Startup and shutdown events in FastAPI - Performance & Optimization

Performance: Startup and shutdown events

MEDIUM IMPACT

This concept affects the initial server readiness time and graceful shutdown, impacting how fast the app starts serving requests and releases resources.

Running heavy blocking code during startup

FastAPI

import asyncio
from fastapi import FastAPI
app = FastAPI()

@app.on_event("startup")
async def startup_event():
    await asyncio.sleep(0)  # Non-blocking placeholder
    print("App started")

Using async non-blocking code allows the server to start quickly without delay.

📈 Performance GainStartup completes immediately, improving LCP and reducing server downtime.

Running heavy blocking code during startup

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.on_event("startup")
def startup_event():
    import time
    time.sleep(5)  # Blocking delay
    print("App started")

Blocking the event loop during startup delays server readiness, increasing time before it can handle requests.

📉 Performance CostBlocks server startup for 5 seconds, delaying LCP and user interaction.

Performance Comparison

Pattern	Blocking Behavior	Startup Delay	Shutdown Delay	Verdict
Blocking synchronous startup code	Blocks event loop	Delays startup by seconds	N/A	[X] Bad
Non-blocking async startup code	Non-blocking	Immediate startup	N/A	[OK] Good
No resource cleanup on shutdown	N/A	N/A	Prolonged shutdown, leaks	[X] Bad
Async resource cleanup on shutdown	Non-blocking	N/A	Fast, clean shutdown	[OK] Good

Rendering Pipeline

Startup and shutdown events run outside the browser rendering pipeline but affect server readiness and resource cleanup, indirectly impacting user experience by controlling server availability.

→Server Initialization

→Resource Management

⚠️ BottleneckBlocking synchronous code during startup or shutdown delays server availability.

Optimization Tips

1Avoid blocking synchronous code in startup events to reduce server readiness delay.

2Use asynchronous code for startup and shutdown handlers to keep the server responsive.

3Always clean up resources properly during shutdown to prevent leaks and slow shutdown.

Performance Quiz - 3 Questions

What is the main performance issue with blocking synchronous code in FastAPI startup events?

A It delays server readiness and blocks handling requests

B It increases CSS paint time

C It causes layout shifts in the browser

D It reduces JavaScript bundle size

DevTools: Network and Console panels

How to check: Use Network panel to see when server starts responding; check Console for startup logs. During shutdown, monitor server logs and resource release messages.

What to look for: Look for delays in server response after startup and any errors or warnings during shutdown indicating resource leaks.