Exercise: Three Correctness Bugs in an Order Pipeline

1. The Pipeline, as Written

A team has an order flow built from three services and two topics. It passes every test, it has been in production for four months, and finance has opened a ticket because a small number of customers were charged twice last week. A different ticket says two orders were paid for and never shipped.

Here is the whole thing. Read it before the sections below, because the exercise is to find the defects yourself.

The order service

def create_order(request):
    with db.transaction():
        order = db