©Bob Godfrey, 2002, 2005 Lecture 17: Transaction Integrity and Concurrency BSA206 Database Management Systems

©Bob Godfrey, 2002, 2005 TRANSACTION A discrete unit of work which must be completely processed or not processed at all

Lecture 17 / Slide 3 Transaction Management  Transactions represent real world events eg enter a customer order  In a database if you read and/or write you make use of a transaction  For a transaction to occur a database must be in a consistent state

Lecture 17 / Slide 4 Transaction Management in SQL  Transaction support is provided through the use of 2 SQL statements: –COMMIT –ROLLBACK  Terminating a transaction: –a COMMIT or ROLLBACK is performed –a DDL command is performed –the user disconnects –the user process is abnormally terminated

Lecture 17 / Slide 5 Transaction Properties  Atomicity requires that all parts of a transaction must be completed and if not the transaction is aborted  Durability indicates the permanence of the database’s consistent state  Serialisability describes the result of the concurrent execution of several transactions  Isolation means that the data used during the execution of a transaction cannot be used by a second transaction, until the first one is completed

Lecture 17 / Slide 6 Transaction Boundaries BEGIN TRANSACTION Action 1 Action 2 etc. COMMIT ROLLBACK

Lecture 17 / Slide 7 Lost Update Transaction ATransaction B Read a/c X balance $150 Increase balance by $50 Decrease balance by $100 Write a/c X with balance $200 Write a/c X with balance $50 * A/c X should be $100 ($150 + $50 - $100 + $100, not $50)

Lecture 17 / Slide 8 Uncommitted Dependency Transaction ATransaction B Read a/c X balance $150 Increase balance by $50 Write a/c X with balance $200 Read a/c X balance $200 Rollback transaction (balance $150)

Lecture 17 / Slide 9 Inconsistent Analysis Three A/Cs with balances of X(150), Y(350) and Z(500) total = $1000 Transaction ATransaction B Read a/c X, add balance of $150 to total = $150 Read a/c X balance $150 Decrease balance by $100 Write a/c X with balance $50 Read a/c Y balance $350 Increase balance by $100 Write a/c X with balance $450 Read a/c Y, add balance of $450 to total = $600 Read a/c Z, add balance of $500 to total = $1100 Print report total of $1100 !!! (Three A/Cs with balances of X(50), Y(450) and Z(500) total = $1000)

Lecture 17 / Slide 10 Record Locking (Pessimistic Control)  A concurrency processing procedure that ensures that only one user at a time can access or alter a record that is currently accessed.  A lock guarantees exclusive use of a data item to a current transaction.  A transaction acquires a lock at access and it will be unlocked when the transaction is completed.

Lecture 17 / Slide 11 Optimistic Concurrency Control  Based on the assumption that the majority of database operations do not conflict.  Each transaction is executed, with no restrictions, until it is committed.  Each transaction moves through phases: read validation write

Lecture 17 / Slide 12 Lock Granularity  Indicates the level of lock use, which can take place at the database, table, record, and field levels.  Locking database or table means only one end user can view or use at a time  Record locking may mean that access to popular items are constantly delayed  Field locking permits more than one person to view but only one to change

Lecture 17 / Slide 13 Locking & SQL LOCK TABLE tablename IN lockmode MODE [NOWAIT]  Lockmode is one of: exclusive share row share/share update row exclusive share row exclusive  NOWAIT If this option is specified then the user is not kept waiting for the lock to be applied

Lecture 17 / Slide 14 Deadlock Transaction ATransaction B Read a/c X balance $150 and place shared(read) lock Read a/c X balance $150 and place shared(read) lock Increase balance by $50 Decrease balance by $100 Write a/c X with balance $200 (denied X-lock so waits) Write a/c X with balance $250 (denied X-lock so waits)

Lecture 17 / Slide 15 Controlling Deadlocks  Deadlock Prevention –a transaction requesting a new lock is aborted if there is a possibility that a deadlock may occur  Deadlock Detection –the DBMS periodically checks for deadlock and if found one of the transactions is aborted and rolled back

Lecture 17 / Slide 16 Controlling Deadlock  Two-phase Locking –all locks required for a transaction are acquired before any locks are released