The problem

A shop sells home electronics online. On a normal day it takes about 30 orders a minute. Next Friday it runs a sale: 2 000 TVs at half price, announced to 400 000 newsletter readers at 9 a.m.

Last year’s sale went wrong in three ways. Two customers got the last TV of a model. One card was charged twice after a timeout. Confirmation emails arrived an hour late.

This exercise puts earlier katas together: stock that cannot go below zero, payments that cannot run twice, and work that leaves the checkout through a queue. Take 15 minutes for it.