Chapters 20

1
Chapters 20

• Transaction Management

2
Agenda

• Properties of Transaction
• Concurrent Processing
• Database Protection
• Recovery

3
Property of Transactions (ACID)

• Atomicity
• Consistency
• Isolation
• Durability

4
Concurrent Processing

• Definition
• Problems
• Control

5
Concurrent Processing

• Multiprogramming
• Interleaved between two transactions
• CPU
• I/O
• Logical unit of work

6
Concurrent Processing Problem

• No problem
• Write different data
• Update different data
• Read the same data
• Problem
• Write the same data
• Update the same data

7
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

8
SERIALIZABILITY

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

9
Concurrency Control

• Locking
• Deadlock
• Two-phase locking
• Timestamping
• Optimistic technique

10
Locking

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

11
Deadlock

• Definition
• Tow or more transactions each wait for locks held
  by other transaction
• Livelock
• Control
• Wait-Die
• Wound-wait

12
Two-phase Locking

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

13
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

14
Optimistic Technique

• Read phase
• Validate phase
• Write phase

15
Database Recovery

• Restoring the database to its correct state in
  the event of a failure
• Why?
• Physical (fire, flood, etc.)
• Sabotage
• Carelessness
• Hardware
• Software (application/system)

16
Database Protection

• Back up
• 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)

17
Recovery Methods

• Reprocessing
• Record all transactions since last backup and
  replay those transactions
• Rollforward
• 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

18
Shadow Paging Method

• Current page table vs. Shadow page table
• Pros cons
• Faster
• Less overhead
• Data fragmentation
• Reclaim inaccessible blocks

19
Failure Recovery

• Aborted transaction
• Rollback
• Incorrect data
• Rollback or restart from checkpoint
• System failure
• Rollback or restart from checkpoint
• Database destroyed
• Rollforward from last backup

20
Points To Remember

• Properties of Transaction
• Concurrent Processing
• Database Protection
• Recovery

21
Assignments

• Review chapters 5-6, 11-13, 18-19, 24-26
• Exam 3
• Date 5/13/04
• Project
• Due date 5/20/04
• Place MIS Department Office

22
End of MIS150

• Exam date 5/13/03
• Study! Study! Study!
• Have a happy and safe holiday!!