System design interview question
"Design a system that consumes order lifecycle events — created, paid, packed, shipped, delivered, cancelled — from many upstream services and maintains the current state of every order. The events arrive out of order, they get duplicated, and occasionally they arrive hours late. Millions of orders a day."
event-processing correctness under at-least-once, out-of-order delivery — the everyday reality of any message-driven system. It is a focused question with no room to hide: the candidate either understands idempotency, ordering, and late data, or they do not. It is a common mid-level screen for exactly that reason.
On Practick another engineer interviews you live for 60 minutes on a question from this bank. They have the full interviewer guide beside the call: the answers to your clarifying questions, the numbers to hand out, a ladder of follow-ups and a scoring rubric. You get the model solution once the session settles. It is free: interview someone for an hour and you have earned your own.
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