WebSocket authentication in FastAPI - Performance & Optimization

Performance: WebSocket authentication

MEDIUM IMPACT

This affects the initial connection setup time and ongoing message responsiveness in WebSocket communication.

Authenticating users on WebSocket connections

FastAPI

async def websocket_endpoint(websocket: WebSocket, token: str):
    user = await authenticate_token(token)  # Authenticate once during handshake
    if not user:
        await websocket.close()
        return
    await websocket.accept()
    while True:
        data = await websocket.receive_text()
        await websocket.send_text(f"Hello {user.name}")

Authenticate once during connection setup, reducing repeated checks and improving message responsiveness.

📈 Performance GainSingle authentication cost upfront, resulting in faster message handling and lower INP

Authenticating users on WebSocket connections

FastAPI

async def websocket_endpoint(websocket: WebSocket):
    await websocket.accept()
    while True:
        data = await websocket.receive_text()
        user = await authenticate_token(data)  # Authenticate on every message
        if not user:
            await websocket.close()
            break
        await websocket.send_text(f"Hello {user.name}")

Authenticating on every message adds latency and CPU load, causing slower message handling and higher INP.

📉 Performance CostBlocks message processing for each message, increasing input delay and CPU usage

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Authenticate on every message	N/A (WebSocket, no DOM)	N/A	N/A	[X] Bad
Authenticate once during handshake	N/A	N/A	N/A	[OK] Good

Rendering Pipeline

WebSocket authentication occurs during the handshake phase before the connection is established. Proper authentication avoids blocking message processing later.

→Connection Setup

→Message Processing

⚠️ BottleneckRepeated authentication during message processing causes input delay and CPU overhead

Core Web Vital Affected

INP

This affects the initial connection setup time and ongoing message responsiveness in WebSocket communication.

Optimization Tips

1Authenticate once during the WebSocket handshake, not on every message.

2Cache user info after authentication to speed up message handling.

3Use DevTools Network panel to monitor WebSocket handshake and message latency.

Performance Quiz - 3 Questions

What is the main performance benefit of authenticating once during the WebSocket handshake?

A Decreases CSS paint cost

B Reduces message processing delay by avoiding repeated authentication

C Improves DOM rendering speed

D Reduces bundle size

DevTools: Network

How to check: Open DevTools Network panel, filter by WS, observe WebSocket handshake timing and message latency.

What to look for: Long handshake time or high latency between messages indicates inefficient authentication.

Practice

1. What is the main purpose of WebSocket authentication in FastAPI?

easy

A. To speed up the WebSocket connection process

B. To confirm the user's identity before allowing the WebSocket connection

C. To encrypt the WebSocket messages automatically

D. To log all WebSocket messages for debugging

Solution

1. Step 1: Understand WebSocket authentication role

   Authentication ensures only authorized users can connect to the WebSocket.

2. Step 2: Identify the main goal in FastAPI context

   FastAPI uses authentication to confirm user identity before opening the connection.

3. Final Answer:

   To confirm the user's identity before allowing the WebSocket connection -> Option B

4. Quick Check:

   Authentication = Confirm identity [OK]

Hint: Authentication means confirming user identity before connection [OK]

Common Mistakes:

• Confusing authentication with encryption
• Thinking authentication speeds up connection
• Assuming authentication logs messages

2. Which of the following is the correct way to accept a WebSocket connection with token authentication in FastAPI?

easy

A. await websocket.accept() without checking token

B. Call await websocket.close() before checking token

C. Use await websocket.send_text() before accepting

D. Check token, then call await websocket.accept() if valid

Solution

1. Step 1: Understand WebSocket connection flow

   You must check the token first to decide if connection is allowed.

2. Step 2: Accept connection only if token is valid

   Call await websocket.accept() only after confirming token validity.

3. Final Answer:

   Check token, then call await websocket.accept() if valid -> Option D

4. Quick Check:

   Accept after token check = C [OK]

Hint: Accept connection only after validating token [OK]

Common Mistakes:

• Accepting connection before token check
• Closing connection before checking token
• Sending messages before accepting connection

3. Given this FastAPI WebSocket code snippet, what will happen if the token is invalid?

async def websocket_endpoint(websocket: WebSocket):
    token = websocket.query_params.get('token')
    if token != 'secret123':
        await websocket.close(code=1008)
    else:
        await websocket.accept()
        await websocket.send_text('Welcome!')

medium

A. The connection closes immediately with code 1008

B. The connection accepts and sends 'Welcome!' anyway

C. The server raises a runtime error

D. The connection stays open but no message is sent

Solution

1. Step 1: Check token validation logic

   If token is not 'secret123', the code calls await websocket.close(code=1008).

2. Step 2: Understand WebSocket close behavior

   Calling close immediately ends the connection with the given code.

3. Final Answer:

   The connection closes immediately with code 1008 -> Option A

4. Quick Check:

   Invalid token triggers close(1008) [OK]

Hint: Invalid token triggers immediate close with code 1008 [OK]

Common Mistakes:

• Assuming connection accepts anyway
• Expecting a runtime error
• Thinking connection stays open silently

4. Identify the error in this FastAPI WebSocket authentication code:

async def websocket_endpoint(websocket: WebSocket):
    token = websocket.headers.get('Authorization')
    if token != 'Bearer secret123':
        await websocket.accept()
        await websocket.send_text('Unauthorized')
        await websocket.close()
    else:
        await websocket.accept()
        await websocket.send_text('Welcome')

medium

A. Using headers.get instead of query_params.get

B. Not closing connection on invalid token

C. Accepting connection before sending 'Unauthorized' message

D. Missing await before websocket.close()

Solution

1. Step 1: Analyze connection flow on invalid token

   The code calls await websocket.accept() before sending 'Unauthorized' and closing.

2. Step 2: Understand best practice for unauthorized access

   It is better to close connection immediately without accepting it to avoid opening unauthorized connection.

3. Final Answer:

   Accepting connection before sending 'Unauthorized' message -> Option C

4. Quick Check:

   Unauthorized should close without accept [OK]

Hint: Do not accept connection before rejecting unauthorized users [OK]

Common Mistakes:

• Accepting connection before rejecting
• Ignoring connection close on invalid token
• Confusing headers and query parameters

5. You want to authenticate WebSocket connections in FastAPI using a token passed as a query parameter. Which approach correctly handles authentication and rejects unauthorized users cleanly?

hard

A. Check token from query params; if invalid, call await websocket.close(1008) before accept; else accept connection

B. Accept connection first, then check token; if invalid, send 'Unauthorized' message and close

C. Ignore token and accept all connections; handle authorization in message handlers

D. Check token in headers only; ignore query parameters

Solution

1. Step 1: Understand token location and validation

   Token is passed as query parameter, so check websocket.query_params for token.

2. Step 2: Handle unauthorized users before accepting connection

   If token is invalid, close connection immediately with code 1008 to reject cleanly.

3. Step 3: Accept connection only if token is valid

   Only call await websocket.accept() after confirming token validity.

4. Final Answer:

   Check token from query params; if invalid, call await websocket.close(1008) before accept; else accept connection -> Option A

5. Quick Check:

   Reject before accept with close(1008) = A [OK]

Hint: Reject unauthorized before accept using close(1008) [OK]

Common Mistakes:

• Accepting connection before token check
• Ignoring query parameters for token
• Handling authorization after connection open