Design an In-Memory Database With Transactions

Problem Implement an in-memory key-value database supporting set(key, value), get(key), and delete(key), with records that can hold multiple fields, plus nested transactions via begin(), commit(), and rollback().

Requirements

• set, get, delete operate on keys (and named fields within a record).
• begin() opens a transaction; transactions may be nested.
• rollback() discards changes made since the most recent begin(), restoring the prior state.
• commit() permanently applies all currently open transactions' changes.

Areas to design

• How to snapshot or log changes so a nested rollback restores exactly the right state.
• Whether commit folds one level into its parent or flushes every open level.
• Isolation: what get sees while a transaction is open.
• Edge cases: rollback/commit with no open transaction; deleting a key that was set in an outer transaction.