CS 775/875: Fall 2000

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CS 775/875 Fall 2000

• Chapter 12 Transactions and Concurrency Control

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Transactions

• What is a transaction?
• ACID properties Atomicity, Consistency,
  Isolation, Durability
• Serializable transactions
• Open, close, commit/abort
• Concurrency control---The lost update problem,
  inconsistent retrieval, serial equivalence
• Conflicting Operations
• Dirty reads, premature writes, cascaded aborts,
  tentative versions
• Nested Transactions
• Locks implementation nested transactions
• Deadlocks

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Optimistic Concurrency Control

• Locks present an overhead, may cause deadlocks,
  reduces concurrency
• Optimistic---assumes that data conflicts are rare
• Kung and Robinson algorithm Working phase,
  validation phase, update phase
• Tentative versions, reads on committed versions
  only, writes on tentative versions
• Validation of transactions Backward validation
  and forward validation transaction numbers
  assigned at the beginning of validation phase
• Only one transaction can be in the validation
  phase until this is completed, no new validation
  begins

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Optimistic Concurrency Control

• See Fig. 12.28 http//www.cdk3.net/ig/slides/Chapt
  er12Slides.pdf
• Backward validation Compare readset with the
  already committed transactions
• Forward validation Compare writeset with the
  readsets of all overlapping active transactions
• Starvation is a problem with optimistic CC

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Timestamp Ordering

• Basic TO rule A transactions request to write
  an object is valid only if that object was last
  read and written by earlier transactions. A
  transactions request to read an object is valid
  only if that object was last written by an
  earlier transaction.
• Transactions are only committed in the order of
  their numbers.
• See Fig. 12.30 and 12.31
• http//www.cdk3.net/ig/slides/Chapter12Slides.pdf
• Multiversion TS ordering