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

Need a 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

Need a 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

Need a 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

Need a hint?

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