Chapter 20

Transaction Management

Thomas Connolly, Carolyn Begg, Database System, A Practical Approach to Design Implementation and

Management, 4th 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