Transcript Chapter 15 Transaction Management

Chapter 20
Transaction Management
Agenda
• 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
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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
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Locking
Deadlock
Two-phase locking
Timestamping
Optimistic technique
Locking
• Types
– Shared Locks vs. Exclusive Locks
– Read Locks vs. Write Locks
– Upgrade vs. Downgrade
• Granularity
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Database
file
page
record
field
Deadlock
• Definition
– Tow or more transactions each wait for locks held by
other transaction
– Livelock
• Control
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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
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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
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–
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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 19-20
• Read chapter
• Exam 3
– Date: 5/17/07
• Project
– Due date: 5/22/07
– Place: Tahoe 2090
– Time: 12 noon
End of MIS150
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