Question

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

Short answer

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

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.

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.

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.