Database session management in FastAPI - Step-by-Step Execution

Concept Flow - Database session management

Start API request

↓

Create DB session

↓

Use session in endpoint

↓

Commit or rollback transaction

↓

Close DB session

↓

Return response

↓

End API request

This flow shows how a database session is created at the start of a request, used during the request, then committed or rolled back, and finally closed before sending the response.

Execution Sample

FastAPI

from fastapi import Depends
from sqlalchemy.orm import Session

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

This code creates a database session for each request, yields it for use, and ensures it closes after the request finishes.

Execution Table

Step	Action	Session State	Result
1	API request starts	No session	Prepare to create session
2	Create DB session with SessionLocal()	Session open	Session ready for queries
3	Yield session to endpoint	Session open	Endpoint uses session
4	Endpoint runs DB operations	Session open	Queries executed, changes staged
5	Commit or rollback transaction	Session open	Changes saved or discarded
6	Close DB session in finally block	Session closed	Resources freed
7	Return response to client	Session closed	Request complete

💡 Session closed after request ends to avoid resource leaks

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 6	Final
db	None	Session object	Session object (yielded)	Closed session	Closed session

Key Moments - 3 Insights

Why do we use 'yield' in the get_db function?

Using 'yield' allows the session to be used during the request and ensures the 'finally' block runs after, closing the session as shown in steps 3 and 6 of the execution_table.

What happens if we forget to close the session?

If the session is not closed (step 6), database connections stay open, causing resource leaks and possible errors in future requests.

When do we commit or rollback the transaction?

After endpoint operations (step 4), before closing the session (step 5), to save or discard changes made during the request.

Visual Quiz - 3 Questions

Look at the execution_table, what is the session state right after creating the DB session?

A Session open

B Session closed

C No session

D Session yielded

💡 Hint

Check Step 2 in the execution_table for session state after creation

At which step does the session get closed?

A Step 4

B Step 6

C Step 5

D Step 7

💡 Hint

Look for the step mentioning 'Close DB session' in the execution_table

If we remove the 'finally' block, what likely changes in the variable_tracker?

A db variable closes earlier at Step 4

B db variable becomes None after Step 3

C db variable stays as 'Session object' after Step 6

D db variable is never created

💡 Hint

Refer to variable_tracker and key_moments about session closing

Concept Snapshot

Database session management in FastAPI:
- Create session at request start
- Use 'yield' to provide session to endpoints
- Commit or rollback after DB operations
- Close session in 'finally' to free resources
- Prevents connection leaks and ensures safe DB access

Full Transcript

In FastAPI, database session management means creating a session when a request starts, using it during the request, then committing or rolling back changes, and finally closing the session to free resources. The get_db function uses 'yield' to provide the session to endpoints and ensures the session closes after the request finishes. This prevents resource leaks and keeps the database connections healthy. The session state changes from no session, to open, to closed as the request progresses. Closing the session is critical and happens in the 'finally' block. Forgetting to close the session can cause errors and resource exhaustion.

Practice

1. What is the main purpose of using a get_db function in FastAPI when working with databases?

easy

A. To create and close a database session for each request safely

B. To store user data permanently in memory

C. To handle HTTP requests directly without a database

D. To generate HTML templates for responses

Solution

1. Step 1: Understand the role of get_db

   The get_db function is designed to open a database session when a request starts and close it when the request ends.

2. Step 2: Recognize safe database session management

   This ensures that each request has its own session, preventing conflicts and resource leaks.

3. Final Answer:

   To create and close a database session for each request safely -> Option A

4. Quick Check:

   Database session management = create and close session [OK]

Hint: Remember: get_db opens and closes sessions per request [OK]

Common Mistakes:

• Thinking get_db stores data permanently
• Confusing get_db with HTTP request handling
• Assuming get_db generates HTML

2. Which of the following is the correct way to declare a dependency for a database session in a FastAPI route using Depends?

easy

A. def read_items(db = Depends(Session)):

B. def read_items(db: get_db = Session()):

C. def read_items(db: Session = get_db()):

D. def read_items(db: Session = Depends(get_db)):

Solution

1. Step 1: Understand FastAPI dependency injection syntax

   FastAPI uses Depends to inject dependencies like database sessions into route functions.

2. Step 2: Correct syntax for session injection

   The correct syntax is to type hint the parameter as Session and assign it Depends(get_db) to call the dependency function.

3. Final Answer:

   def read_items(db: Session = Depends(get_db)): -> Option D

4. Quick Check:

   Dependency injection = parameter: Type = Depends(function) [OK]

Hint: Use parameter: Type = Depends(function) for dependencies [OK]

Common Mistakes:

• Calling get_db() directly in parameter default
• Using Depends with a class instead of a function
• Swapping parameter and default values

3. Given this FastAPI route code snippet, what will be the output if the database session is correctly managed?

from fastapi import FastAPI, Depends
from sqlalchemy.orm import Session

app = FastAPI()

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

@app.get('/items')
def read_items(db: Session = Depends(get_db)):
    items = db.query(Item).all()
    return items

medium

A. An error because the session is not closed

B. An empty list because the query is missing

C. A list of all items from the database

D. A syntax error due to wrong yield usage

Solution

1. Step 1: Analyze the get_db function behavior

   The get_db function creates a session, yields it for use, then closes it safely after the request.

2. Step 2: Understand the route's database query

   The route uses the session to query all Item records and returns them as a list.

3. Final Answer:

   A list of all items from the database -> Option C

4. Quick Check:

   Yielded session + query = list of items [OK]

Hint: Yielded session allows safe query and close after use [OK]

Common Mistakes:

• Assuming session is not closed causing error
• Thinking yield causes syntax error
• Believing query returns empty without data

4. Identify the error in this FastAPI database session management code:

def get_db():
    db = Session()
    yield db
    db.close()

@app.post('/add')
def add_item(item: Item, db: Session = Depends(get_db)):
    db.add(item)
    db.commit()

medium

A. The item parameter should be inside get_db

B. The session is closed after yield, so it may not close if an exception occurs

C. The Depends is used incorrectly in the route

D. The db.commit() is missing

Solution

1. Step 1: Review session closing in get_db

   The db.close() is called after yield without a try-finally block, so if an exception happens, the session may never close.

2. Step 2: Understand proper session cleanup

   Using try-finally ensures the session closes even if errors occur during request handling.

3. Final Answer:

   The session is closed after yield, so it may not close if an exception occurs -> Option B

4. Quick Check:

   Session close needs try-finally for safety [OK]

Hint: Always use try-finally to close sessions safely [OK]

Common Mistakes:

• Ignoring try-finally for session cleanup
• Forgetting to commit changes
• Misplacing Depends usage

5. You want to ensure that your FastAPI app's database sessions are properly managed and that any changes are committed only if no exceptions occur. Which of the following get_db implementations best achieves this?

hard

A. def get_db():
db = Session()
try:
yield db
db.commit()
except:
db.rollback()
raise
finally:
db.close()

B. def get_db():
db = Session()
yield db
db.commit()
db.close()

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

D. def get_db():
db = Session()
yield db
db.rollback()
db.close()

Solution

1. Step 1: Understand transaction management needs

   We want to commit changes only if no errors occur, otherwise rollback to avoid partial changes.

2. Step 2: Analyze each get_db implementation

   def get_db(): db = Session() try: yield db db.commit() except: db.rollback() raise finally: db.close() uses try-except-finally to commit on success, rollback on error, and always close the session, which is the safest approach.

3. Final Answer:

   def get_db(): db = Session() try: yield db db.commit() except: db.rollback() raise finally: db.close() -> Option A

4. Quick Check:

   Commit on success, rollback on error, always close [OK]

Hint: Use try-except-finally to commit, rollback, and close sessions [OK]

Common Mistakes:

• Committing after yield without error handling
• Not rolling back on exceptions
• Closing session without try-finally