ClientSubscription

type ClientSubscription struct {
	// contains filtered or unexported fields

A ClientSubscription represents a subscription established through EthSubscribe.

package main

import (


// In this example, our client whishes to track the latest 'block number'
// known to the server. The server supports two methods:
// eth_getBlockByNumber("latest", {})
//    returns the latest block object.
// eth_subscribe("newBlocks")
//    creates a subscription which fires block objects when new blocks arrive.

type Block struct {
	Number *big.Int

func main() {
	// Connect the client.
	client, _ := rpc.Dial("ws://")
	subch := make(chan Block)

	// Ensure that subch receives the latest block.
	go func() {
		for i := 0; ; i++ {
			if i > 0 {
				time.Sleep(2 * time.Second)
			subscribeBlocks(client, subch)

	// Print events from the subscription as they arrive.
	for block := range subch {
		fmt.Println("latest block:", block.Number)

// subscribeBlocks runs in its own goroutine and maintains
// a subscription for new blocks.
func subscribeBlocks(client *rpc.Client, subch chan Block) {
	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
	defer cancel()

	// Subscribe to new blocks.
	sub, err := client.EthSubscribe(ctx, subch, "newBlocks")
	if err != nil {
		fmt.Println("subscribe error:", err)

	// The connection is established now.
	// Update the channel with the current block.
	var lastBlock Block
	if err := client.CallContext(ctx, &lastBlock, "eth_getBlockByNumber", "latest"); err != nil {
		fmt.Println("can't get latest block:", err)
	subch <- lastBlock

	// The subscription will deliver events to the channel. Wait for the
	// subscription to end for any reason, then loop around to re-establish
	// the connection.
	fmt.Println("connection lost: ", <-sub.Err())

func (*ClientSubscription) Err

func (sub *ClientSubscription) Err() <-chan error

Err returns the subscription error channel. The intended use of Err is to schedule resubscription when the client connection is closed unexpectedly.

The error channel receives a value when the subscription has ended due to an error. The received error is nil if Close has been called on the underlying client and no other error has occurred.

The error channel is closed when Unsubscribe is called on the subscription.

func (*ClientSubscription) Unsubscribe

func (sub *ClientSubscription) Unsubscribe()

Unsubscribe unsubscribes the notification and closes the error channel. It can safely be called more than once.

type CodecOption

type CodecOption int

CodecOption specifies which type of messages this codec supports

const (
	// OptionMethodInvocation is an indication that the codec supports RPC method calls
	OptionMethodInvocation CodecOption = 1 << iota

	// OptionSubscriptions is an indication that the codec suports RPC notifications
	OptionSubscriptions = 1 << iota // support pub sub

type Error

type Error interface {
	Error() string  // returns the message
	ErrorCode() int // returns the code

Error wraps RPC errors, which contain an error code in addition to the message.

type ID

type ID string

ID defines a pseudo random number that is used to identify RPC subscriptions.

func NewID

func NewID() ID

NewID generates a identifier that can be used as an identifier in the RPC interface. e.g. filter and subscription identifier.

type Notifier

type Notifier struct {
	// contains filtered or unexported fields

Notifier is tight to a RPC connection that supports subscriptions. Server callbacks use the notifier to send notifications.

func NotifierFromContext

func NotifierFromContext(ctx context.Context) (*Notifier, bool)

NotifierFromContext returns the Notifier value stored in ctx, if any.

func (*Notifier) Closed

func (n *Notifier) Closed() <-chan interface{}

Closed returns a channel that is closed when the RPC connection is closed.

func (*Notifier) CreateSubscription

func (n *Notifier) CreateSubscription() *Subscription

CreateSubscription returns a new subscription that is coupled to the RPC connection. By default subscriptions are inactive and notifications are dropped until the subscription is marked as active. This is done by the RPC server after the subscription ID is send to the client.

func (*Notifier) Notify

func (n *Notifier) Notify(id ID, data interface{}) error

Notify sends a notification to the client with the given data as payload. If an error occurs the RPC connection is closed and the error is returned.

type RPCService

type RPCService struct {
	// contains filtered or unexported fields

RPCService gives meta information about the server. e.g. gives information about the loaded modules.

func (*RPCService) Modules

func (s *RPCService) Modules() map[string]string

Modules returns the list of RPC services with their version number

type Server

type Server struct {
	// contains filtered or unexported fields

Server represents a RPC server

func NewServer

func NewServer() *Server

NewServer will create a new server instance with no registered handlers.

func StartHTTPEndpoint

func StartHTTPEndpoint(endpoint string, apis []API, modules []string, cors []string, vhosts []string) (net.Listener, *Server, error)

StartHTTPEndpoint starts the HTTP RPC endpoint, configured with cors/vhosts/modules

func StartIPCEndpoint

func StartIPCEndpoint(ipcEndpoint string, apis []API) (net.Listener, *Server, error)

StartIPCEndpoint starts an IPC endpoint.

func StartWSEndpoint

func StartWSEndpoint(endpoint string, apis []API, modules []string, wsOrigins []string, exposeAll bool) (net.Listener, *Server, error)

StartWSEndpoint starts a websocket endpoint

func (*Server) RegisterName

func (s *Server) RegisterName(name string, rcvr interface{}) error

RegisterName will create a service for the given rcvr type under the given name. When no methods on the given rcvr match the criteria to be either a RPC method or a subscription an error is returned. Otherwise a new service is created and added to the service collection this server instance serves.

func (*Server) ServeCodec

func (s *Server) ServeCodec(codec ServerCodec, options CodecOption)

ServeCodec reads incoming requests from codec, calls the appropriate callback and writes the response back using the given codec. It will block until the codec is closed or the server is stopped. In either case the codec is closed.

func (*Server) ServeHTTP

func (srv *Server) ServeHTTP(w http.ResponseWriter, r *http.Request)

ServeHTTP serves JSON-RPC requests over HTTP.

func (*Server) ServeListener

func (srv *Server) ServeListener(l net.Listener) error

ServeListener accepts connections on l, serving JSON-RPC on them.

func (*Server) ServeSingleRequest

func (s *Server) ServeSingleRequest(ctx context.Context, codec ServerCodec, options CodecOption)

ServeSingleRequest reads and processes a single RPC request from the given codec. It will not close the codec unless a non-recoverable error has occurred. Note, this method will return after a single request has been processed!

func (*Server) Stop

func (s *Server) Stop()

Stop will stop reading new requests, wait for stopPendingRequestTimeout to allow pending requests to finish, close all codecs which will cancel pending requests/subscriptions.

func (*Server) WebsocketHandler

func (srv *Server) WebsocketHandler(allowedOrigins []string) http.Handler

WebsocketHandler returns a handler that serves JSON-RPC to WebSocket connections.

allowedOrigins should be a comma-separated list of allowed origin URLs. To allow connections with any origin, pass "*".

type ServerCodec

type ServerCodec interface {
	// Read next request
	ReadRequestHeaders() ([]rpcRequest, bool, Error)
	// Parse request argument to the given types
	ParseRequestArguments(argTypes []reflect.Type, params interface{}) ([]reflect.Value, Error)
	// Assemble success response, expects response id and payload
	CreateResponse(id interface{}, reply interface{}) interface{}
	// Assemble error response, expects response id and error
	CreateErrorResponse(id interface{}, err Error) interface{}
	// Assemble error response with extra information about the error through info
	CreateErrorResponseWithInfo(id interface{}, err Error, info interface{}) interface{}
	// Create notification response
	CreateNotification(id, namespace string, event interface{}) interface{}
	// Write msg to client.
	Write(msg interface{}) error
	// Close underlying data stream
	// Closed when underlying connection is closed
	Closed() <-chan interface{}

ServerCodec implements reading, parsing and writing RPC messages for the server side of a RPC session. Implementations must be go-routine safe since the codec can be called in multiple go-routines concurrently.

func NewCodec

func NewCodec(rwc io.ReadWriteCloser, encode, decode func(v interface{}) error) ServerCodec

NewCodec creates a new RPC server codec with support for JSON-RPC 2.0 based on explicitly given encoding and decoding methods.

func NewJSONCodec

func NewJSONCodec(rwc io.ReadWriteCloser) ServerCodec

NewJSONCodec creates a new RPC server codec with support for JSON-RPC 2.0.

type StdIOConn

type StdIOConn struct{}

func (StdIOConn) Close

func (io StdIOConn) Close() error

func (StdIOConn) LocalAddr

func (io StdIOConn) LocalAddr() net.Addr

func (StdIOConn) Read

func (io StdIOConn) Read(b []byte) (n int, err error)

func (StdIOConn) RemoteAddr

func (io StdIOConn) RemoteAddr() net.Addr

func (StdIOConn) SetDeadline

func (io StdIOConn) SetDeadline(t time.Time) error

func (StdIOConn) SetReadDeadline

func (io StdIOConn) SetReadDeadline(t time.Time) error

func (StdIOConn) SetWriteDeadline

func (io StdIOConn) SetWriteDeadline(t time.Time) error

func (StdIOConn) Write

func (io StdIOConn) Write(b []byte) (n int, err error)

type Subscription

type Subscription struct {
	// contains filtered or unexported fields

a Subscription is created by a notifier and tight to that notifier. The client can use this subscription to wait for an unsubscribe request for the client, see Err().

func (*Subscription) Err

func (s *Subscription) Err() <-chan error

Err returns a channel that is closed when the client send an unsubscribe request.

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