Transaction Locking and Row Versioning Guide

Transaction Locking and Row Versioning Guide

  • 03/10/2020
  • 138 minutes to read

Applies to: 

SQL Server (all supported versions) 

Azure SQL Database 

Azure SQL Managed Instance 

Azure Synapse Analytics 

Parallel Data Warehouse

In any database, mismanagement of transactions often leads to contention and performance problems in systems that have many users. As the number of users that access the data increases, it becomes important to have applications that use transactions efficiently. This guide describes the locking and row versioning mechanisms the SQL Server Database Engine uses to ensure the physical integrity of each transaction and provides information on how applications can control transactions efficiently.

Applies to: SQL Server (SQL Server 2005 (9.x) through SQL Server 2019 (15.x), unless noted otherwise) and Azure SQL Database.

Transaction Basics

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 fully durable transaction has completed, its effects are permanently in place in the system. The modifications persist even in the event of a system failure. SQL Server 2014 (12.x) and later enable delayed durable transactions. Delayed durable

transactions commit before the transaction log record is persisted to disk. For more information on delayed transaction durability see the topic Transaction Durability.

SQL programmers are responsible for starting and ending transactions at points that enforce the logical consistency of the data. The programmer must define the sequence of data modifications that leave the data in a consistent state relative to the organization's business rules. The programmer includes these modification statements in a single transaction so that the SQL Server Database Engine can enforce the physical integrity of the transaction.

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