Connection lifecycle management in FastAPI - Step-by-Step Execution

Concept Flow - Connection lifecycle management

Start Request

↓

Open Connection

↓

Process Request

↓

Close Connection

↓

End Request

This flow shows how FastAPI opens a connection at the start of a request, processes it, then closes the connection at the end.

Execution Sample

FastAPI

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

app = FastAPI()

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

This code manages a database connection lifecycle by opening a session before a request and closing it after.

Execution Table

Step	Action	Connection State	Yielded Value	Notes
1	Request received	No connection	None	Start handling request
2	Call get_db()	Open connection	db session object	Connection opened and yielded
3	Process endpoint using db	Connection open	db session object	Use connection for queries
4	Request processing done	Connection open	db session object	Ready to close connection
5	Exit get_db() generator	Connection closed	None	Connection closed in finally block
6	Response sent	No connection	None	Request fully handled

💡 Request ends after connection is closed and response is sent

Variable Tracker

Variable	Start	After Step 2	After Step 5	Final
db	None	Session object (open)	None (closed)	None (closed)

Key Moments - 2 Insights

Why does the connection stay open during request processing?

Because the connection is yielded by the generator at Step 2 and only closed after the request finishes at Step 5, allowing the endpoint to use it.

What ensures the connection is closed even if an error occurs?

The finally block in the generator function always runs after yielding, closing the connection regardless of errors, as shown in Step 5.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the connection opened?

A Step 1

B Step 4

C Step 2

D Step 5

💡 Hint

Check the 'Connection State' column in the execution table at Step 2

At which step does the connection close?

A Step 5

B Step 4

C Step 3

D Step 6

💡 Hint

Look for 'Connection closed' in the 'Connection State' column

If the finally block was removed, what would happen?

A Connection would never open

B Connection might stay open after request ends

C Connection would close earlier

D No change in connection lifecycle

💡 Hint

Refer to Step 5 where connection is closed in finally block

Concept Snapshot

FastAPI connection lifecycle:
- Open connection before request (yield in dependency)
- Use connection during request
- Close connection after request (finally block)
- Ensures resources free even on errors
- Use generator dependencies for clean lifecycle

Full Transcript

In FastAPI, connection lifecycle management means opening a connection at the start of a request and closing it at the end. This is often done using a generator function dependency that yields a connection object. When the request starts, FastAPI calls the generator, opening the connection and yielding it to the endpoint. The endpoint uses this connection to process the request. After the request finishes, FastAPI continues the generator, which runs the finally block to close the connection. This ensures connections do not stay open and resources are freed properly, even if errors occur during processing.