Managing Transaction and Lock Vu Tuyet Trinh Hanoi University of Technology 1

Microsoft Transaction A sequence of tasks that together constitute a logical unit of work Ensuring the highest level of integrity and consistency within a database Used when statements are processed

Microsoft ACID Properties Atomicity  All or none Consistency  Leave data in a consistent state Isolation  Don’t show data that’s ½ done Durability  Modifications are permanent

Microsoft Overview  Transaction Review of Transaction Log Introduction to Transactions and Locks Managing Transactions SQL Server Locking & Managing Locks

Microsoft Transaction Log Architecture Write-Ahead transaction log  Flushing the page and dirty Pages Use of checkpoints  Minimize what must be processed to recover Truncating the transaction log Shrinking the transaction log  DBCC SHRINKDATABASE  DBCC SHRINKFILE

Microsoft Transaction Recovery and Checkpoints Transaction RecoveryAction Required None CheckpointSystem Failure Roll forward Roll back Roll forward Roll back

Microsoft Concurrency Control Pessimistic concurrency control  High contention for data Optimistic concurrency control  Low contention for data

Microsoft Lockable Resources Item Item Description Description RIDRow identifier KeyRow lock within an index Page Extent Table Data page or index page Group of pages Entire table DatabaseEntire database

Microsoft Types of Locks Basic locks  Shared (LS)  Exclusive (LX) Special situation locks  Intent  Update  Schema  Bulk update

Microsoft 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 LSLX LStruefalse LXfalse

Microsoft Managing Locks Session-level locking options – READ UNCOMMITTED – READ COMMITTED (default) – REPEATABLE READ – SERIALIZABLE – SNAPSHOT Locking timeout – Limits time waiting for a locked resource – SET LOCK_TIMEOUT

Microsoft Viewing Locking Information Activity Monitor sys.dm_tran_locks Dynamic Management View EnumLocks SQL Server Profiler System Monitor

Microsoft 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 to determine nesting level

Microsoft Transaction Types Explicit Transaction  Explicitly define start and end Autocommit Transactions  Every statement is committed or rolled back Implicit Transactions  Statement after “Commit Transaction” or Rollback Transaction” starts a new transaction  SET IMPLICIT_TRANSACTIONS ON

Microsoft Explicit Transaction Defining BEGIN TRANSACTION COMMIT TRANSACTION ROLLBACK TRANSACTION BEGIN TRANSACTION INSERT INTO Extensions (PrimaryExt) VALUES ('555–1212') INSERT INTO Extensions (Voic Ext) VALUES (' ') ROLLBACK TRANSACTION

Microsoft Implicit Transaction Using An implicit transaction starts when one of the following statements is executed  Transaction must be explicitly completed with COMMIT or ROLLBACK TRANSACTION SET IMPLICIT_TRANSACTIONS ON ∙ ALTER DATABASE ∙ CREATE ∙ DELETE ∙ DROP ∙ FETCH ∙ GRANT ∙ INSERT ∙ OPEN ∙ REVOKE ∙ SELECT ∙ TRUNCATE TABLE ∙ UPDATE

Microsoft SQL Server Locking Concurrency Problems Prevented by Locks Dealing with Deadlocks

Microsoft Concurrency Problems Prevented by Locks Lost update  Two simultaneous updates Uncommitted dependency (Dirty Read)  Selecting data that’s being updated Inconsistent analysis (Nonrepeatable Read)  Reading data that changed between multiple reads Phantoms reads  Reading data that doesn’t exist on the second read

Microsoft Concurrency Control Optimistic concurrency control  Low contention for data Pessimistic concurrency control  High contention for data Isolation level issues  Lower isolation level: increases concurrency; increases chance of reading bad data  Higher isolation level: ensure data is correct; negatively affects concurrency

Microsoft Transaction Isolation Level READ COMMITTED (DEFAULT)  Prevents SELECT statements from reading dirty, or uncommitted data READ UNCOMMITTED  Allows dirty reads REPEATABLE READ  No other rows can update/delete rows in the result set until released; inserting news rows is okay SERIALIZABLE  No other transaction can make any changes

Microsoft Dynamic Locking Architecture Dynamic Locking TablePageRow Cost Granularity Locking Cost Concurrency Cost

Microsoft Blocks …

Microsoft … Deadlocks  Occurs when two transactions are blocking each other

Microsoft Deadlock How SQL server ends a deadlock How to minimize deadlocks  Access objects in same order  Keep transactions short  Use low isolation level  Use bound connections How to customize the lock time-out setting  SET LOCK_TIMEOUT function

Microsoft Remarks Keep transactions short Design transactions to minimize deadlocks Use SQL Server defaults for locking Be careful when use locking options

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