Week 8 Session: The Fix That Isn't Safe

How do you decide whether an available fix is acceptable, not just whether it makes the symptom go away? The diagnosis this week is deliberately easy to reach; the risk is entirely in the fix.

19 April 2027

Before the session#

The diagnosis in this week’s system is cheap to establish on purpose, so attention shifts to a different question: what else could the obvious fix disturb?

The incident#

Popular items in fulfil occasionally show negative available stock. The diagnosis takes little effort to reach: when an order is cancelled and then immediately resubmitted, the cancellation path re-allocates inventory before it restores the stock the original order had reserved, so the resubmitted order allocates against stock that was never actually freed. The obvious fix is to call the stock-restoration function earlier in the cancellation path, before re-allocation runs.

The question before the patch#

That same restoration function is also called, deliberately, from the partial-refund path — where it does not run immediately. During high-traffic sales, refund-driven restocking is batched overnight on purpose, so that available-stock counts don’t fluctuate faster than the reconciliation job can keep up with. Moving restoration earlier in one caller’s path is a one-line change; whether it also changes when the other caller’s stock becomes available, and whether that’s tolerable, is not something the symptom or the one-line diff will tell you.

In the session#

Before applying the fix, you state what it’s meant to do, name at least one other path or invariant it could plausibly disturb — the refund flow’s deferred restocking is one candidate, not the only possible one — and design a check for that risk before you make the change. You apply the smallest version of the fix that resolves the cancellation bug, then check both the original symptom and the risk you identified, and you say plainly what you still don’t know about other effects.

Afterwards#

Verifying that this week’s fix actually addressed the diagnosed cause, rather than just removed the symptom, is next week’s problem.