Broadcasting to multiple clients in FastAPI - Performance & Optimization

Performance: Broadcasting to multiple clients

MEDIUM IMPACT

This concept affects the responsiveness and scalability of real-time updates sent to multiple clients, impacting interaction speed and server load.

Sending real-time messages to multiple WebSocket clients

FastAPI

await asyncio.gather(*(client.send_text(message) for client in clients))

Sends messages concurrently, reducing total wait time and improving responsiveness for all clients.

📈 Performance GainNon-blocking sends reduce INP latency, scales better with many clients

Sending real-time messages to multiple WebSocket clients

FastAPI

for client in clients:
    await client.send_text(message)

Sequentially sending messages causes delays as each send waits for the previous to finish, increasing latency for all clients.

📉 Performance CostBlocks event loop per client, increasing INP latency linearly with client count

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Sequential send in loop	N/A (server-side)	N/A	N/A	[X] Bad
Concurrent send with asyncio.gather	N/A (server-side)	N/A	N/A	[OK] Good

Rendering Pipeline

Broadcasting messages to clients involves server-side event loop scheduling and network I/O, which affects how quickly clients receive updates and can render changes.

→Server Event Loop

→Network I/O

→Client Rendering

⚠️ BottleneckServer Event Loop blocking during sequential sends

Core Web Vital Affected

INP

This concept affects the responsiveness and scalability of real-time updates sent to multiple clients, impacting interaction speed and server load.

Optimization Tips

1Avoid sequential blocking calls when sending messages to multiple clients.

2Use asynchronous concurrency (e.g., asyncio.gather) to send messages simultaneously.

3Monitor network timing in DevTools to ensure messages are delivered promptly.

Performance Quiz - 3 Questions

What is the main performance issue with sending messages to multiple clients sequentially in FastAPI?

A It blocks the event loop causing higher latency for all clients

B It increases the bundle size of the application

C It causes layout shifts in the browser

D It reduces the number of connected clients

DevTools: Network

How to check: Open DevTools Network panel, filter WebSocket frames, observe message delivery timing to multiple clients.

What to look for: Look for delays between messages sent to clients; shorter and simultaneous delivery indicates better performance.