Chapter 20 Transaction Management Thomas Connolly, Carolyn Begg, Database System, A Practical Approach to Design Implementation and Management, 4 th Edition, Addison Wesley Pg 572 ~ 678

Learning Outcomes Transaction Concurrent Processing Backup and Recovery

Transaction - I Definition –An action or actions to read or update the contents of the database Types –Committed –Aborted –Compensating

Transaction - II Property of Transactions (ACID) –Atomicity (all or nothing) –Consistency (state by database constraints and applications) –Isolation (independent) –Durability (permanent)

Concurrent Processing Definition Problems Control

Concurrent Processing Multiprogramming Interleaved between two transactions –CPU –I/O Logical unit of work

Concurrent Processing Problem No problem –Write different data –Update different data –Read the same data Problem –Write the same data –Update the same data

Concurrent Processing Problems Lost update –Two transactions simultaneously update the same files Uncommitted update –Transaction 2 uses the result updated by transaction 1 –Transaction 1 aborts and rolls back –Transaction 2 commits Inconsistent Analysis –Transaction 1 reads –Transaction 2 reads and uses for calculation –Transaction 1 updates and commits –Transaction 2 updates and commits

SERIALIZABILITY Transaction results form concurrent processing are the same as if stand-alone sequential processing was used Ensure no anomalies arise from concurrent processing

Concurrency Control Locking Deadlock Two-phase locking Timestamping Optimistic technique

Locking Types –Shared Locks vs. Exclusive Locks –Read Locks vs. Write Locks –Upgrade vs. Downgrade Granularity –Database –file –page –record –field

Deadlock Definition –Tow or more transactions each wait for locks held by other transaction –Livelock Control –Wait-Die –Wound-wait –Time out –Conservative 2PL

Two-phase Locking Growing phase –Get all locks –Upgrade locks Shrinking phase –Downgrade locks –Once starting to release a lock - no more new locks

Timestamping Timestamp –unique identifier as relative starting time of a transaction –Read-timestamp & write timestamp Timestamp protocol –Transactions with smaller timestamps get priority in the event of conflict –Transaction is only allowed on the item with smaller read-timestamp or write timestamp

Optimistic Technique Read phase Validate phase Write phase

Database Recovery Definition –Restoring the database to its correct state in the event of a failure Reasons –Physical (fire, flood, etc.) –Sabotage –Carelessness –Hardware –Software (application/system)

Database Backup Backup –Copy of the database Transaction log –Transaction ID, time, operation, object, before image, after image, prior pointer, next pointer Checkpoint –Synchronize transaction log and the database –Write data from buffers to database on the disk –Write checkpoint to log identify current transaction(s)

Recovery Methods Reprocessing –Record all transactions since last backup and replay the following transactions Rollfoward –Use the transaction log to change any committed transactions on the database or since last checkpoint Rollback –Use transaction log to undo any aborted transactions

Shadow Paging Method Current page table vs. Shadow page table Pros & cons –Faster –Less overhead –Data fragmentation –Reclaim inaccessible blocks

Points To Remember Properties of Transaction Concurrent Processing Backup and Recovery

Assignments Review chapters 5-6, 11-14, and Read chapter Exam 3 –Date: 5/17/07 Project –Due date: 5/22/07 –Place: Tahoe 2090 –Time: 12 noon