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Chapter 19: Problem 14

What is the difference between the UNDO/REDO and the UNDO/NO-REDO algorithms for recovery with immediate update? Develop the outline for an UNDO/NOREDO algorithm.

Short Answer

Expert verified
The UNDO/REDO algorithm undoes incomplete transactions and redoes completed ones after failure. The UNDO/NOREDO algorithm only undoes incomplete transactions. This is mainly because in UNDO/NOREDO, all changes of a transaction are executed in the main memory first and only committed to disk upon completion.

Step by step solution

01

Understanding UNDO/REDO Algorithm

The UNDO/REDO algorithm is used in database recovery to maintain the atomicity and durability of transactions. In case of failure, this algorithm can both 'undo' uncommitted transactions that were in the process when the failure occurred and 'redo' committed transactions that were not written into the database.
02

Understanding UNDO/NOREDO Algorithm

The UNDO/NOREDO algorithm, on the other hand, is able to undo incomplete transactions but doesn't redo completed transactions. This is used when all changes of a transaction are executed in main memory first and are written to disk only upon commit.
03

Differences between UNDO/REDO and UNDO/NOREDO

The main difference between the two algorithms lies in their response to committed transactions. UNDO/REDO algorithm, after a failure, reapplies already written changes while UNDO/NOREDO algorithm does not.
04

Outline of UNDO/NOREDO Algorithm

To develop an UNDO/NOREDO algorithm, consider this basic outline: \n 1) Start from the last log entry and move backwards. \n 2) If a 'transaction commit' entry is encountered, ignore it. \n 3) If a 'transaction start' entry is encountered, ensure all changes from this transaction are undone (using the UNDO operation). \n 4) Continue this process until the beginning of the log. In the end, all incomplete transactions should be undone, and no completed transactions should be redone.

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Most popular questions from this chapter

Choose the correct answer for each of the following multiple-choice questions: In case of transaction failure under a deferred update incremental logging scheme, which of the following will be needed: a. an undo operation. b. a redo operation. c. an undo and redo operation. d. none of the above.

Choose the correct answer for each of the following multiple-choice questions: When a failure occurs, the log is consulted and each operation is either undone or redone. This is a problem because a. searching the entire log is time consuming. b. many redo's are unnecessary. c. both (a) and (b). d. none of the above.

Discuss the UNDO and REDO operations and the recovery techniques that use each.

What are UNDO-type and REDO-type log entries?

What are log sequence numbers (LSNs) in ARIES? How are they used? What information does the Dirty Page Table and Transaction Table contain? Describe how fuzzy checkpointing is used in ARIES.
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