WebSocket

WebSocket is a protocol that keeps a single connection open so the client and server can send messages to each other at any time. Normal HTTP is request-response: the client asks, the server answers, the connection closes. WebSocket breaks that pattern and enables real-time, bidirectional communication.

The problem it solves

With plain HTTP, the server cannot push data to the client. To get updates, clients often poll: ask again and again.

HTTP polling (wasteful):
  Client: any news?  Server: no
  Client: any news?  Server: no
  Client: any news?  Server: yes! here it is

WebSocket (efficient):
  Client <==== open connection ====> Server
  Server: here is an update (whenever it happens)

The handshake

A WebSocket starts life as an HTTP request that asks to “upgrade” the connection.

GET /chat HTTP/1.1
Host: example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade

After the 101 response, the TCP connection stays open and both sides exchange messages freely.

Using it

const socket = new WebSocket("wss://example.com/chat");

socket.onmessage = (event) => {
  console.log("server says:", event.data);
};

socket.onopen = () => socket.send("hello");

WebSocket vs HTTP polling

                  Polling        WebSocket
Direction         client asks    both ways
Latency           high           low
Overhead          repeated reqs  one connection
Server push       no             yes

Advantages

• Real-time, low-latency, two-way messages.
• Less overhead than constant polling.
• The server can push without being asked.

Disadvantages

• A persistent connection uses server resources per client.
• Scaling many open connections needs care.
• Must handle reconnects when the network drops.
• Not ideal for simple request-response, where REST is simpler.

When to use it

Use WebSocket for chat, live dashboards, collaborative editing, multiplayer games, and live notifications. For occasional data fetches, stick with regular HTTP requests.