Implement an in-memory key-value store with transactions: begin, commit, rollback, and nesting.

A classic build screen: a key-value store where writes inside a transaction can be committed or thrown away, and transactions nest. The trap is mutating the base store directly. Here is the overlay-stack design that makes rollback O(1).

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