ITPEC IP April 2025 Question 29

A log records the transaction history of a database — the only description that matches its term.

The log (journal) records each update a transaction makes, with the values before it (before-image) and after it (after-image), and when each transaction commits. After a failure the DBMS uses it to recover: before-images to undo unfinished transactions (rollback), after-images to reapply committed ones to a backup (roll-forward).

Answer (d)

Why not others:

• •(a) — returning the database to its state before an unsuccessful transaction is rollback; commit makes a successful transaction's updates permanent
• •(b) — confirming the updates of a successful transaction is commit; exclusive control is locking
• •(c) — preventing inconsistencies when several transactions update the same data at once is exclusive control (locking), not rollback

Key rule: Commit confirms, rollback undoes, locking keeps concurrent updates from clashing, and the log is what makes both rollback and roll-forward possible.

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