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Chapter 19: Problem 15

Describe the shadow paging recovery technique. Under what circumstances does it not require a log?

Short Answer

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Shadow paging is a recovery technique in databases where changes during a transaction are made in a memory-based shadow page, with the current page being replaced only after a successful transaction, and the shadow page discarded after an unsuccessful one. Under normal circumstances, shadow paging doesn't require a log since the atomicity and durability properties of transactions are preserved by just discarding shadow pages in the event of a crash. However, when managing concurrent transactions, a log is often used to record the order of transactions to prevent conflicts and ensure consistency.

Step by step solution

01

Understand Shadow Paging

Shadow Paging is a technique used in database systems for achieving atomicity and durability of transactions. During the transaction, changes are made in a separate area in memory, creating a shadow page. The original page, or current page, stays unchanged. Once the transaction is completed successfully, the shadow page replaces the current page. If the transaction is not successful, the shadow page is simply discarded.
02

Shadow Paging and Logs

In Shadow Paging, logs might not be always necessary. This is because the recovery system can achieve atomicity and durability properties of transactions without needing to keep a log. In the event of a system crash, the database can recover by simply discarding the shadow pages and retaining the original pages. Thus, there is no need to rollback any transactions recorded in a log.
03

Note the Exceptions

However, there are certain exceptions. For instance, when the database system operates concurrently with several transactions, logging may be necessary to record the order of transactions. This log helps in preventing conflicts, ensuring consistency when two or more transactions are trying to access the same data element. Hence, while shadow paging doesn't necessarily demand a log, it is often employed for managing concurrent transactions more efficiently.

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Most popular questions from this chapter

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

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?

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.

Choose the correct answer for each of the following multiple-choice questions: The write ahead logging (WAL) protocol simply means that a. the writing of a data item should be done ahead of any logging operation. b. the log record for an operation should be written before the actual data is written. c. all log records should be written before a new transaction begins execution. d. the log never needs to be written to disk.

Choose the correct answer for each of the following multiple-choice questions: For correct behavior during recovery, undo and redo operations must be a. commutative. b. associative. c. idempotent. d. distributive.
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