Recovery Techniques — What Is Backward Recovery? In database recovery terminology, which option correctly defines backward recovery (also called rollback)?

Introduction / Context:Recovery procedures must both roll forward committed work and roll back uncommitted work. The term “backward recovery” refers to undoing changes to restore a consistent state after failures or transaction aborts.

Given Data / Assumptions:

• Before-images capture values prior to modification.
• After-images capture values after modification.
• We need the definition that maps to “backward recovery.”

Concept / Approach:Backward recovery, or rollback, uses before-images to undo the effects of incomplete or erroneous transactions. In contrast, forward recovery (rollforward) uses after-images to reapply committed changes after a crash. Combining both is part of comprehensive crash recovery, but the term “backward” specifically means applying before-images.

Step-by-Step Solution:Identify that undo requires the old values.Map undo to application of before-images.Exclude after-images, which are used for redo.Select the option describing before-image application.

Verification / Alternative check:ARIES and other recovery algorithms distinguish UNDO and REDO phases; UNDO corresponds to backward recovery and uses before-images or compensation log records.

Why Other Options Are Wrong:

• After-images apply to forward recovery (redo), not backward recovery.
• Applying both together describes a full crash recovery sequence, not specifically backward recovery.
• Switching to a copy is failover or restore, not rollback.

Common Pitfalls:Confusing backup restoration (media recovery) with logical transaction rollback; both can be involved but are distinct steps.

Final Answer:Where the before-images are applied to the database