Async database queries in FastAPI - Performance & Optimization

Performance: Async database queries

HIGH IMPACT

This concept affects how quickly the server can respond to requests by not blocking while waiting for database results.

Fetching data from a database in a web API endpoint

FastAPI

from fastapi import FastAPI
from databases import Database

app = FastAPI()
database = Database('sqlite:///test.db')

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

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

@app.get('/items')
async def read_items():
    query = 'SELECT * FROM items'
    items = await database.fetch_all(query)  # async non-blocking call
    return items

The async call does not block the event loop, allowing the server to handle other requests while waiting for the database.

📈 Performance GainNon-blocking query improves concurrency and reduces request latency under load

Fetching data from a database in a web API endpoint

FastAPI

from fastapi import FastAPI, Depends
from sqlalchemy.orm import Session

app = FastAPI()

def get_db():
    db = Session()
    try:
        yield db
    finally:
        db.close()

@app.get('/items')
def read_items(db: Session = Depends(get_db)):
    items = db.query(Item).all()  # synchronous blocking call
    return items

The synchronous database call blocks the server thread until the query finishes, reducing concurrency and increasing response time under load.

📉 Performance CostBlocks event loop during query, increasing response latency and reducing throughput

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Synchronous DB query	N/A (server-side)	N/A	N/A	[X] Bad
Async DB query	N/A (server-side)	N/A	N/A	[OK] Good

Rendering Pipeline

Async database queries affect the server-side response time, which impacts how quickly the browser receives data to render. The browser rendering pipeline starts only after the server sends the response.

→Server Processing

→Network Transfer

→Browser Rendering

⚠️ BottleneckServer Processing (waiting for database response)

Core Web Vital Affected

INP

This concept affects how quickly the server can respond to requests by not blocking while waiting for database results.

Optimization Tips

1Always use async database drivers and queries in async FastAPI endpoints.

2Avoid blocking calls inside async functions to keep the event loop free.

3Monitor server response times in DevTools Network tab to detect blocking.

Performance Quiz - 3 Questions

What is the main performance benefit of using async database queries in FastAPI?

A They prevent blocking the server event loop, improving concurrency.

B They reduce the size of the database.

C They automatically cache query results in the browser.

D They increase the number of database connections.

DevTools: Network

How to check: Open DevTools, go to the Network tab, reload the page, and inspect the timing of the API request to see server response time.

What to look for: Look for long waiting (TTFB) times indicating slow server response due to blocking database calls.