Connection pooling in FastAPI - Step-by-Step Execution

Concept Flow - Connection pooling

App starts

↓

Create connection pool

↓

Request comes in

↓

Get connection from pool

↓

Use connection for query

↓

Return connection to pool

↓

Request ends

↓

Repeat for next request

↓

App stops -> close pool

The app creates a pool of reusable connections. Each request borrows a connection, uses it, then returns it for reuse.

Execution Sample

FastAPI

from fastapi import FastAPI
from databases import Database

app = FastAPI()
db = Database('sqlite:///test.db')

@app.on_event('startup')
async def startup():
    await db.connect()

@app.on_event('shutdown')
async def shutdown():
    await db.disconnect()

@app.get('/')
async def read_root():
    query = 'SELECT 1'
    return await db.fetch_one(query)

This FastAPI app uses a database connection pool to handle queries efficiently on each request.

Execution Table

Step	Event	Pool State	Connection Action	Result
1	App starts	Pool created, empty	No connection used yet	Ready to accept requests
2	Startup event	Pool initialized with connections	Connections opened and pooled	Pool ready with open connections
3	Request 1 arrives	Pool has free connections	Connection borrowed from pool	Connection assigned to request
4	Request 1 query	Pool minus 1 free connection	Connection used to run query	Query executed, result ready
5	Request 1 ends	Connection returned to pool	Connection marked free	Pool restored to previous state
6	Request 2 arrives	Pool has free connections	Connection borrowed from pool	Connection assigned to request
7	Request 2 query	Pool minus 1 free connection	Connection used to run query	Query executed, result ready
8	Request 2 ends	Connection returned to pool	Connection marked free	Pool restored to previous state
9	App shutdown	Pool closing	All connections closed	Pool destroyed, app stops

💡 App stops and closes all connections, ending the pool lifecycle.

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 6	After Step 8	Final
Pool free connections	0 (before startup)	N-1 (connection borrowed)	N (connection returned)	N-1 (connection borrowed)	N (connection returned)	0 (after shutdown)
Connection borrowed	None	1 connection	None	1 connection	None	None

Key Moments - 3 Insights

Why does the pool have fewer free connections during a request?

Because a connection is borrowed from the pool to handle the request, reducing the free count until it is returned (see execution_table steps 3 and 4).

What happens to connections when the app shuts down?

All connections in the pool are closed and the pool is destroyed to free resources (see execution_table step 9).

Why reuse connections instead of opening a new one each request?

Opening connections is slow and costly. Reusing from the pool saves time and resources, improving performance (concept_flow explanation).

Visual Quiz - 3 Questions

Look at the execution_table, what is the pool state after step 5?

A Pool is empty

B Pool has one connection borrowed

C Pool has all connections free

D Pool is closed

💡 Hint

Check the 'Pool State' column at step 5 in the execution_table.

At which step does the app close all connections?

A Step 5

B Step 9

C Step 7

D Step 3

💡 Hint

Look for the 'App shutdown' event in the execution_table.

If a request borrows a connection but never returns it, what happens to the pool free connections?

A They decrease and stay low

B They stay the same

C They increase

D They reset automatically

💡 Hint

Refer to variable_tracker for 'Pool free connections' during borrowing and returning.

Concept Snapshot

Connection Pooling in FastAPI:
- Create a pool of reusable DB connections at startup
- Borrow a connection per request
- Use connection for queries
- Return connection to pool after use
- Close all connections on shutdown
Improves performance by avoiding repeated connection setup.

Full Transcript

Connection pooling in FastAPI means the app creates a set of database connections at startup. When a request comes, it borrows one connection from this pool to run queries. After the request finishes, the connection is returned to the pool for reuse. This saves time and resources compared to opening a new connection each time. When the app stops, all connections are closed properly. The execution table shows each step: app start, pool creation, requests borrowing and returning connections, and app shutdown closing the pool. The variable tracker follows how many connections are free or borrowed over time. Key moments include understanding why connections are borrowed and returned, and what happens on shutdown. The visual quiz tests understanding of pool state changes and connection lifecycle. This approach helps FastAPI apps handle many requests efficiently by reusing database connections.