Design and implement an in-memory key-value store that supports set, transactional begin, commit, and abort operations.

Focus on outlining the architecture of your in-memory key-value store, emphasizing the data structures you'll use to store key-value pairs and manage transactions. Discuss how you will handle concurrency, ensuring that operations like set and transaction management (begin, commit, abort) are thread-safe. Be prepared to explain your choice of algorithms for transaction management, such as two-phase commit or locking mechanisms, and how they impact performance and consistency. Additionally, consider edge cases like handling failures during transactions and how you will ensure data integrity.