WebSocket endpoint creation in FastAPI - Step-by-Step Execution

Concept Flow - WebSocket endpoint creation

Client connects to WebSocket URL

↓

Server accepts connection

↓

Server listens for messages

↓

Client sends message

→Server receives message

↓

Server processes message

↓

Connection stays open or closes

↓

End

This flow shows how a WebSocket connection is established, messages are exchanged, and the connection is maintained or closed.

Execution Sample

FastAPI

from fastapi import FastAPI, WebSocket
app = FastAPI()

@app.websocket("/ws")
async def websocket_endpoint(websocket: WebSocket):
    await websocket.accept()
    data = await websocket.receive_text()
    await websocket.send_text(f"Message text was: {data}")

This code creates a WebSocket endpoint that accepts a connection, receives a text message, and sends back a response with the received message.

Execution Table

Step	Action	Evaluation	Result
1	Client connects to /ws	Connection request received	Server prepares to accept
2	Server calls websocket.accept()	Connection accepted	WebSocket connection established
3	Server waits for message	Waiting for client message	Paused until message arrives
4	Client sends message 'Hello'	Message received	data = 'Hello'
5	Server processes message	Formats response string	Response = 'Message text was: Hello'
6	Server sends response	Response sent to client	Client receives response
7	Connection remains open or closes	Depends on client/server	Connection stays or ends

💡 Execution stops when connection closes or client disconnects

Variable Tracker

Variable	Start	After Step 4	After Step 5	Final
websocket	WebSocket object	Connected WebSocket	Connected WebSocket	Connected or closed
data	None	'Hello'	'Hello'	'Hello'
response	None	None	'Message text was: Hello'	'Message text was: Hello'

Key Moments - 3 Insights

Why do we need to call websocket.accept()?

The server must accept the WebSocket connection explicitly; without calling websocket.accept(), the connection is not established. See execution_table step 2.

What happens if the client sends no message?

The server waits at await websocket.receive_text() until a message arrives, so the code pauses at step 3 until the client sends something.

Can the connection stay open after sending a response?

Yes, WebSocket connections stay open for continuous communication unless closed by client or server, as shown in step 7.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'data' after step 4?

A 'Hello'

B None

C 'Message text was: Hello'

D WebSocket object

💡 Hint

Check the variable_tracker row for 'data' at 'After Step 4'

At which step does the server accept the WebSocket connection?

A Step 1

B Step 4

C Step 2

D Step 6

💡 Hint

Look at execution_table step describing websocket.accept()

If the client never sends a message, what happens at step 3?

A Server sends a default message

B Server waits indefinitely for a message

C Server closes connection immediately

D Server throws an error

💡 Hint

Refer to key_moments about waiting for client message

Concept Snapshot

WebSocket endpoint creation in FastAPI:
- Use @app.websocket decorator with URL path
- Accept connection with await websocket.accept()
- Receive messages with await websocket.receive_text()
- Send messages with await websocket.send_text()
- Connection stays open for ongoing communication
- Close connection when done or on error

Full Transcript

This visual execution trace shows how to create a WebSocket endpoint in FastAPI. The client connects to the server's WebSocket URL. The server accepts the connection explicitly using websocket.accept(). Then the server waits for a message from the client. When the client sends a message, the server receives it and processes it. The server sends back a response message to the client. The connection can stay open for more messages or close when communication ends. Variables like 'data' hold the received message, and 'response' holds the message sent back. Key points include the need to accept the connection and that the server waits for messages asynchronously. This step-by-step flow helps beginners understand how WebSocket communication works in FastAPI.