07 - Socket.IO

Code for this chapter available here.

💡 Socket.IO is a library to easily deal with Websockets. It provides a convenient API and fallback for browsers that don't support Websockets.

In this chapter, we are going to set up a basic message exchange between the client and the server. In order to not add more pages and components – which would be unrelated to the core feature we're interested in here – we are going to make this exchange happen in the browser console. No UI stuff in this chapter.

Run yarn add socket.io socket.io-client

Server-side

Edit your src/server/index.js like so:

// @flow

import compression from 'compression'
import express from 'express'
import { Server } from 'http'
import socketIO from 'socket.io'

import routing from './routing'
import { WEB_PORT, STATIC_PATH } from '../shared/config'
import { isProd } from '../shared/util'
import setUpSocket from './socket'

const app = express()
// flow-disable-next-line
const http = Server(app)
const io = socketIO(http)
setUpSocket(io)

app.use(compression())
app.use(STATIC_PATH, express.static('dist'))
app.use(STATIC_PATH, express.static('public'))

routing(app)

http.listen(WEB_PORT, () => {
  // eslint-disable-next-line no-console
  console.log(`Server running on port ${WEB_PORT} ${isProd ? '(production)' :
    '(development).\nKeep "yarn dev:wds" running in an other terminal'}.`)
})

Note that in order for Socket.IO to work, you need to use Server from http to listen to incoming requests, and not the Express app. Fortunately, that doesn't change much of the code. All the Websocket details are externalized in a different file, called with setUpSocket.

Add the following constants to src/shared/config.js:

export const IO_CONNECT = 'connect'
export const IO_DISCONNECT = 'disconnect'
export const IO_CLIENT_HELLO = 'IO_CLIENT_HELLO'
export const IO_CLIENT_JOIN_ROOM = 'IO_CLIENT_JOIN_ROOM'
export const IO_SERVER_HELLO = 'IO_SERVER_HELLO'

These are the type of messages your client and your server will exchange. I suggest prefixing them with either IO_CLIENT or IO_SERVER to make it clearer who is sending the message. Otherwise, things can get pretty confusing when you have a lot of message types.

As you can see, we have a IO_CLIENT_JOIN_ROOM, because for the sake of demonstration, we are going to make clients join a room (like a chatroom). Rooms are useful to broadcast messages to specific groups of users.

Create a src/server/socket.js file containing:

// @flow

import {
  IO_CONNECT,
  IO_DISCONNECT,
  IO_CLIENT_JOIN_ROOM,
  IO_CLIENT_HELLO,
  IO_SERVER_HELLO,
} from '../shared/config'

/* eslint-disable no-console */
const setUpSocket = (io: Object) => {
  io.on(IO_CONNECT, (socket) => {
    console.log('[socket.io] A client connected.')

    socket.on(IO_CLIENT_JOIN_ROOM, (room) => {
      socket.join(room)
      console.log(`[socket.io] A client joined room ${room}.`)

      io.emit(IO_SERVER_HELLO, 'Hello everyone!')
      io.to(room).emit(IO_SERVER_HELLO, `Hello clients of room ${room}!`)
      socket.emit(IO_SERVER_HELLO, 'Hello you!')
    })

    socket.on(IO_CLIENT_HELLO, (clientMessage) => {
      console.log(`[socket.io] Client: ${clientMessage}`)
    })

    socket.on(IO_DISCONNECT, () => {
      console.log('[socket.io] A client disconnected.')
    })
  })
}
/* eslint-enable no-console */

export default setUpSocket

Okay, so in this file, we implement how our server should react when clients connect and send messages to it:

When the client connects, we log it in the server console, and get access to the socket object, which we can use to communicate back with that client.
When a client sends IO_CLIENT_JOIN_ROOM, we make it join the room it wants. Once it has joined a room, we send 3 demo messages: 1 message to every user, 1 message to users in that room, 1 message to that client only.
When the client sends IO_CLIENT_HELLO, we log its message in the server console.
When the client disconnects, we log it as well.

Client-side

The client-side of things is going to look very similar.

Edit src/client/index.jsx like so:

// [...]
import setUpSocket from './socket'

// [at the very end of the file]
setUpSocket(store)

As you can see, we pass the Redux store to setUpSocket. This way whenever a Websocket message coming from the server should alter the client's Redux state, we can dispatch actions. We are not going to dispatch anything in this example though.

Create a src/client/socket.js file containing:

// @flow

import socketIOClient from 'socket.io-client'

import {
  IO_CONNECT,
  IO_DISCONNECT,
  IO_CLIENT_HELLO,
  IO_CLIENT_JOIN_ROOM,
  IO_SERVER_HELLO,
  } from '../shared/config'

const socket = socketIOClient(window.location.host)

/* eslint-disable no-console */
// eslint-disable-next-line no-unused-vars
const setUpSocket = (store: Object) => {
  socket.on(IO_CONNECT, () => {
    console.log('[socket.io] Connected.')
    socket.emit(IO_CLIENT_JOIN_ROOM, 'hello-1234')
    socket.emit(IO_CLIENT_HELLO, 'Hello!')
  })

  socket.on(IO_SERVER_HELLO, (serverMessage) => {
    console.log(`[socket.io] Server: ${serverMessage}`)
  })

  socket.on(IO_DISCONNECT, () => {
    console.log('[socket.io] Disconnected.')
  })
}
/* eslint-enable no-console */

export default setUpSocket

What happens here should not be surprising if you understood well what we did on the server:

As soon as the client is connected, we log it in the browser console and join the room hello-1234 with a IO_CLIENT_JOIN_ROOM message.
We then send Hello! with a IO_CLIENT_HELLO message.
If the server sends us a IO_SERVER_HELLO message, we log it in the browser console.
We also log any disconnection.

🏁 Run yarn start and yarn dev:wds, open https://localhost:8000. Then, open your browser console, and also look at the terminal of your Express server. You should see the Websocket communication between your client and server.

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