Create and run commands over a RPC protocol stream
Last updated a year ago by bret .
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$ cnpm install rpc-protocol 
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Create and run commands over a RPC protocol stream


$ npm install rpc-protocol


Echo server over web sockets:

const WebSocketServer = require('simple-websocket/server')
const protocol = require('rpc-protocol')
const server = new WebSocketServer({ port: 3000 })

server.on('connection', onconnection)

function onconnection(socket) {
  const rpc = protocol({ stream: socket })
  rpc.command('echo', (req) => {
    return req.arguments

Echo client over web sockets

const WebSocket = require('simple-websocket')
const protocol = require('rpc-protocol')
const socket = new WebSocket('ws://localhost:3000')

socket.on('connect', onconnect)

function onconnect() {
  const rpc = protocol({ stream: socket })
  rpc.call('echo', 'hello world', (err, res) => {
    console.log(res) // [ 'hello world' ]


rpc = require('rpc-protocol')(opts)

Creates a new RPC protocol Duplex stream where opts can be:

  • opts.encoding is an optional encoding object that contains an encode(value) and decode(buffer) functions for converting values to and from buffers that can be sent over a binary stream
  • opts.stream is an optional stream to pump data in and out of into the protocol stream

rpc.command(commandName, callback)

Create a callback for a command given by the string commandName that is called when rpc.call(commandName, ...) is called from the client and where callback contains a command request object. Results can be returned to the caller by returning a value, which can be a Promise.


Below is a simple example of a command that proxies a fetch() call over the stream. The caller would simply need to call rpc.call('fetch', ['https://github.com', { mode: 'cors' }, ], callback).

rpc.command('fetch', async (req) => {
  const [ resource, init ] = req.arguments
  return fetch(resource, init).then(async (res) => {
    return {
      arrayBuffer: Buffer.from(await res.clone().arrayBuffer),
      text: Buffer.from(await res.clone().text),
      json: Buffer.from(await res.clone().json),

      statusText: res.statusText,
      redirected: res.redirected,
      bodyUsed: res.bodyUsed,
      headers: res.headers,
      status: res.status,
      type: res.type,
      url: res.url,
      ok: res.ok,

Command Streams

Commands can stream responses by to the client by making use of the second argument given to the command callback called reply(err, results). It is a function that accepts an optional error and an array of results to send back to the client.

Below is an example of a simple counter stream. Given a start and end range with an interval the command will reply with an incremented i value at some interval.

rpc.command('counter', (req, reply) => {
  const [ start, end, interval = 100] = req.arguments
  let i = start


  function ontick() {
    if (i <= end) {
      // reply returns true that means we can write again
      if (reply(null, i++)) {
        setTimeout(ontick, interval)
    } else {
      // signal end of stream
      reply(null, null)
didWrite = reply(err, response)

The reply() function replies to the caller with an error err and an array of results as a response. The command can continue to call this function with more results or errors. The return value of the reply function will indicate if the stream is still open to write to.

command = rpc.call(commandName, arguments, callback)

A command can be invoked by calling rpc.call() with a command name and an optional array of arguments along with a callback(err, res) function that will be called when the command responds with a reply. Response results are always given as an array as the command may return more than value. A command can only throw one error at a time.

rpc.call('echo', ['hello', 'world'], (err, res) => {
  if (err) {
    // handle Error
  } else {
    console.log(res) // ['hello', 'world']

stream = rpc.createReadStream(command)

Read streams can be created from an existing command. They are useful if the command can reply with multiple values over a period of time.

rpc.command('fs.createReadStream', (req, reply) => {
  const [ path ] = req.arguments
  const stream = fs.createReadStream(path)
  stream.on('data', (data) => reply(null, data))
  stream.on('end', () => reply(null, null)) // 'null' signals end of stream

const command = rpc.call('fs.createReadStream', ['/path/to/file.txt'])
const stream = rpc.createReadStream(command)
const chunks []

stream.on('data', (data) => { chunks.push(data) })
stream.on('end', () => console.log(Buffer.concat(chunks).toString()))


rpc.send() is the low level function that makes a request for an existing command, an extension, or an arbitrary buffer.

Sending An Extension

Sending an extension message requires you use the extension type and some value that the extension encodes.

rpc.send(EXTENSION_TYPE, 'some value', (err, res) => {
  // called when extension replies with a response

Sending A Command

Sending a command requires you create an instance of Command with an encoding, command name, array of arguments, and a callback that is called when the command replies with a response

const command = new Command(rpc.encoding, 'echo', ['hello'], (err, res) => {
  // called when the command replies with a response

rpc.send(command.id, command, command.pack())

Sending An Arbitrary Message

Sending an arbitrary message is possible but replies cannot be linked to the sent message and there is no guarantee that the message was read.

rpc.send(Buffer.from('hello'), (err) => {
  // It is impossible for a reply as this is an arbitrary message


An alias to rpc.fin(command).


Send a Fin packet to the stream tied to the command request.

const command = rpc.call('echo', ['hello', 'world'], console.log)

rpc.extension(extensionType, encoding)

Extensions provide a way to extend the protocol with user supplied binary encodings. The rpc.extension(extensionType, encoding) function accepts an integer extensionType and an encoding object that contains encode(value) and decode(buffer) functions for encoding values to and from buffers.

Extensions can be used by making use of the rpc.send() function that expects an extensionType, an array of arguments that will be encoded by the extension encoding and a callback that will be called when the extension replies with a response.

Extension Wire Interface

Extensions must provide a way of encoding an id into the extension encoding which must be available after decode(buffer) is called on the return value. The id is used internally to track requests and response from callers.

If an extension provides a way to encode and decode Error properties, then they will be propagated to the err argument in the reply(err) function.

Example Extension

const keyPair = require('hypercore-crypto')
const pbs = require('protocol-buffers')
const KEY_PAIR_EXTENSION = 0xfed
const { KeyPair } = pbc(`
  message KeyPair {
    bytes id = 1;
    bytes publicKey = 2;
    bytes secretKey = 3;

const server = protocol()
const client = protocol()

server.extension(KEY_PAIR_EXTENSION, KeyPair)
client.extension(KEY_PAIR_EXTENSION, KeyPair)

server.on('extension', (req, type, buffer, reply) => {
  if (KEY_PAIR_EXTENSION === type) {
    reply(null, keyPair())

client.send(KEY_PAIR_EXTENSION, (err, res) => {
  console.log(res) // { publicKey: <Buffer ...>, secretKey: <Buffer ...>



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