The Scenario

Your FastAPI service is crashing with a cryptic error:

FATAL: remaining connection slots are reserved for non-replication superuser connections

Your database has a maximum of 100 connections, but your app keeps opening new ones without closing them. After processing 100 requests, your database refuses all new connections.

The Problem

Your code opens a new database connection for each user inside a loop. This is inefficient - the TCP handshake for each connection adds 10-50ms overhead. With 100 users, that's up to 5 seconds wasted just establishing connections!

The Goal

Implement connection pooling by reusing a single connection.

Requirements:

• Open the database connection once before the loop
• Reuse the same connection for all queries
• Close the connection after all processing is complete
• The test will verify that sqlite3.connect() is called exactly once

Note: We simulate with SQLite for simplicity, but this applies to Postgres, MySQL, etc.