System design interview question
"Add a Favorites feature to a food delivery app. Users tap a heart on a restaurant or a dish; they see a Favorites list; and favourited items should be surfaced elsewhere in the app. 100 million users. Seems simple — design it and tell me where it gets interesting."
Whether the candidate can find the depth in a deliberately simple-sounding feature. The interesting parts are: the write path is trivial but the read path is everywhere in the app (so it becomes a latency and caching problem), favourited entities change or disappear, and the "is this favourited?" check appears on every restaurant card in every list.
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