Async database with databases library in FastAPI - Performance & Optimization

Performance: Async database with databases library

HIGH IMPACT

This affects how quickly the server can handle database queries without blocking other requests, improving responsiveness and throughput.

Handling database queries in a FastAPI app

FastAPI

import databases
import sqlalchemy
from fastapi import FastAPI

db = databases.Database('sqlite:///test.db')

app = FastAPI()

@app.on_event('startup')
async def startup():
    await db.connect()

@app.on_event('shutdown')
async def shutdown():
    await db.disconnect()

@app.get('/items')
async def read_items():
    query = 'SELECT * FROM items'
    results = await db.fetch_all(query)
    return results

Async calls free the event loop during database operations, allowing other requests to be processed concurrently.

📈 Performance GainNon-blocking queries improve throughput and reduce request latency

Handling database queries in a FastAPI app

FastAPI

import databases
import sqlalchemy
from fastapi import FastAPI

db = databases.Database('sqlite:///test.db')

app = FastAPI()

@app.get('/items')
def read_items():
    query = 'SELECT * FROM items'
    results = db.fetch_all(query)
    return results

Using synchronous database calls blocks the async event loop, causing delays in handling other requests.

📉 Performance CostBlocks event loop during query, increasing response time and reducing concurrency

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Synchronous DB calls in FastAPI	N/A	N/A	N/A	[X] Bad
Async DB calls with databases library	N/A	N/A	N/A	[OK] Good

Rendering Pipeline

Async database calls do not directly affect browser rendering but improve server response time, which reduces time to first byte and interaction delays.

→Server Processing

→Network Response

⚠️ BottleneckServer Processing when synchronous blocking occurs

Core Web Vital Affected

INP

This affects how quickly the server can handle database queries without blocking other requests, improving responsiveness and throughput.

Optimization Tips

1Always use async database calls in async frameworks to avoid blocking.

2Connect and disconnect the database asynchronously during app startup and shutdown.

3Avoid mixing synchronous DB calls inside async endpoints to keep event loop free.

Performance Quiz - 3 Questions

Why is using async database calls important in a FastAPI app?

A It automatically caches all queries.

B It reduces the size of the database.

C It prevents blocking the event loop, improving concurrency and responsiveness.

D It makes the database run faster internally.

DevTools: Network

How to check: Open DevTools, go to Network tab, make a request to the API endpoint, and observe the response time.

What to look for: Look for lower response times and no long blocking periods indicating server-side delays.