Ran out of time before I had a clean solution.
Start by clarifying requirements and constraints, then propose a design using a stack of transaction layers where each layer maintains a map of key-value changes. Explain how begin pushes a new layer, commit merges the top layer into the one below, and rollback discards the top layer. Discuss trade-offs between simplicity and performance, and consider edge cases like nested transactions and read-your-writes consistency.
Pro tip: Demonstrate awareness of real-world database semantics by discussing isolation levels and how your in-memory design could be extended to support them, showing you think beyond the basic implementation.
Ask about expected operations (get, set, delete), transaction nesting, concurrency, and performance requirements. Confirm whether reads should see uncommitted changes within the same transaction.
Propose a stack of transaction layers, each containing a hash map for key-value changes. The base layer holds committed data, and each begin pushes a new layer.
Explain how begin creates a new layer, commit merges the top layer into the previous one (handling conflicts), and rollback simply pops the top layer.
Describe how get searches from the top layer down to the base, and set/delete write to the top layer. Mention read-your-writes consistency within a transaction.
Talk about time/space complexity, alternatives like copy-on-write or undo logs, and how to extend for concurrency, isolation levels, or persistence.
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