WebSocket Full Duplex Communication Explained

WebSocket Full Duplex Communication Explained | Generated by AI

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Question: How does WebSocket work?

Answer:

WebSocket is a communication protocol that provides full-duplex (two-way), persistent connection between a client (usually browser) and a server over a single, long-lived TCP connection.

Here’s how WebSocket actually works, step by step:

1. The WebSocket Handshake (HTTP → WebSocket upgrade)

Everything starts with a normal HTTP request — this is the only way browsers allow us to switch to WebSocket.

Client → Server

GET /chat HTTP/1.1
Host: server.example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13
Origin: https://client.example.com

Important headers:

Upgrade: websocket — I want to switch to WebSocket
Connection: Upgrade — please don’t treat this as normal HTTP
Sec-WebSocket-Key — random base64 value (browser generates it)
Sec-WebSocket-Version: 13 — current standard version

Server must reply with:

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=

101 Switching Protocols = “okay, switching now”
Sec-WebSocket-Accept = special hash:
base64( SHA-1( client-key + "258EAFA5-E914-47DA-95CA-C5AB0DC85B11" ) )

After this response — the connection is no longer HTTP. It is now a WebSocket connection on the same TCP socket.

2. WebSocket framing (after handshake)

Once upgraded, both sides can send frames at any time.

Most common frame types:

Opcode	Meaning	Typical use
0x1	Text frame	JSON, chat messages
0x2	Binary frame	images, audio, protobuf, etc.
0x8	Connection close	polite goodbye
0x9	Ping	keep-alive check
0xA	Pong	reply to ping

Small real-world frame example (text message “Hello”):

FIN=1, Opcode=0x1 (text)
Mask=1 (client → server messages are always masked)
Payload length = 5
Masking-key: 4 random bytes
Payload: "Hello" XORed with the 4-byte mask (repeated)

→ Server unmasks it using the same key → gets “Hello”

3. Key properties of WebSocket

Property	WebSocket	HTTP polling / long-polling
Connection	persistent, single TCP	new connection each time
Direction	full-duplex (both ways anytime)	usually client → server request
Overhead per message	~2–14 bytes	200–800+ bytes (HTTP headers)
Latency	very low	high (new TCP + HTTP overhead)
Browser support	excellent (all modern browsers)	works everywhere

4. Typical lifecycle in JavaScript

// Client side
const ws = new WebSocket("wss://server.example.com/chat");

ws.onopen = () => {
  console.log("Connected!");
  ws.send(JSON.stringify({ type: "join", user: "alice" }));
};

ws.onmessage = (event) => {
  const data = JSON.parse(event.data);
  console.log("Received:", data);
};

ws.onclose = () => console.log("Disconnected");
ws.onerror = (err) => console.error("Error", err);

// Later
ws.send("Hello everyone!");
ws.close(1000, "User left");

5. Most common server implementations (2025)

Node.js → ws, uWebSockets.js (very fast), Socket.IO (adds fallback + features)
Python → websockets, FastAPI + WebSockets, Django Channels
Go → gorilla/websocket, native since Go 1.22 is very good
Java → Spring WebSocket, Jakarta WebSocket
Rust → tokio-tungstenite, warp, axum

Quick summary — WebSocket in one sentence

WebSocket takes one normal HTTP request, upgrades it to a long-lived bidirectional binary-safe channel, adds very little framing overhead, and then lets both client and server send messages at any time without asking permission again.

References:

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