Global dependencies in FastAPI - Performance & Optimization

Performance: Global dependencies

MEDIUM IMPACT

Global dependencies affect the server response time and resource usage by running shared logic once per request or application lifecycle.

Sharing common logic or resources across multiple routes

FastAPI

from fastapi import Depends, FastAPI

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

app = FastAPI(dependencies=[Depends(get_db)])

@app.get("/items/")
async def read_items(db=Depends()):
    return "Use shared db connection"

@app.get("/users/")
async def read_users(db=Depends()):
    return "Use shared db connection"

The database connection is created once per request globally and reused in all routes, reducing overhead.

📈 Performance Gainsingle resource initialization per request, lowering latency

Sharing common logic or resources across multiple routes

FastAPI

from fastapi import Depends, FastAPI

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

app = FastAPI()

@app.get("/items/")
async def read_items(db=Depends(get_db)):
    return db.query_items()

@app.get("/users/")
async def read_users(db=Depends(get_db)):
    return db.query_users()

The database connection is created and closed separately for each route call, causing repeated overhead.

📉 Performance Costtriggers multiple resource initializations per request, increasing latency

Performance Comparison

Pattern	Resource Initialization	Request Latency	Reusability	Verdict
Per-route dependency	Multiple times per request	Higher latency	Low	[X] Bad
Global dependency	Once per request	Lower latency	High	[OK] Good

Rendering Pipeline

Global dependencies run before route handlers, preparing shared resources once per request, reducing repeated setup work.

→Request Handling

→Dependency Injection

⚠️ BottleneckRepeated resource initialization in each route handler

Optimization Tips

1Declare shared dependencies globally on the FastAPI app to reduce repeated setup.

2Avoid creating expensive resources separately in each route handler.

3Use global dependencies to improve request handling speed and resource reuse.

Performance Quiz - 3 Questions

What is the main performance benefit of using global dependencies in FastAPI?

A They increase bundle size but improve caching

B They reduce client-side rendering time

C Shared setup runs once per request, reducing repeated overhead

D They eliminate the need for database connections

DevTools: Network

How to check: Open DevTools, go to Network tab, make repeated requests to different routes, and compare response times.

What to look for: Look for consistent and lower response times when using global dependencies versus per-route dependencies.