Failure Recovery
Introduction
Undo Logging
Redo Logging
1
Docsity.com
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Guidelines and tips
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Failure Recovery: Undo and Redo Logging for Database Consistency, Slides of Database Management Systems (DBMS)
The importance of data integrity in databases and the role of undo and redo logging in maintaining consistency. It covers concepts such as consistent states, constraints, and transaction execution, as well as potential causes of inconsistencies and methods for prevention and recovery. The document also touches upon the difference between undo and redo logging and their respective rules.
Typology: Slides
2012/2013
1 / 37
This page cannot be seen from the preview
Don't miss anything!
Related documents
Partial preview of the text
Download Failure Recovery: Undo and Redo Logging for Database Consistency and more Slides Database Management Systems (DBMS) in PDF only on Docsity!
Failure Recovery
Introduction Undo Logging Redo Logging
Integrity or correctness of data
- Would like data to be “accurate” or “correct” at all times
EMP
2
Name White Green Gray
Age 52 3421 1
Definition:
- Consistent state: satisfies all constraints
- Consistent DB: DB in consistent state
Constraints ( as we use here) may not capture “full correctness”
Example 1 Transaction constraints
- When salary is updated,
new salary > old salary
- When account record is deleted,
balance = 0
Constraints ( as we use here) may not capture “full correctness”
Example 2 Database should reflect real world
7
DB
Reality
in any case, continue with constraints...
Observation: DB cannot be consistent always! Example: a 1 + a 2 +…. an = TOT ( constraint ) Deposit $100 in a 2 : a 2 ← a 2 + 100 TOT ← TOT + 100
Transaction: collection of actions that preserve consistency
10
Consistent DB T Consistent DB’
Big assumption:
If T starts with consistent state + T executes in isolation ⇒ T leaves consistent state
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
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
Chapter 17: Recovery
- First order of business: Failure Model
Events Desired
Undesired Expected Unexpected
Desired events: see product manuals….
Undesired expected events: System crash
- memory lost
- cpu halts, resets
19
Undesired Unexpected: Everything else!
that’s it!!
Examples:
- Disk data is lost
- Memory lost without CPU halt
- CPU implodes wiping out universe….
20
Undesired Unexpected: Everything else!