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Advanced Database Systems
Notes 10: Failure Recovery
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Advanced Database Systems: Notes on Data Integrity and Constraints, Transaction Recovery, Slides of Database Management Systems (DBMS)
Notes on advanced database systems, focusing on data integrity and constraints, transaction recovery, and related topics. It covers concepts such as crash recovery, consistency, constraints, transaction constraints, undo and redo logging, and recovery rules. The document also discusses the importance of preventing and fixing violations of constraints and the role of undo and redo logging in ensuring database consistency.
Typology: Slides
2012/2013
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Advanced Database Systems
Notes 10: Failure Recovery
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Schedule
- Crash recovery (1 lect.) Ch. 17
- Concurrency control (1.5 lect.) Ch. 18
- More transaction proc. (1.5 lect.) Ch. 19
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Integrity or consistency constraints
- Predicates data must satisfy
- Examples:
- x is key of relation R
- x ā y holds in R (functional dependency)
- Domain(x) = {Red, Blue, Green} ā Ī± is valid index for attribute x of R
- no employee should make more than twice the average salary
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Definition:
- Consistent state: satisfies all constraints
- Consistent DB: DB in consistent state
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Note: could be āemulatedā by simple constraints, e.g.,
account (^) Acct # ā¦. balance deleted?
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Example 2 Database should reflect real world
DB
Reality
Constraints ( as we use here) may
not capture āfull correctnessā
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a 2
TOT
.. 50 .. 1000
.. 150 .. 1000
.. 150 .. 1100
Example: a 1 + a 2 +ā¦. a n = TOT ( constraint )
Deposit $100 in a 2 : a 2 ā a 2 + 100 TOT ā TOT + 100
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Transaction: collection of actions
that preserve consistency
Consistent DB T Consistent DBā
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Correctness (informally)
- If we stop running transactions, DB left consistent
- Each transaction sees a consistent DB
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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 ā systems analysts
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Will not consider:
- How to write correct transactions
- How to write correct DBMS
- Constraint checking & repair That is, solutions studied here do not need to know constraints
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Chapter 17: Recovery
- First order of business: Failure Model
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Our failure model
processor
memory disk
CPU
M (^) D
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Desired events: see product manualsā¦.
Undesired expected events: System crash
- memory lost
- cpu halts, resets
Undesired Unexpected: Everything else!
thatās it!!