What Is Socket.IO? A Deep Dive into Features, Versions, and Cross‑Language Implementations

What is Socket.IO

Socket.IO is a library that enables low‑latency, bi‑directional, event‑driven communication between client and server.

Official site:

https://socket.io

Version differences

Since its inception Socket.IO has released major versions 1.x, 2.x, 3.x and 4.x, each adding new features and performance improvements.

1.x : initial release with basic real‑time communication.

2.x : better error handling, protocol improvements, more stable connections.

3.x : higher performance, lower latency, TypeScript support, better framework integration.

4.x : WebSocket protocol updated for HTTP/2, modern JavaScript, API changes, dropped support for old Node.js versions.

Language implementations

Server side

JavaScript (Node.js) – https://github.com/socketio/socket.io JavaScript (Deno) – https://github.com/socketio/socket.io-deno Java – https://github.com/mrniko/netty-socketio Java – https://github.com/trinopoty/socket.io-server-java Python – https://github.com/miguelgrinberg/python-socketio Golang – https://github.com/googollee/go-socket.io Rust – https://github.com/Totodore/socketioxide PHP –

https://github.com/walkor/phpsocket.io

Client side

JavaScript (browser, Node.js, React Native) – installation steps, API, source code.

JavaScript (WeChat Mini‑Programs) – https://github.com/weapp-socketio/weapp.socket.io Java – https://github.com/socketio/socket.io-client-java C++ – https://github.com/socketio/socket.io-client-cpp Swift – https://github.com/socketio/socket.io-client-swift Dart – https://github.com/rikulo/socket.io-client-dart Python – https://github.com/miguelgrinberg/python-socketio.Net – https://github.com/doghappy/socket.io-client-csharp Rust – https://github.com/1c3t3a/rust-socketio Kotlin – https://github.com/icerockdev/moko-socket-io PHP –

https://github.com/ElephantIO/elephant.io

What Socket.IO is not

Socket.IO is not a pure WebSocket implementation.

Although it falls back to WebSocket when possible, it adds extra metadata to each packet, making direct WebSocket‑to‑Socket.IO connections incompatible.

// WARNING: the client will NOT be able to connect!
const socket = io("ws://echo.websocket.org");

Socket.IO is not suitable for background services in mobile apps.

The library keeps an open TCP connection, which can drain battery; use dedicated push services such as FCM instead.

Key features

HTTP long‑polling fallback

If a WebSocket connection cannot be established, Socket.IO automatically falls back to HTTP long‑polling, a feature still useful for users behind misconfigured proxies.

Automatic reconnection

Socket.IO includes a heartbeat mechanism that detects broken connections and automatically attempts reconnection with exponential back‑off.

Packet buffering

When the client disconnects, outgoing packets are buffered and sent once the connection is restored.

Acknowledgments (acks)

Events can be emitted with a callback that receives a response from the other side.

socket.emit("hello", "world", (response) => {
  console.log(response); // "got it"
});

A timeout can be added:

socket.timeout(5000).emit("hello", "world", (err, response) => {
  if (err) {
    // the other side did not acknowledge within the delay
  } else {
    console.log(response); // "got it"
  }
});

Broadcast

Servers can emit to all clients or to a specific room.

// to all connected clients
io.emit("hello");

// to all clients in the "news" room
io.to("news").emit("hello");

Multiplexing (namespaces)

Namespaces let you split application logic over a single shared connection, e.g., an admin channel.

io.on("connection", (socket) => { /* classic users */ });
io.of("/admin").on("connection", (socket) => { /* admin users */ });

Practical example – PHP server with PHPSocket.IO

Installation

composer require workerman/phpsocket.io

Server code (server.php)

<?php
declare(strict_types=1);
require_once '../vendor/autoload.php';
use Workerman\Worker;
use PHPSocketIO\SocketIO;

// create socket.io server listening on port 2024
$io = new SocketIO(2024);
$io->on('connection', function ($socket) use ($io) {
    echo "[x] new connection coming".PHP_EOL;
});
Worker::runAll();

Start the server:

php server.php start

Client HTML

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>phpsocket.io client</title>
</head>
<body>
  <script src="https://cdn.bootcss.com/socket.io/2.0.3/socket.io.js"></script>
  <script>
    var socket = io('http://127.0.0.1:2024');
    socket.on('connect', function() {
      console.log('connect success');
    });
  </script>
</body>
</html>

When the page loads, the server logs new connections.

Custom events

Beyond the built‑in connect, message, and disconnect events, developers can define their own events using emit and on.

<?php
require_once __DIR__.'/vendor/autoload.php';
use Workerman\Worker;
use PHPSocketIO\SocketIO;

$io = new SocketIO(2024);
$io->on('connection', function ($socket) use ($io) {
    $socket->on('chat message', function ($msg) use ($io) {
        $io->emit('chat message from server', $msg);
    });
});
Worker::runAll();

Client triggers the event:

<script src="//cdn.bootcss.com/socket.io/1.3.7/socket.io.js"></script>
<script>
var socket = io('http://127.0.0.1:2024');
socket.emit('chat message', 'this is the message...');
socket.on('chat message from server', function(msg){
  console.log('get message:' + msg + ' from server');
});
</script>

workerStart event

The workerStart callback runs once when the process starts, allowing you to set up additional workers, such as an HTTP endpoint for pushing messages.

require_once __DIR__.'/vendor/autoload.php';
use Workerman\Worker;
use PHPSocketIO\SocketIO;

$io = new SocketIO(9120);
$io->on('workerStart', function() use ($io) {
    $inner_http_worker = new Worker('http://0.0.0.0:9191');
    $inner_http_worker->onMessage = function($http_connection, $data) use ($io){
        if(!isset($_GET['msg'])) {
            return $http_connection->send('fail, $_GET["msg"] not found');
        }
        $io->emit('chat message', $_GET['msg']);
        $http_connection->send('ok');
    };
    $inner_http_worker->listen();
});
$io->on('connection', function($socket) use ($io){
    $socket->on('chat message', function($msg) use ($io){
        $io->emit('chat message from server', $msg);
    });
});
Worker::runAll();