Connection pool exhausted during external HTTP call

The code that's running in prod

# checkout_api.py
# POST /checkout opens a DB session, loads the cart, calls the
# external billing service, then commits the result.

import requests
from sqlalchemy.orm import sessionmaker

SessionLocal = sessionmaker(bind=engine, autocommit=False)


def checkout(cart_id):
    session = SessionLocal()
    try:
        cart = session.query(Cart).get(cart_id)
        if cart is None or cart.empty():
            return {"error": "empty cart"}, 400

        # External call to billing service. Typical: 2-5 seconds.
        resp = requests.post(
            "https://billing.example.com/charge",
            json={"cart_id": cart_id, "amount": cart.total},
            timeout=10,
        )

        cart.charge_id = resp.json()["charge_id"]
        cart.status = "paid"
        session.commit()
        return {"status": "ok", "charge_id": cart.charge_id}
    finally:
        session.close()

Log dashboard (last 60 seconds)

Pick the root cause

Four candidates. Three are plausible but mistake symptom for cause. Pick what you'd write in the postmortem timeline.