ITPEC FE Morning April 2020 Question 30

Roll back — undoing the changes made since the last commit, returning the database to that committed state.

The DBMS records the before images of changed data in its log. When a transaction fails or is cancelled before it commits, rollback writes those before images back, so none of its uncommitted changes stay in the database.

Answer (b)

Why not others:

• •(a) Commit — the opposite: it makes a transaction's changes permanent and ends it
• •(c) Roll forward — redoes committed changes from the after images in the log, for example after a backup is restored; it reapplies work instead of undoing it
• •(d) View — a virtual table defined by a query; it has nothing to do with transaction control

Key rule: Rollback uses before images to undo uncommitted work; roll forward uses after images to redo committed work.

Databases · AI-written, may contain errors