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CS4432: Database Systems II

CS4432: Database Systems II. Lecture #19 Database Consistency and Transactions. Professor Elke A. Rundensteiner. Transactions, etc. Crash recovery Ch. [17] Concurrency control Ch. [18] Transaction processing Ch. [19]. Name. Age. White Green Gray. 52 3421 1.

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CS4432: Database Systems II

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  1. CS4432: Database Systems II Lecture #19 Database Consistency and Transactions Professor Elke A. Rundensteiner

  2. Transactions, etc. • Crash recovery Ch. [17] • Concurrency control Ch. [18] • Transaction processing Ch. [19]

  3. Name Age White Green Gray 52 3421 1 Integrity or correctness of data ? • We would like data in our database to be “accurate” ( “correct” ) at all times. EMP • How DBMS decides if data is consistent?

  4. Integrity or consistency constraints • Utilize predicates data must satisfy • Examples: - x is key of relation R - x  y holds in R - Domain(x) = {Red, Blue, Green} - no employee should make more than twice the average salary

  5. Such constraints may not capture “full correctness” Example 1 :Transaction constraints • When salary is updated, new salary > old salary • When account record is deleted, balance = 0

  6. in any case, continue with constraints ... Observation: DB cannot be consistent always Example: Constraint : a1 + a2 +…. an = TOT Action: Deposit $100 in a2: a2 a2 + 100 TOT  TOT + 100

  7. Example: a1 + a2 +…. an = TOT (constraint) Deposit $100 in a2: a2 a2 + 100 TOT  TOT + 100 a2 TOT . . . . . . 50 150 150 . . . . . . 1000 1000 1100

  8. Transaction: a collection of actions that preserve consistency Consistent DB Consistent DB’ T

  9. Big assumption: If T starts with consistent state AND T executes in isolation  T leaves consistent state

  10. Correctness(informally) • If we stop running transaction(s), DB left consistent • Each transaction sees a consistent DB

  11. How can constraints be violated? • Transaction bug • DBMS bug • Hardware failure e.g., disk crash alters balance of account • Data sharing e.g.: T1: give 10% raise to programmers T2: change programmers  systems analysts

  12. Will not consider: • How to write correct transactions • How to write correct DBMS system • Constraint checking & repair

  13. How can we prevent/fix violations? • Chapter 17: due to failures only • Chapter 18: due to data sharing only • Chapter 19: due to failures and sharing

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