Stock is the least forgiving thing you can put in a database. A number that is wrong by one is not a rounding error to the warehouse; it is an order that ships short and a customer who stops trusting the panel.
The system I worked on kept its truth in a chain: a movement row was written, a trigger updated the warehouse total, and a second trigger pushed that up into the product. It was elegant right up to the day a bundled product entered the catalogue and the chain had to run the other way too — the components decide the bundle, and the bundle must never decide itself.
The fix was not clever code. It was admitting that a bundle is a different kind of thing from a product, saying so in the schema, and then giving the recalculation a session flag so it could not fall into its own loop. Once the model was honest the queries got shorter, not longer.
That is the part I would tell anyone starting on a commerce system. You are not choosing column types. You are deciding what your business is allowed to mean, and every screen built on top inherits that decision whether it likes it or not.
