Broadcasting to multiple clients in FastAPI - Mini Project: Build & Apply

Broadcasting to Multiple Clients with FastAPI

📖 Scenario: You are building a simple chat server where messages sent by one client are broadcast to all connected clients in real time.This is like a group chat where everyone sees the messages instantly.

🎯 Goal: Create a FastAPI app that can broadcast messages to multiple connected WebSocket clients.Clients connect, send messages, and receive messages from others.

📋 What You'll Learn

Create a list to hold connected WebSocket clients

Add a helper variable to count connected clients

Write a function to broadcast messages to all clients

Complete the WebSocket endpoint to accept connections and broadcast messages

💡 Why This Matters

🌍 Real World

Real-time chat apps, live notifications, multiplayer games, and collaborative tools use broadcasting to keep all users updated instantly.

💼 Career

Understanding WebSocket broadcasting is key for backend developers working on interactive web applications and real-time communication features.

1

Create a list to hold connected WebSocket clients

Create a list called connected_clients and set it to an empty list to store WebSocket connections.

FastAPI

# Create a list called connected_clients to hold WebSocket connections
# Your code here

Hint

Think of this list as a waiting room where all connected clients stay until they disconnect.

2

Add a helper variable to count connected clients

Create a variable called client_count and set it to 0 to keep track of how many clients are connected.

FastAPI

connected_clients = []
# Create a variable called client_count and set it to 0
# Your code here

Hint

This variable helps us know how many people are in the chat room at any time.

3

Write a function to broadcast messages to all clients

Define an async function called broadcast_message that takes a message parameter and sends it to every WebSocket in connected_clients using await client.send_text(message) inside a for loop.

FastAPI

connected_clients = []
client_count = 0

# Define async function broadcast_message(message) to send message to all clients
# Your code here

Hint

This function is like a loudspeaker that repeats the message to everyone in the room.

4

Complete the WebSocket endpoint to accept connections and broadcast messages

Create a FastAPI app instance called app. Then write a WebSocket endpoint at /ws using @app.websocket("/ws"). Inside the endpoint function websocket_endpoint, accept the connection with await websocket.accept(), add the websocket to connected_clients, increase client_count by 1, then use a while True loop to receive messages with await websocket.receive_text() and call await broadcast_message(message). Use try and finally to remove the client from connected_clients and decrease client_count when the client disconnects.

FastAPI

connected_clients = []
client_count = 0

async def broadcast_message(message):
    for client in connected_clients:
        await client.send_text(message)

# Create FastAPI app and websocket endpoint /ws
# Your code here

Hint

This endpoint lets clients join the chat, send messages, and receive messages from others.

Practice

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

easy

A. To send the same message to multiple connected clients at once

B. To send a message to only one client at a time

C. To close all client connections simultaneously

D. To authenticate clients before connection

Solution

1. Step 1: Understand broadcasting concept

   Broadcasting means sending one message to many clients simultaneously.

2. Step 2: Compare options with broadcasting definition

   Only 'To send the same message to multiple connected clients at once' accurately describes broadcasting.

3. Final Answer:

   To send the same message to multiple connected clients at once -> Option A

4. Quick Check:

   Broadcasting = send to many clients [OK]

Hint: Broadcasting means sending one message to many clients [OK]

Common Mistakes:

• Confusing broadcasting with single client messaging
• Thinking broadcasting closes connections
• Mixing broadcasting with authentication

2. Which of the following is the correct way to send a message to all clients in a FastAPI WebSocket manager class?

easy

A. self.connections.send(message)

B. await self.connections.send_text(message)

C. for client in self.connections: client.send(message)

D. for client in self.connections: await client.send_text(message)

Solution

1. Step 1: Identify correct syntax for sending messages

   In FastAPI WebSocket, sending text requires await and send_text method on each client.

2. Step 2: Check each option for correct usage

   for client in self.connections: await client.send_text(message) loops through connections and awaits send_text, which is correct. Others miss await or use wrong methods.

3. Final Answer:

   for client in self.connections: await client.send_text(message) -> Option D

4. Quick Check:

   Use await and send_text per client [OK]

Hint: Use await and send_text inside a loop for all clients [OK]

Common Mistakes:

• Forgetting to await send_text
• Trying to send to all clients at once without loop
• Using send instead of send_text

3. Given this snippet inside a FastAPI WebSocket manager class:

async def broadcast(self, message: str):
    for connection in self.active_connections:
        await connection.send_text(message)

What happens if self.active_connections is empty when broadcast is called?

medium

A. An IndexError is raised

B. The server crashes with a runtime error

C. No messages are sent and no error occurs

D. The message is sent to a default client

Solution

1. Step 1: Understand for-loop behavior on empty list

   A for-loop over an empty list runs zero times and does nothing without error.

2. Step 2: Apply to broadcast method

   If active_connections is empty, the loop sends no messages but does not raise errors.

3. Final Answer:

   No messages are sent and no error occurs -> Option C

4. Quick Check:

   Empty list loop = no action, no error [OK]

Hint: Empty loops run zero times without errors [OK]

Common Mistakes:

• Assuming empty list causes errors
• Thinking a default client receives message
• Confusing IndexError with empty iteration

4. You have this code snippet in your FastAPI WebSocket manager:

async def broadcast(self, message: str):
    for connection in self.active_connections:
        connection.send_text(message)

Why does this code cause an error when broadcasting?

medium

A. Missing await before connection.send_text causes coroutine not awaited error

B. send_text is not a valid method on WebSocket connections

C. active_connections should be a dictionary, not a list

D. The for loop syntax is incorrect

Solution

1. Step 1: Identify async method usage

   send_text is an async method and must be awaited to run properly.

2. Step 2: Check code for await keyword

   The code calls send_text without await, causing coroutine not awaited error.

3. Final Answer:

   Missing await before connection.send_text causes coroutine not awaited error -> Option A

4. Quick Check:

   Async calls need await [OK]

Hint: Always await async send_text calls [OK]

Common Mistakes:

• Forgetting await on async methods
• Thinking send_text is synchronous
• Misunderstanding active_connections type

5. You want to broadcast messages to multiple clients but also remove disconnected clients automatically. Which approach best solves this in a FastAPI WebSocket manager?

hard

A. Use a global variable to track client count instead of connections

B. Wrap send_text in try-except; on exception remove client from active_connections

C. Clear active_connections list before every broadcast

D. Send messages without error handling; disconnected clients are ignored

Solution

1. Step 1: Understand client disconnection handling

   Disconnected clients cause send_text to raise exceptions when sending messages.

2. Step 2: Use try-except to catch exceptions and remove clients

   Wrapping send_text in try-except allows removing disconnected clients from active_connections.

3. Step 3: Evaluate other options

   Other approaches are inadequate: tracking client count with a global variable doesn't manage connections, sending without error handling lets exceptions propagate without cleanup, clearing the list discards active connections too.

4. Final Answer:

   Wrap send_text in try-except; on exception remove client from active_connections -> Option B

5. Quick Check:

   Handle errors to remove disconnected clients [OK]

Hint: Use try-except to remove disconnected clients during broadcast [OK]

Common Mistakes:

• Ignoring exceptions during send
• Clearing all clients mistakenly
• Tracking count instead of connections