Did you write the service that applies these transactions? What exactly did it do?

Yes, I wrote a service in Python that used transactions to ensure data integrity when working with a database. For example, the service processed orders in an online store: when creating an order, it was necessary to simultaneously deduct the item from the stock and record the order information. To avoid data inconsistency, operations were wrapped in a transaction — if something went wrong, all changes were rolled back.

An example using SQLAlchemy:

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

engine = create_engine('postgresql://user:password@localhost/db')
Session = sessionmaker(bind=engine)

session = Session()
try:
    # deduct the product from stock
    product = session.query(Product).filter_by(id=product_id).one()
    product.stock -= quantity

    # create an order
    order = Order(user_id=user_id, product_id=product_id, quantity=quantity)
    session.add(order)

    session.commit()
except Exception:
    session.rollback()
    raise
finally:
    session.close()

Thus, the service guaranteed that either all operations would be applied or none.