- MongoDB's concurrency control uses multiple-granularity locking at the instance, database, and collection level. This allows finer-grained locking than previous approaches. - The storage engine handles concurrency control at lower levels like the document level, using either MVCC or locking depending on the engine. WiredTiger uses MVCC while MMAPv1 uses locking at the collection level. - Intents signal the intention to access lower levels without acquiring locks upfront, improving concurrency compared to directly acquiring locks. The lock manager enforces the locking protocol and ensures consistency.