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Chapter 19: Problem 11

Discuss the deferred update technique of recovery. What are the advantages and disadvantages of this technique? Why is it called the NO-UNDO/REDO method?

Short Answer

Expert verified
The deferred update or NO-UNDO/REDO technique in database recovery involves recording changes in a logging buffer, and applying to the database after the transaction has been committed, eliminating the need for 'undo' operations due to the delay in updating the database. Its advantages include cost and time efficiency in case of system failure, and maintaining atomicity of transactions. On the downside, it requires more logging space, results in higher wait times, and can provide obsolete data to transactions due to the delay in updating.

Step by step solution

01

Explanation of the Deferred Update Technique

Deferred update, also known as NO-UNDO/REDO or write-ahead logging, is a technique used in database recovery. In this method, changes made in a transaction are not immediately updated in the database. Instead, the changes are recorded in the system's logging buffer. The changes are applied to the database only after the transaction reaches its commit point. If a system failure occurs before the transaction reaches that point, the changes aren't made to the database, making this a 'no-undo' method. The recorded changes, however, are used to redo the transaction after recovery, hence the term 'redo' method.
02

Advantages of the Deferred Update Technique

There are several benefits of this technique. Firstly, as we don't need to undo transactions it saves the cost of undoing. Secondly, it greatly simplifies crash recovery by applying only redo operations. Lastly, there are no modifications to the database until the transaction is committed, this ensures the atomicity property of the transactions are maintained.
03

Disadvantages of the Deferred Update Technique

Despite its advantages, the deferred update technique has certain drawbacks. Firstly, there is a higher wait time because the database isn't updated immediately. Also, it requires a substantial amount of logging space as all changes are recorded. In addition, delay in reflecting the changes to the database might lead to obsolete data being provided to other transactions.
04

Why it's Known as the NO-UNDO/REDO Technique

This method gets its name because changes to the database aren't made until after the commit point meaning that if a system failure occurs, there's no need for an 'undo' operation as no changes have been made to the database yet. Therefore, it is called a 'NO-UNDO' method. After recovery from a system failure, the changes that were stored in the log are then made in the database, hence the 'REDO' part of the description.

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Most popular questions from this chapter

Choose the correct answer for each of the following multiple-choice questions: If the shadowing approach is used for flushing a data item back to disk, then a. the item is written to disk only after the transaction commits. b. the item is written to a different location on disk. c. the item is written to disk before the transaction commits. d. the item is written to the same disk location from which it was read.

(Review from Chapter 17 ) What is the system log used for? What are the typical kinds of entries in a system log? What are checkpoints, and why are they important? What are transaction commit points, and why are they important?

How can recovery handle transaction operations that do not affect the database, such as the printing of reports by a transaction?

What is meant by transaction rollback? What is meant by cascading rollback? Why do practical recovery methods use protocols that do not permit cascading rollback? Which recovery techniques do not require any rollback?

What do the terms steal/no-steal and force/no-force mean with regard to buffer management for transaction processing.
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