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Version: 4.x

Ending notes

Final server code​

index.js
const express = require('express');
const { createServer } = require('node:http');
const { join } = require('node:path');
const { Server } = require('socket.io');
const sqlite3 = require('sqlite3');
const { open } = require('sqlite');
const { availableParallelism } = require('node:os');
const cluster = require('node:cluster');
const { createAdapter, setupPrimary } = require('@socket.io/cluster-adapter');

if (cluster.isPrimary) {
  const numCPUs = availableParallelism();
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork({
      PORT: 3000 + i
    });
  }

  return setupPrimary();
}

async function main() {
  const db = await open({
    filename: 'chat.db',
    driver: sqlite3.Database
  });

  await db.exec(`
    CREATE TABLE IF NOT EXISTS messages (
      id INTEGER PRIMARY KEY AUTOINCREMENT,
      client_offset TEXT UNIQUE,
      content TEXT
    );
  `);

  const app = express();
  const server = createServer(app);
  const io = new Server(server, {
    connectionStateRecovery: {},
    adapter: createAdapter()
  });

  app.get('/', (req, res) => {
    res.sendFile(join(__dirname, 'index.html'));
  });

  io.on('connection', async (socket) => {
    socket.on('chat message', async (msg, clientOffset, callback) => {
      let result;
      try {
        result = await db.run('INSERT INTO messages (content, client_offset) VALUES (?, ?)', msg, clientOffset);
      } catch (e) {
        if (e.errno === 19 /* SQLITE_CONSTRAINT */ ) {
          callback();
        } else {
          // nothing to do, just let the client retry
        }
        return;
      }
      io.emit('chat message', msg, result.lastID);
      callback();
    });

    if (!socket.recovered) {
      try {
        await db.each('SELECT id, content FROM messages WHERE id > ?',
          [socket.handshake.auth.serverOffset || 0],
          (_err, row) => {
            socket.emit('chat message', row.content, row.id);
          }
        )
      } catch (e) {
        // something went wrong
      }
    }
  });

  const port = process.env.PORT;

  server.listen(port, () => {
    console.log(`server running at http://localhost:${port}`);
  });
}

main();

Final client code​

index.html
<!DOCTYPE html>
<html>
  <head>
    <meta name="viewport" content="width=device-width,initial-scale=1.0">
    <title>Socket.IO chat</title>
    <style>
      body { margin: 0; padding-bottom: 3rem; font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif; }

      #form { background: rgba(0, 0, 0, 0.15); padding: 0.25rem; position: fixed; bottom: 0; left: 0; right: 0; display: flex; height: 3rem; box-sizing: border-box; backdrop-filter: blur(10px); }
      #input { border: none; padding: 0 1rem; flex-grow: 1; border-radius: 2rem; margin: 0.25rem; }
      #input:focus { outline: none; }
      #form > button { background: #333; border: none; padding: 0 1rem; margin: 0.25rem; border-radius: 3px; outline: none; color: #fff; }

      #messages { list-style-type: none; margin: 0; padding: 0; }
      #messages > li { padding: 0.5rem 1rem; }
      #messages > li:nth-child(odd) { background: #efefef; }
    </style>
  </head>
  <body>
    <ul id="messages"></ul>
    <form id="form" action="">
      <input id="input" autocomplete="off" /><button>Send</button>
    </form>
    <script src="/socket.io/socket.io.js"></script>
    <script>
      let counter = 0;
  
      const socket = io({
        ackTimeout: 10000,
        retries: 3,
        auth: {
          serverOffset: 0
        }
      });
  
      const form = document.getElementById('form');
      const input = document.getElementById('input');
      const messages = document.getElementById('messages');
  
      form.addEventListener('submit', (e) => {
        e.preventDefault();
        if (input.value) {
          const clientOffset = `${socket.id}-${counter++}`;
          socket.emit('chat message', input.value, clientOffset);
          input.value = '';
        }
      });
  
      socket.on('chat message', (msg, serverOffset) => {
        const item = document.createElement('li');
        item.textContent = msg;
        messages.appendChild(item);
        window.scrollTo(0, document.body.scrollHeight);
        socket.auth.serverOffset = serverOffset;
      });
    </script>
  </body>
</html>

Homework​

Here are some ideas to improve the application:

  • Broadcast a message to connected users when someone connects or disconnects.
  • Add support for nicknames.
  • Don’t send the same message to the user that sent it. Instead, append the message directly as soon as they press enter.
  • Add “{user} is typing” functionality.
  • Show who’s online.
  • Add private messaging.
  • Share your improvements!

Getting this example​

You can find it on GitHub here.

git clone https://github.com/socketio/chat-example.git

Next steps​

Please check out: