Accepting connections in FastAPI - Performance & Optimization

Performance: Accepting connections

MEDIUM IMPACT

This affects how quickly the server can start handling incoming user requests, impacting initial response time and throughput.

Handling incoming HTTP connections in FastAPI

FastAPI

import uvicorn

if __name__ == "__main__":
    uvicorn.run("app:app", host="0.0.0.0", port=8000, workers=4, loop="uvloop")

Multiple workers and uvloop improve concurrency and event loop efficiency, accepting connections faster.

📈 Performance GainHandles more connections concurrently, reducing wait time and improving LCP.

Handling incoming HTTP connections in FastAPI

FastAPI

import uvicorn

if __name__ == "__main__":
    uvicorn.run("app:app", host="0.0.0.0", port=8000, workers=1, loop="asyncio")

Using a single worker and default asyncio loop can limit concurrent connection handling, causing slower acceptance under load.

📉 Performance CostLimits concurrent connections, increasing request queue and delaying response start.

Performance Comparison

Pattern	Concurrent Connections	Request Queue	Response Start Delay	Verdict
Single worker, default loop	Low	High under load	High delay	[X] Bad
Multiple workers, uvloop	High	Low	Low delay	[OK] Good

Rendering Pipeline

Accepting connections happens before the browser rendering pipeline but affects when the server can start sending data, impacting Largest Contentful Paint (LCP).

→Network

→Server Processing

→Response Start

⚠️ BottleneckServer's ability to accept and process connections quickly

Core Web Vital Affected

LCP

This affects how quickly the server can start handling incoming user requests, impacting initial response time and throughput.

Optimization Tips

1Use multiple worker processes to handle more connections concurrently.

2Prefer uvloop over the default asyncio loop for better event loop performance.

3Keep connections alive to reduce overhead of new connection setups.

Performance Quiz - 3 Questions

What improves FastAPI's ability to accept more connections quickly?

A Disabling keep-alive connections

B Using a single worker with default asyncio loop

C Increasing the number of worker processes

D Serving static files from the same worker

DevTools: Network

How to check: Open DevTools > Network tab, reload page, and check the 'Waiting (TTFB)' time for the main document request.

What to look for: Long 'Waiting' times indicate slow connection acceptance or server response start delays.