Lock modes are an essential aspect of managing concurrency and data integrity in Microsoft SQL Server. They determine how resources (such as tables, rows, or pages) are locked and controlled during transactions. SQL Server offers various lock modes to balance concurrency and data consistency:
- Shared Lock (SHARED):
- Multiple transactions can acquire shared locks on a resource concurrently.
- Allows for simultaneous read access to the locked resource.
- Prevents other transactions from acquiring an exclusive lock on the same resource.
- Shared locks are compatible with other shared locks but incompatible with exclusive locks.
- Exclusive Lock (EXCLUSIVE):
- Only one transaction can acquire an exclusive lock on a resource at a time.
- Provides exclusive write access to the locked resource.
- Prevents other transactions from acquiring shared or exclusive locks on the same resource.
- An exclusive lock is incompatible with all other locks.
- Update Lock (UPDLOCK):
- A specialized lock mode used when a transaction intends to update a resource.
- Allows for concurrent shared locks but prevents other transactions from acquiring exclusive locks on the same resource.
- Primarily used to minimize deadlocks when an update operation follows a read.
- Intent Locks:
- Higher-level locks used to indicate the intention of a transaction to acquire locks on lower-level resources.
- Include Intent Shared (IS) and Intent Exclusive (IX) locks.
- Prevents conflicting lock modes on higher-level resources while allowing compatible locks on lower-level resources.
- Schema Modification Lock (Sch-M):
- Acquired when a transaction is modifying the schema of a table (e.g., adding or dropping columns).
- Prevents other transactions from acquiring any locks on the affected table during schema modification.
- Allows the transaction to complete schema changes without interference.
- Schema Stability Lock (Sch-S):
- Acquired when a transaction is reading the schema of a table.
- Prevents concurrent schema modification locks (Sch-M) but allows other transactions to acquire schema stability locks.
- Shared Intent Lock (SCH-S):
- A combination of shared lock (SHARED) and schema stability lock (Sch-S).
- Used to indicate an intention to read both data and schema information concurrently.
- Prevents other transactions from acquiring schema modification locks (Sch-M).
- Bulk Update Lock (BU):
- Used during bulk update operations (e.g., BULK INSERT, BCP).
- Allows concurrent read operations but prevents other transactions from acquiring any locks (including shared locks) on the same resource.
Understanding the different lock modes and their compatibility is essential for designing efficient and deadlock-resistant SQL Server applications. Properly selecting lock modes and optimizing queries can significantly impact database performance and concurrency.