Concurrency: Document-Level Locking / MVCC Simulation

Problem

Simulate document-level concurrency: multiple writers update different documents concurrently without blocking; write-write conflicts on the same doc retry.

Example

writers on different docs proceed; same-doc conflict -> retry

Constraints

• Snapshot isolation semantics

What evaluators look for

Reason about MVCC + document-level locking, snapshot isolation, and conflict retries.