by michaelt | 2025-12-29 14:07:57+00:00
I've always felt it's unrealistic to separate upfront architecture from implementation, because my experience is a lot of systems turn out to have requirements that are a lot more complex in reality than they might seem at first, even if you think quite hard about the requirements.

Imagine if you worked for an online retailer like Amazon, and you were assigned to architect a change so you can add free sample items into customers' orders. Take a moment to think about how you'd architect such a system, and what requirements you'd anticipate fulfilling. In the next paragraph, I'll tell you what the requirements are. Or you can skip the next paragraph, the size of which should tell you the requirements are more complex than they seem.

The samples must be items in the basket, so the warehouse knows to pick them. They must be added at the moment of checkout, because that's when the order contents and weight can change. Often a customer should receive a sample only once, even if they check out multiple orders - so a record should be kept of which customers have already been allocated a given sample. It should be possible to assign a customer the same sample multiple times, in which case they should receive it once per order until they've received the assigned number. Some samples go out of stock regularly, so the sample items should not be visible to the customer when they view their order on the website, but if shipped it should appear on their receipt to assure them they haven't been charged for it. Samples should never be charged for, even if their barcode is identical to something we normally charge for. If the warehouse is unable to ship the sample, the customer should not receive a missing-item apology or a separate shipment, and the record saying that customer has had that sample already should be decremented. If the warehouse can't ship anything except the sample, the entire orders should be delayed/cancelled, never shipping the sample alone. If a customer ordered three of an item and was assigned one sample item with the same barcode but the warehouse only had three items with that barcode in stock, something sensible should happen. One key type of 'sample' is first-time-customer gifts; internal documentation should explain that if the first order a customer places is on 14-day delivery and their second order is on faster delivery and arrives first, the first-order gift will be in the second order to arrive but that's expected because it's assigned at checkout. If the first-order-checked-out is cancelled, either by the customer or the warehouse, the new-customer gift should be added to the next order they check out. Some customers will want to opt out of free samples, those who do should not be assigned any samples. But the free sample system is also used by customer services to give out token apology gifts to customers whose orders have had problems, customers who've been promised a gift should receive it even if they've opted out of free samples.

No reasonable person can design such a system upfront, because things like 'opt-out mechanism sometimes shouldn't opt you out' and 'more than one definition of a customer's first order' do not occur to reasonable people.