Including routers in main app in FastAPI - Performance & Optimization

Performance: Including routers in main app

MEDIUM IMPACT

This affects the initial server startup time and the routing lookup speed during request handling.

Organizing API endpoints in a FastAPI application

FastAPI

from fastapi import FastAPI, APIRouter

items_router = APIRouter()

@items_router.get('/items')
def read_items():
    return {'items': []}

users_router = APIRouter()

@users_router.get('/users')
def read_users():
    return {'users': []}

app = FastAPI()
app.include_router(items_router)
app.include_router(users_router)

Routes are grouped logically in routers and included in the main app, improving code organization and allowing FastAPI to optimize routing internally.

📈 Performance GainRouting lookup is more efficient with modular routers; startup time is better managed; code is easier to maintain.

Organizing API endpoints in a FastAPI application

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get('/items')
def read_items():
    return {'items': []}

@app.get('/users')
def read_users():
    return {'users': []}

All routes are defined directly on the main app, which can make the app large and routing slower as the number of endpoints grows.

📉 Performance CostRouting lookup time increases linearly with number of routes; can cause slower request handling for large apps.

Performance Comparison

Pattern	Routing Table Size	Startup Time	Request Routing Speed	Verdict
All routes on main app	Large (all routes in one table)	Longer (registers all routes at once)	Slower (linear search through routes)	[X] Bad
Routes organized in routers	Smaller per router, combined efficiently	Shorter (modular registration)	Faster (optimized routing lookup)	[OK] Good

Rendering Pipeline

When the FastAPI app starts, it registers all routes from included routers into its internal routing table. During a request, the routing system matches the URL path to the correct endpoint handler.

→Startup Initialization

→Routing Lookup

→Request Handling

⚠️ BottleneckRouting Lookup stage can become slower if too many routes are registered directly without modular grouping.

Optimization Tips

1Group related routes using APIRouter to keep routing tables manageable.

2Include routers in the main app to improve routing lookup speed.

3Avoid defining all routes directly on the main app to prevent slower request handling.

Performance Quiz - 3 Questions

What is a main performance benefit of including routers in a FastAPI main app?

A Improves routing lookup speed by grouping routes

B Reduces network latency between client and server

C Decreases database query time

D Automatically caches responses

DevTools: Network and Logs

How to check: Use server logs to measure startup time and response time for requests; use profiling tools to check routing lookup performance.

What to look for: Look for lower startup time and faster response times when using routers; check logs for routing errors or delays.

Practice

1. What is the main purpose of using include_router in a FastAPI application?

easy

A. To handle user authentication automatically.

B. To automatically generate HTML templates for the app.

C. To connect the app to a database.

D. To split the app into smaller, manageable parts for better organization.

Solution

1. Step 1: Understand the role of routers in FastAPI

   Routers help organize routes by grouping related endpoints together.

2. Step 2: Recognize what include_router does

   It adds these grouped routes into the main app, keeping code clean and modular.

3. Final Answer:

   To split the app into smaller, manageable parts for better organization. -> Option D

4. Quick Check:

   Router inclusion = app organization [OK]

Hint: Routers help organize routes; include_router adds them to main app [OK]

Common Mistakes:

• Thinking include_router generates HTML templates
• Confusing routers with database connections
• Assuming include_router handles authentication

2. Which of the following is the correct way to include a router named user_router with prefix /users in a FastAPI app named app?

easy

A. app.add_router(user_router, prefix='/users')

B. app.include_router(user_router, prefix='/users')

C. app.include(user_router, prefix='/users')

D. app.router_include(user_router, prefix='/users')

Solution

1. Step 1: Recall the correct method name for adding routers

   The method is include_router, not add_router or others.

2. Step 2: Check the syntax for prefix argument

   The prefix is passed as a keyword argument prefix='/users'.

3. Final Answer:

   app.include_router(user_router, prefix='/users') -> Option B

4. Quick Check:

   Method name is include_router with prefix argument [OK]

Hint: Use include_router with prefix keyword to add routers [OK]

Common Mistakes:

• Using wrong method names like add_router
• Passing prefix as positional argument
• Misspelling include_router

3. Given this code snippet, what will be the URL path to access the get_items endpoint?

from fastapi import FastAPI, APIRouter

app = FastAPI()
router = APIRouter()

@router.get('/items')
async def get_items():
    return {'items': [1, 2, 3]}

app.include_router(router, prefix='/api')

medium

A. /api/items

B. /items

C. /router/items

D. /api/router/items

Solution

1. Step 1: Identify the route path in the router

   The endpoint is defined at /items inside the router.

2. Step 2: Understand the prefix added when including the router

   The router is included with prefix /api, so the full path is /api/items.

3. Final Answer:

   /api/items -> Option A

4. Quick Check:

   Router prefix + endpoint path = full URL [OK]

Hint: Full path = prefix + router path [OK]

Common Mistakes:

• Ignoring the prefix when accessing the endpoint
• Assuming router name appears in URL
• Confusing endpoint path with prefix

4. What is wrong with this FastAPI code snippet?

from fastapi import FastAPI, APIRouter

app = FastAPI()
router = APIRouter()

@router.get('/hello')
async def say_hello():
    return {'message': 'Hello'}

app.include_router(router, prefix='/greet')
app.include_router(router, prefix='/greet')

medium

A. Including the same router twice with the same prefix causes duplicate routes.

B. The router must be included before defining endpoints.

C. The prefix argument must be omitted when including routers.

D. The router variable should be named app_router instead of router.

Solution

1. Step 1: Check how many times the router is included

   The router is included twice with the same prefix /greet.

2. Step 2: Understand the effect of including the same router twice

   This causes duplicate routes, which can lead to errors or unexpected behavior.

3. Final Answer:

   Including the same router twice with the same prefix causes duplicate routes. -> Option A

4. Quick Check:

   Duplicate router inclusion = duplicate routes [OK]

Hint: Avoid including the same router twice with same prefix [OK]

Common Mistakes:

• Thinking router must be included before endpoints
• Believing prefix is mandatory or must be omitted
• Confusing variable naming with functionality

5. You have two routers: user_router with prefix /users and item_router with prefix /items. You want to include both in your FastAPI app and add tags for automatic docs grouping. Which code snippet correctly includes both routers with tags?

hard

A. app.include_router(user_router, prefix='/users', tags='users')
app.include_router(item_router, prefix='/items', tags='items')

B. app.include_router(user_router, prefix='/users')
app.include_router(item_router, prefix='/items')
app.tags = ['users', 'items']

C. app.include_router(user_router, prefix='/users', tags=['users'])
app.include_router(item_router, prefix='/items', tags=['items'])

D. app.include_router(user_router, tags=['users'])
app.include_router(item_router, tags=['items'])

Solution

1. Step 1: Check how to add tags when including routers

   Tags are passed as a list to the tags argument in include_router.

2. Step 2: Verify prefix and tags usage for both routers

   Each router is included with its prefix and tags as lists, e.g., tags=['users'].

3. Final Answer:

   app.include_router(user_router, prefix='/users', tags=['users']) app.include_router(item_router, prefix='/items', tags=['items']) -> Option C

4. Quick Check:

   Tags must be lists in include_router calls [OK]

Hint: Pass tags as list when including routers for docs grouping [OK]

Common Mistakes:

• Passing tags as string instead of list
• Setting tags on app instead of routers
• Omitting tags argument entirely