Transactions are used to ensure data integrity and manage concurrent access in SQL Server. There are two types of transactions: implicit, which automatically commit after each statement, and explicit, which require BEGIN, COMMIT, or ROLLBACK statements. Transactions have ACID properties including atomicity, consistency, isolation, and durability. Isolation levels like read committed and serializable control how transactions see concurrent data modifications. Snapshot isolation is an alternative to locking that uses row versioning to provide consistency.

1. SQL Server Transaction Management Denise McInerney San Francisco SQL Server User Group April 9, 2008

2. Introduction What is a transaction Auto-commit Explicit transactions Nested transactions Catching errors Coding efficient transactions Isolation levels Conclusion Agenda

3. Introduction 10 years SQL Server DBA MCDBA/MCTS Web-based OLTP applications Focus on performance tuning Development DBA www.intuitmarket.com PASS volunteer Women in Technology SIG Chairperson

4. Single, logical unit of work Sequence of operations Dictated by business rules Debit/credit Order header/order detail Why do we care? Data integrity Performance What is a transaction?

5. A tomicity All or Nothing; everything commits, or everything rolls back C onsistency All constraints and rules are honored and all data structures are intact, even if system fails ACID Properties

6. ACID Properties continued I solation Concurrent transactions are kept separate, aka “serialized” Separate transactions never see data in an intermediate state Managed with locking D urability After commit, effects of transaction persist even if system fails

7. Default mode for SQL Server Every T-SQL command that is successfully completed commits If command encounters an error, transaction is rolled back Over-ridden with explicit transaction BEGIN TRAN Auto-Commit

8. Defined by the user Automatically abort only in case of fatal error TSQL Begin Tran Commit Tran/Commit Work Rollback Tran/Rollback Work OLEDB ITransactionLocal::StartTransaction ITransaction::Commit/ITransaction::Abort ADO.NET SqlConnection object BeginTransaction Commit()/Rollback() ADO Connection object BeginTrans CommitTrans/Rollbacktrans Explicit Transactions

9. TSQL Transaction DEMO

10. Crucial to managing transactions properly In TSQL Check @@ERROR System error message True for SQL command immediately proceeding Pass back to calling program and take action Take action in SQL code Return user-defined error message via RAISERROR Store in sysmessages Build dynamically Can be logged Can take parameters TRY…CATCH SET XACT_ABORT ON Provides protection, but silently Errors During Transactions

11. Error Handling DEMO

12. Possible to nest transaction blocks BUT! Committing inner transactions is ignored Only outermost COMMIT will have effect ROLLBACK will rollback all levels @@TRANCOUNT Returns nesting level of current transaction block Each BEGIN TRAN increments by 1 COMMIT decrements by 1 ROLLBACK decrements to 0 Nested Transactions

13. Nested Transaction DEMO

14. Requires planning Keep as short as possible Access the least amount of data possible Do not wait for user input during a transaction Make intelligent use of isolation levels Coding Efficient Transactions

15. Isolation Levels Read Uncommitted aka (NOLOCK) Does not issue shared lock, does not honor exclusive lock Rows can be updated/inserted/deleted before transaction ends Least restrictive Read Committed SQL Server default Holds shared lock Cannot read uncommitted data, but data can be changed before end of transaction, resulting in nonrepeatable read or phantom rows

16. Isolation Levels continued Repeatable Read Locks data being read, prevents updates/deletes New rows can be inserted during transaction, will be included in later reads Serializable aka HOLDLOCK on all tables in SELECT Locks range of data being read, no modifications are possible Prevents updates/deletes/inserts Most restrictive

17. Isolation Level DEMO

18. Isolation Levels Summary From Books Online “Managing Concurrent Data Access” No No No Serializable Yes No No Repeatable Read Yes Yes No Read committed Yes Yes Yes Read Uncommitted Phantom Rows Non-Repeatable Read Dirty Read Isolation Level

19. Isolation Levels Added in SS2005 READ_COMMITTED_SNAPSHOT New implementation of READ COMMITTED Uses row versioning instead of locking ALTER DATABASE option When turned ON, transactions setting the read committed isolation level use row versioning SNAPSHOT Uses row versioning instead of locking Only recognize data modifications that were committed before the start of the transaction Data modifications made by others after start of transaction are ignored Both Use TEMPDB heavily Minimize contention Provide read consistency

20. Snapshot Isolation DEMO

21. Snapshot Isolation Summary From Books Online “Row Versioning-based Isolation Levels in the Database Engine” Uses row versions to select rows to update. Tries to acquire an exclusive lock on the actual data row to be modified, and if the data has been modified by another transaction, an update conflict occurs and the snapshot transaction is terminated. Reverts from row versions to actual data to select rows to update and uses update locks on the data rows selected. Acquires exclusive locks on actual data rows to be modified. No update conflict detection. How updates are handled. Integrated support. Cannot be disabled. None. Update conflict detection. All data that was committed before the start of each transaction. All data that was committed before the start of each statement. The version of data read by statements. Requires the execution of SET TRANSACTION ISOLATION LEVEL to specify the SNAPSHOT isolation level before the start of the transaction. Use the default read-committed isolation level, or run the SET TRANSACTION ISOLATION LEVEL statement to specify the READ COMMITTED isolation level. This can be done after the transaction starts. How a session requests the specific type of row versioning. ALLOW_SNAPSHOT_ISOLATION READ_COMMITTED_SNAPSHOT The database option that must be set to ON to enable the required support. Snapshot isolation level Read-committed snapshot isolation level Property

22. Conclusion Transaction management crucial Guaranteeing data integrity Performance Understand how transactions work Understand what’s acceptable for your system Choose appropriate isolation level Assume nothing! DBA & developer collaboration key Email: denise.mcinerney@sqlpass.org