Fixing bugs in Event Sourcing is hard, for real?

Every system ends up with bad data. Some of it comes from integrations, some from users doing things nobody predicted, and a good part of it comes from us, because we shipped a change that looked fine in review.

This is about what we can do on Tuesday morning, when someone from support writes: “the price on this reservation looks wrong”.

The bug

Imagine we’re working on a hotel reservations system. Pricing there is fiddly: rate plans get revised, extras are added after booking, and the city sets the tourist tax on its own schedule.

Let’s say that the tourist tax used to be a single value in configuration, 4.50 per person per night, multiplied by nights and guests. That was enough for years, because the rate never moved.

Then the city announced an increase to 7.00 from 1 April. It appeared that one value can’t express that: a stay from 30 March to 2 April has nights on both sides of the boundary. So we added rates with validity periods and a lookup to resolve them, and shipped it on 12 March.…