Why databases persist data in FastAPI - See It in Action

Why Databases Persist Data

📖 Scenario: You are building a simple FastAPI app to store and retrieve user notes. This app will show how data is saved persistently using a database instead of temporary memory.

🎯 Goal: Create a FastAPI app that saves notes in a dictionary and then add a configuration to simulate saving notes persistently. Finally, implement a route to add notes and a route to get all saved notes.

📋 What You'll Learn

Create a dictionary called notes_db to store notes with keys as note IDs and values as note texts

Add a variable called next_id starting at 1 to assign unique IDs to notes

Write a POST route /add_note that accepts a note text and saves it in notes_db with the current next_id, then increments next_id

Write a GET route /notes that returns all saved notes from notes_db

💡 Why This Matters

🌍 Real World

Many web apps need to save user data like notes, messages, or profiles so it stays available after the app restarts or users come back later.

💼 Career

Understanding how to persist data is essential for backend developers building APIs and services that manage user data reliably.

1

Create the initial data structure

Create a dictionary called notes_db initialized as empty and a variable called next_id set to 1 to hold note IDs.

FastAPI

# Create notes_db dictionary and next_id variable
# Your code here

Hint

Use notes_db = {} to create an empty dictionary and next_id = 1 to start IDs from 1.

2

Add configuration variable

Add a FastAPI app instance called app by importing FastAPI and creating app = FastAPI().

FastAPI

notes_db = {}
next_id = 1

# Create FastAPI app instance
# Your code here

Hint

Import FastAPI and create an app instance with app = FastAPI().

3

Implement core logic to add notes

Write a POST route /add_note using @app.post("/add_note") that accepts a JSON body with a text field. Inside the function, add the note text to notes_db with the current next_id as key, then increment next_id by 1. Return the note ID in the response.

FastAPI

from fastapi import FastAPI

notes_db = {}
next_id = 1

app = FastAPI()

# Add POST route to add notes
# Your code here

Hint

Use @app.post("/add_note") decorator and an async function that reads JSON from the request. Use global next_id to modify the variable.

4

Add route to get all notes

Write a GET route /notes using @app.get("/notes") that returns the entire notes_db dictionary.

FastAPI

from fastapi import FastAPI, Request

notes_db = {}
next_id = 1

app = FastAPI()

@app.post("/add_note")
async def add_note(request: Request):
    global next_id
    data = await request.json()
    text = data.get("text")
    notes_db[next_id] = text
    note_id = next_id
    next_id += 1
    return {"note_id": note_id}

# Add GET route to return all notes
# Your code here

Hint

Use @app.get("/notes") decorator and return the notes_db dictionary directly.

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