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.

Practice

1. What is the main purpose of global dependencies in FastAPI?

easy

A. To create database models

B. To define route-specific logic only

C. To style the API responses

D. To run shared logic automatically for every request

Solution

1. Step 1: Understand what global dependencies do

   Global dependencies are functions or classes that run for every request to apply shared logic.

2. Step 2: Identify their main use

   They help with tasks like authentication, database connections, or validations that apply to all routes.

3. Final Answer:

   To run shared logic automatically for every request -> Option D

4. Quick Check:

   Global dependencies = shared logic for all requests [OK]

Hint: Global dependencies run once per request for all routes [OK]

Common Mistakes:

• Thinking global dependencies are for styling
• Confusing global with route-specific dependencies
• Assuming they create database models

2. Which of the following is the correct way to add a global dependency in FastAPI?

easy

A. app = FastAPI(dependencies=[Depends(common_dependency)])

B. app = FastAPI(global_dep=Depends(common_dependency))

C. app = FastAPI(dependency=common_dependency())

D. app = FastAPI(use_global=common_dependency)

Solution

1. Step 1: Recall FastAPI global dependency syntax

   Global dependencies are passed as a list to the 'dependencies' parameter when creating the FastAPI app.

2. Step 2: Match the correct syntax

   app = FastAPI(dependencies=[Depends(common_dependency)]) uses 'dependencies=[Depends(common_dependency)]', which is the correct pattern.

3. Final Answer:

   app = FastAPI(dependencies=[Depends(common_dependency)]) -> Option A

4. Quick Check:

   Global dependencies use dependencies=[Depends(...)] [OK]

Hint: Use dependencies=[Depends(...)] in FastAPI() constructor [OK]

Common Mistakes:

• Using wrong parameter names like global_dep or use_global
• Calling the dependency function instead of passing Depends()
• Passing a single dependency without a list

3. Given this code snippet, what will be printed when a request is made to /items/42?

from fastapi import FastAPI, Depends

app = FastAPI()

def common_dep():
    print("Global dependency called")

@app.get("/items/{item_id}")
def read_item(item_id: int, dep=Depends(common_dep)):
    return {"item_id": item_id}

medium

A. No output printed because common_dep is not global

B. Global dependency called printed once per request

C. Global dependency called printed twice per request

D. SyntaxError due to missing global dependency

Solution

1. Step 1: Check how common_dep is used

   common_dep is used as a route dependency only on the read_item function, not as a global dependency.

2. Step 2: Understand when common_dep runs

   It runs only when the /items/{item_id} route is called, printing the message once per request to that route.

3. Final Answer:

   Global dependency called printed once per request -> Option B

4. Quick Check:

   Route dependency prints only when route is called, not global [OK]

Hint: Route dependencies run per route, not global unless set globally [OK]

Common Mistakes:

• Assuming common_dep is global when it's route-specific
• Expecting multiple prints per request
• Confusing global and route dependencies

4. Identify the error in this FastAPI app code that tries to use a global dependency:

from fastapi import FastAPI, Depends

def common_dep():
    print("Running global dependency")

app = FastAPI(dependencies=Depends(common_dep))

medium

A. common_dep is not defined before use

B. Missing parentheses when calling FastAPI()

C. dependencies parameter expects a list, not a single Depends

D. Depends cannot be used as a global dependency

Solution

1. Step 1: Check the dependencies parameter type

   The 'dependencies' argument expects a list of Depends instances, not a single Depends object.

2. Step 2: Identify the fix

   Wrap Depends(common_dep) inside a list: dependencies=[Depends(common_dep)]

3. Final Answer:

   dependencies parameter expects a list, not a single Depends -> Option C

4. Quick Check:

   dependencies=[Depends(...)] requires a list [OK]

Hint: Always pass dependencies as a list, even if one item [OK]

Common Mistakes:

• Passing Depends(...) directly without list
• Defining dependency after app creation
• Confusing Depends usage with function call

5. You want to add a global dependency that checks user authentication and also logs request info. Which is the best way to combine these in FastAPI?

hard

A. Add both auth check and logging as separate global dependencies in the dependencies list

B. Add auth check as global dependency and logging inside each route

C. Create one dependency function that calls both auth check and logging, then add it globally

D. Use middleware for auth and global dependency for logging

Solution

1. Step 1: Understand global dependencies list

   FastAPI allows multiple global dependencies by passing a list to the dependencies parameter.

2. Step 2: Combine auth and logging as separate dependencies

   Adding both as separate Depends in the list keeps concerns separated and runs both for every request.

3. Step 3: Evaluate other options

   Create one dependency function that calls both auth check and logging, then add it globally mixes concerns in one function, B is inconsistent, D mixes middleware and dependencies unnecessarily.

4. Final Answer:

   Add both auth check and logging as separate global dependencies in the dependencies list -> Option A

5. Quick Check:

   Multiple global dependencies = dependencies=[Depends(...), Depends(...)] [OK]

Hint: Use a list of Depends for multiple global dependencies [OK]

Common Mistakes:

• Combining unrelated logic in one dependency
• Using middleware instead of dependencies for all tasks
• Adding some logic only per route, not globally