Compare Locks and MVCC in a Versioned Store

A key–value store keeps multiple versions and supports versioned reads, writes, and deletion while preserving earlier history. During a discussion of race conditions, a mutex and a read/write lock are proposed, followed by multi-version concurrency control (MVCC). Explain how MVCC could support concurrent access and how it compares with the locking approaches. Distinguish application-visible version numbers from the concurrency mechanism's snapshot or commit versions. Identify the lookup and deletion semantics that must be clarified before implementing the complete store. ### What a Strong Answer Covers - Stable reader views, safe publication of updates, and coordination between concurrent writers. - The difference between retaining value history and providing a correct concurrency protocol. - Memory reclamation and visibility of deletions to old and new readers. - Workload-dependent costs, with no blanket claim that MVCC is lock-free or always faster. ### Follow-up Questions - Can two writers overwrite each other's changes even when all values are immutable? - When is it safe to reclaim an older version that a long-running reader might still use?