SQL Server Transaction Locking and Row Versioning Guide

SQL Server Transaction Locking and Row Versioning Guide

A transaction is a sequence of operations performed as a single logical unit of work. A logical unit of work must exhibit four properties, called the atomicity, consistency, isolation, and durability (ACID) properties, to qualify as a transaction.

Atomicity

A transaction must be an atomic unit of work; either all of its data modifications are performed, or none of them are performed.

Consistency

When completed, a transaction must leave all data in a consistent state. In a relational database, all rules must be applied to the transaction's modifications to maintain all data integrity. All internal data structures, such as B-tree indexes or doubly-linked lists, must be correct at the end of the transaction.

Isolation

Modifications made by concurrent transactions must be isolated from the modifications made by any other concurrent transactions. A transaction either recognizes data in the state it was in before another concurrent transaction modified it, or it recognizes the data after the second transaction has completed, but it does not recognize an intermediate state. This is referred to as serializability because it results in the ability to reload the starting data and replay a series of transactions to end up with the data in the same state it was in after the original transactions were performed.

Durability

After a transaction has completed, its effects are permanently in place in the system. The modifications persist even in the event of a system failure.

The following table shows the resource lock modes that the Database Engine uses.

Lock mode

Description

Shared (S)

Used for read operations that do not change or update data, such as a SELECT statement.

Update (U)

Used on resources that can be updated. Prevents a common form of deadlock that occurs when multiple sessions are reading, locking, and potentially updating resources later.

Exclusive (X)

Used for data-modification operations, such as INSERT, UPDATE, or DELETE. Ensures that multiple updates cannot be made to the same resource at the same time.

Intent

Used to establish a lock hierarchy. The types of intent locks are: intent shared (IS), intent exclusive (IX), and shared with intent exclusive (SIX).

Schema

Used when an operation dependent on the schema of a table is executing. The types of schema locks are: schema modification (Sch-M) and schema stability (Sch-S).

Bulk Update (BU)

Used when bulk copying data into a table and the TABLOCK hint is specified.

Key-range

Protects the range of rows read by a query when using the serializable transaction isolation level. Ensures that other transactions cannot insert rows that would qualify for the queries of the serializable transaction if the queries were run again.

Use the following table to determine the compatibility of all the lock modes available in SQL Server.