Build an in-memory key-value database, then extend it across stages: TTL, transactions, snapshots.

The canonical multi-round build screen: a simple key-value store that grows new requirements every stage (TTL, transactions, scans). The signal is not stage 1, it is whether your code absorbs stage 4 without a rewrite. Here is how to design for it.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.