Capstone: The Payment Service Is Down

1. The Exercise

"It's your order flow from before: gateway, Checkout, Inventory, Payments, and the event consumers. At 2:00 PM, Payments stops responding: calls hang, then time out. Walk me through what happens, minute by minute, and what you'd have built so the answer isn't 'the site goes down.'"

This is the most common shape of a resilience question. The interviewer takes one concrete outage and points it at the design you just drew.

Before you read on, narrate it yourself. Tell the story twice. First with no protections at all. Then with this module's toolkit in place.