WebSocket protocol concept in Node.js - Step-by-Step Execution

Concept Flow - WebSocket protocol concept

Client sends HTTP Upgrade request

↓

Server receives Upgrade request

↓

Server sends HTTP 101 Switching Protocols

↓

WebSocket connection established

↓

Bidirectional message exchange

↓

Connection close initiated by client or server

↓

Connection closed

This flow shows how a WebSocket connection starts with an HTTP upgrade, then allows two-way messages until closed.

Execution Sample

Node.js

const ws = new WebSocket('ws://localhost:8080');
ws.on('open', () => ws.send('Hello'));
ws.on('message', msg => console.log(msg));

This code connects to a WebSocket server, sends 'Hello' when open, and logs incoming messages.

Execution Table

Step	Action	Message Sent	Message Received	Connection State
1	Client sends HTTP Upgrade request	Upgrade request		Connecting
2	Server receives Upgrade request			Connecting
3	Server sends HTTP 101 Switching Protocols	101 Switching Protocols		Upgrading
4	Connection established			Open
5	Client sends 'Hello'	Hello		Open
6	Server sends 'Welcome'		Welcome	Open
7	Client sends 'How are you?'	How are you?		Open
8	Server sends 'Good, thanks!'		Good, thanks!	Open
9	Client initiates close	Close frame		Closing
10	Server acknowledges close		Close frame	Closed

💡 Connection closes after client and server exchange close frames.

Variable Tracker

Variable	Start	After Step 4	After Step 5	After Step 6	After Step 9	Final
Connection State	Connecting	Open	Open	Open	Closing	Closed
Last Message Sent	None	None	Hello	Hello	Close frame	Close frame
Last Message Received	None	None	None	Welcome	Good, thanks!	Close frame

Key Moments - 3 Insights

Why does the connection start as HTTP before switching to WebSocket?

Because WebSocket begins as a normal HTTP request with an Upgrade header (see Step 1 and 3 in execution_table), allowing servers and proxies to handle it like HTTP before switching protocols.

What does '101 Switching Protocols' mean in the process?

It is the server's response (Step 3) telling the client it agrees to switch from HTTP to WebSocket, enabling the full-duplex communication.

How do client and server know when to close the connection?

They send special 'Close frame' messages (Steps 9 and 10) signaling intent to close, ensuring both sides agree before closing.

Visual Quiz - 3 Questions

Look at the execution_table, what is the connection state after Step 4?

A Open

B Connecting

C Closing

D Closed

💡 Hint

Check the 'Connection State' column at Step 4 in execution_table.

At which step does the client send the first message after connection is open?

A Step 3

B Step 7

C Step 5

D Step 9

💡 Hint

Look for 'Client sends' actions in the 'Action' column in execution_table.

If the server never sends '101 Switching Protocols', what happens to the connection?

A Connection becomes Open anyway

B Connection stays in Connecting state

C Connection immediately closes

D Client sends Close frame

💡 Hint

Refer to Step 3 and the importance of the 101 response in execution_table.

Concept Snapshot

WebSocket starts as an HTTP request with an Upgrade header.
Server responds with 101 Switching Protocols to agree.
Connection becomes full-duplex for bidirectional messages.
Messages are sent as frames over the open connection.
Close frames signal connection termination by either side.

Full Transcript

The WebSocket protocol begins with the client sending an HTTP request asking to upgrade the connection. The server responds with a 101 Switching Protocols status to confirm the upgrade. Once established, the connection is open for two-way communication where client and server can send messages independently. When either side wants to end the connection, they send a close frame to signal termination. This process ensures a smooth transition from HTTP to a persistent, bidirectional WebSocket connection.