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Module 14: Managing Transactions and Locks

Module 14: Managing Transactions and Locks. Overview. Introducing Transactions and Locks Managing Transactions Understanding SQL Server Locking Architecture Managing Locks. Lesson: Introducing Transactions and Locks. What Are Transactions and Locks? What Is Concurrency Control?.

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Module 14: Managing Transactions and Locks

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  1. Module 14: Managing Transactions and Locks

  2. Overview • Introducing Transactions and Locks • Managing Transactions • Understanding SQL Server Locking Architecture • Managing Locks

  3. Lesson: Introducing Transactions and Locks • What Are Transactions and Locks? • What Is Concurrency Control?

  4. What Are Transactions and Locks? • Transactions Ensure That Multiple Data Modifications Are Processed Together • Locks Prevent Update Conflicts • Transactions are serializable • Locking is automatic • Locks allow concurrent use of data

  5. What Is Concurrency Control? • Pessimistic • Locks data when data is read in preparation for an update • Other users are blocked until lock is released • Use where high contention for data exists • Optimistic • Locks data when an update is performed • Error received if data was changed since initial read • Use when low contention for data exists

  6. Lesson: Managing Transactions • Multimedia Presentation: SQL Server Transactions • How Transaction Recovery Works • Considerations for Using Transactions • The Implicit Transactions Option • Restricted Statements

  7. Multimedia Presentation: SQL Server Transactions Concepts covered in this presentation: • SQL Server Transaction Processing • Two Types of Transactions • Implicit • Explicit • Rolling Back a Transaction • The Transaction Log • Checkpoints • The Automatic Recovery Process

  8. How Transaction Recovery Works TransactionRecovery Action Required 1 None 2 Roll forward 3 Roll back 4 Roll forward 5 Roll back Checkpoint System Failure

  9. Considerations for Using Transactions • Transaction Guidelines • Keep transactions as short as possible • Use caution with certain Transact-SQL statements • Avoid transactions that require user interaction • Issues in Nesting Transactions • Allowed, but not recommended • Use @@trancount to determine nesting level

  10. The Implicit Transactions Option • Automatically Starts a Transaction When You Execute Certain Statements • Nested Transactions Are Not Allowed • Transaction Must Be Explicitly Completed with COMMIT or ROLLBACK TRANSACTION • By Default, Setting Is Off SET IMPLICIT_TRANSACTIONS ON

  11. Restricted Statements • Certain Statements May Not Be Included • ALTER DATABASE • BACKUP LOG • CREATE DATABASE • DROP DATABASE • RECONFIGURE • RESTORE DATABASE • RESTORE LOG • UPDATE STATISTICS

  12. Lesson: Understanding SQL Server Locking Architecture • What Concurrency Problems Are Prevented by Locks? • Lockable Resources • Types of Locks • Lock Compatibility

  13. What Concurrency Problems Are Prevented by Locks? • Lost Update • Uncommitted Dependency (Dirty Read) • Inconsistent Analysis (Nonrepeatable Read) • Phantom Reads

  14. Lockable Resources

  15. Types of Locks • Basic Locks • Shared • Exclusive • Special Situation Locks • Intent • Update • Schema • Bulk update

  16. Lock Compatibility • Locks May or May Not Be Compatible with Other Locks • Examples • Shared locks are compatible with all locks except exclusive • Exclusive locks are not compatible with any other locks • Update locks are compatible only with shared locks

  17. Lesson: Managing Locks • Session-Level Locking Options • What Is Dynamic Locking Architecture? • What Are Table-Level Locking Options? • What Are Deadlocks? • Methods to View Locking Information • Best Practices

  18. Session-Level Locking Options • Transaction Isolation Level • READ UNCOMMITTED • READ COMMITTED (default) • REPEATABLE READ • SERIALIZABLE • Locking Timeout • Limits time waiting for a locked resource • Use SET LOCK_TIMEOUT

  19. What Is Dynamic Locking Architecture? Locking Cost Concurrency Cost DynamicLocking Cost Row Page Table Granularity

  20. What Are Table-Level Locking Options? • Use with Caution • Can Specify One or More Locking Options for a Table • Use optimizer_hints Portion of FROM Clause inSELECT or UPDATE Statement • Overrides Session-Level Locking Options

  21. What Are Deadlocks? • How SQL Server Ends a Deadlock • How to Minimize Deadlocks • How to Customize the Lock Time-Out Setting

  22. Methods to View Locking Information • Current Activity Window • sp_lock System Stored Procedure • SQL Profiler • Windows 2000 System Monitor • Additional Information

  23. Best Practices ü Keep Transactions Short ü Design Transactions to Minimize Deadlocks ü Use SQL Server Defaults for Locking ü Be Careful When You Use Locking Options

  24. Lab A: Managing Transactions and Locks • Exercise 1: Creating and Executing a Transaction • Exercise 2: Rolling Back a Transaction • Exercise 3: Viewing Locking Information • Exercise 4: Setting Locking Options

  25. Course Evaluation

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