Why databases persist data in FastAPI - Performance Evidence

Performance: Why databases persist data

MEDIUM IMPACT

This concept affects how data storage impacts application responsiveness and load times when interacting with databases.

Saving user input data reliably in a FastAPI app

FastAPI

async def save_user(data):
    await db.insert_async(data)  # non-blocking async call
    return {'status': 'saved'}

Using async database calls keeps the event loop free, improving responsiveness and user experience.

📈 Performance Gainnon-blocking calls reduce input delay and improve INP

Saving user input data reliably in a FastAPI app

FastAPI

async def save_user(data):
    # Save data without async DB driver
    db.insert(data)  # blocking call
    return {'status': 'saved'}

Blocking database calls freeze the event loop, delaying responses and reducing app responsiveness.

📉 Performance Costblocks event loop causing slow response times and poor INP

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Blocking DB calls in FastAPI	0 (server-side)	0	0	[X] Bad
Async DB calls in FastAPI	0 (server-side)	0	0	[OK] Good

Rendering Pipeline

When a FastAPI app persists data, the request triggers network I/O and disk writes outside the browser. This affects the interaction phase as the server processes and stores data before responding.

→Network Request

→Server Processing

→Database I/O

⚠️ BottleneckDatabase I/O latency is the main bottleneck affecting response time.

Core Web Vital Affected

INP

This concept affects how data storage impacts application responsiveness and load times when interacting with databases.

Optimization Tips

1Use asynchronous database calls to avoid blocking the server event loop.

2Database persistence adds network and disk latency affecting interaction speed.

3Optimizing database I/O improves user input responsiveness (INP).

Performance Quiz - 3 Questions

Why does persisting data in a database affect user interaction speed?

A Because it changes the page layout causing reflows

B Because database writes add network and disk latency before response

C Because it increases CSS selector complexity

D Because it blocks browser painting

DevTools: Network

How to check: Open DevTools, go to Network tab, make a request that saves data, and observe the request timing and response time.

What to look for: Look for long server response times indicating slow database persistence affecting interaction speed.

Practice

1. Why do databases persist data in applications like FastAPI?

easy

A. To keep data safe even if the app stops or restarts

B. To make the app run faster

C. To delete old data automatically

D. To prevent users from accessing the app

Solution

1. Step 1: Understand what persistence means

   Persistence means data stays saved even after the program stops running.

2. Step 2: Connect persistence to databases in FastAPI

   Databases store data on disk, so FastAPI can retrieve it later, even after restarts.

3. Final Answer:

   To keep data safe even if the app stops or restarts -> Option A

4. Quick Check:

   Persistence means data stays saved [OK]

Hint: Persistence means data stays saved after app stops [OK]

Common Mistakes:

• Thinking databases speed up the app only
• Confusing persistence with data deletion
• Believing databases block user access

2. Which of the following is the correct way to save data to a database in FastAPI?

easy

A. Use a database session to add and commit the data

B. Print the data to the console

C. Store data in a local variable only

D. Use a global variable to hold data

Solution

1. Step 1: Identify how FastAPI interacts with databases

   FastAPI uses database sessions to add and commit data to save it permanently.

2. Step 2: Compare options for saving data

   Printing or using variables does not save data persistently; only committing via session does.

3. Final Answer:

   Use a database session to add and commit the data -> Option A

4. Quick Check:

   Commit data with session to save [OK]

Hint: Commit data with session to save persistently [OK]

Common Mistakes:

• Thinking printing saves data
• Using variables instead of database commit
• Skipping the commit step

3. Given this FastAPI code snippet, what will happen when the app restarts?

from fastapi import FastAPI
from sqlalchemy.orm import Session

app = FastAPI()

@app.post('/items/')
def create_item(session: Session, item: Item):
    session.add(item)
    session.commit()
    return item

medium

A. The items will be saved only in memory

B. The saved items will be lost after restart

C. The app will crash on restart

D. The saved items will still be available after restart

Solution

1. Step 1: Understand what session.commit() does

   Calling commit() saves data permanently to the database storage.

2. Step 2: Consider app restart effect on database data

   Since data is saved in the database, it remains after the app restarts.

3. Final Answer:

   The saved items will still be available after restart -> Option D

4. Quick Check:

   Committed data persists after restart [OK]

Hint: Committed data stays after app restarts [OK]

Common Mistakes:

• Confusing commit with temporary memory storage
• Assuming app restart clears database
• Thinking session.add alone saves data

4. What is wrong with this FastAPI code that tries to save data?

from fastapi import FastAPI

app = FastAPI()

@app.post('/users/')
def create_user(user: dict):
    user['id'] = 1
    return user

medium

A. It uses the wrong HTTP method

B. It does not save data to a database, so data is lost on restart

C. It has a syntax error in the function

D. It commits data twice

Solution

1. Step 1: Check if data is saved to a database

   The code only modifies and returns a dictionary; it does not save to any database.

2. Step 2: Understand consequence of no database saving

   Without saving to a database, data is lost when the app stops or restarts.

3. Final Answer:

   It does not save data to a database, so data is lost on restart -> Option B

4. Quick Check:

   No database save means no persistence [OK]

Hint: Data must be saved to database for persistence [OK]

Common Mistakes:

• Thinking returning dict saves data
• Confusing HTTP method with persistence
• Assuming syntax error when none exists

5. You want to ensure user data is saved permanently in FastAPI. Which approach correctly combines FastAPI and database persistence?

hard

A. Store user data in a global list variable inside the app

B. Save user data only in request body without database interaction

C. Use FastAPI with SQLAlchemy session to add and commit user data to a database

D. Print user data to console and rely on logs for storage

Solution

1. Step 1: Identify how to save data permanently

   Permanent saving requires writing data to a database, not just memory or logs.

2. Step 2: Match FastAPI with database usage

   FastAPI works well with SQLAlchemy sessions to add and commit data to databases.

3. Step 3: Eliminate incorrect options

   Global variables and printing do not persist data after app stops; request body alone is temporary.

4. Final Answer:

   Use FastAPI with SQLAlchemy session to add and commit user data to a database -> Option C

5. Quick Check:

   Database commit ensures permanent data [OK]

Hint: Use database session commit for permanent save [OK]

Common Mistakes:

• Using global variables for persistence
• Relying on request data without saving
• Thinking console logs save data