ITPEC IP October 2025 Question 28

Reverting the database to its state before the failed transaction started — that is a rollback.

A transaction is a unit of work that must take effect completely or not at all (atomicity). If it fails part-way, some of its updates may already have been written. Rollback uses the before-images (pre-update values) recorded in the log to undo those updates, returning the database to the state it was in when the transaction began. A transaction that finishes normally ends with a commit instead.

Answer (c)

Why not others:

• •(a) — describes exclusive locking, the concurrency control that stops two transactions updating the same data at once
• •(b) — describes commit, which makes a successful transaction's updates permanent
• •(d) — describes a join, the relational operation that combines tables on matching columns

Key rule: Rollback undoes an unfinished transaction with before-images; roll-forward, after a media failure, reapplies committed updates to the backup with after-images.

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