Build an in-memory database: set/get/delete → field operations and prefix scan → TTL → backup and restore at timestamps
The OpenAI FDE signature question, reported near-verbatim for two years: four levels in ~60 minutes, where L4 silently breaks every naive L3 implementation. The lazy-expiry design that survives all four levels, with the pacing plan.
Updated Sep 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
The OpenAI FDE signature question, reported near-verbatim for two years: four levels in ~60 minutes, where L4 silently breaks every naive L3 implementation. The lazy-expiry design that survives all four levels, with the pacing plan.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
