What is SQL Server latch vs spinlock vs lock, and how are they different internally?

Locks control transaction concurrency, latches protect in-memory structures, and spinlocks protect ultra-short critical sections.

SQL Server uses three synchronization primitives at different layers. Locks are transaction-level and respect isolation levels (row/page/table). Latches are lightweight protections for internal structures like pages and allocation maps. Spinlocks are extremely low-level CPU constructs used when contention is expected to be very short; threads spin instead of sleeping. Understanding all three is essential for diagnosing performance issues that are not purely query-related.