ITPEC FE Subject A April 2024 Question 22

Rollback — undoing one of the deadlocked transactions releases its locks so the others can continue.

In a deadlock, two or more transactions each hold a lock the other is waiting for, so none can proceed. The DBMS detects the cycle, picks a victim transaction and rolls it back: its updates are undone using the before-images in the log and its locks are released. The remaining transactions then get their locks and run normally, and the victim can be restarted.

Answer (c)

Why not others:

• •(a) Commit — a deadlocked transaction is waiting for a lock and cannot reach its commit; commit only ends a transaction that has finished its work
• •(b) Drop — a DDL statement that deletes a database object such as a table; it has nothing to do with transaction control
• •(d) Roll Forward — reapplies committed updates from the log (after-images) to a restored backup after a media failure; it redoes work rather than breaking a lock wait

Key rule: Rollback undoes an unfinished transaction with before-images (deadlock, transaction failure); roll forward redoes committed ones with after-images (media failure).

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