DP-900 Describe core data concepts Practice Question
A banking application processes a funds transfer transaction consisting of two steps: debit $100 from Account A and credit $100 to Account B. If the system crashes after debiting Account A but before crediting Account B, the database automatically reverts the debit, restoring Account A to its original balance. Which ACID property guarantees this behavior?
⚠ Common exam trap
Many candidates confuse atomicity with consistency, thinking that 'restoring the original balance' is about maintaining data rules, when in fact it is the rollback of an incomplete transaction that demonstrates atomicity.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
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Atomicity
Atomicity ensures that a transaction is treated as a single, indivisible unit of work. In this scenario, the debit and credit are part of one transaction; if the system crashes after the debit but before the credit, the database management system (DBMS) automatically rolls back the entire transaction, undoing the debit to restore Account A's original balance. This all-or-nothing behavior is the defining characteristic of atomicity.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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Atomicity
Why this is correct
Atomicity is the correct property here because it enforces the all-or-nothing execution of a transaction. In this funds transfer, the debit step was written, but the corresponding credit never completed before the crash. Atomicity requires the entire transaction to be treated as a single, indivisible unit, so the partial debit must be rolled back and the account restored to its original state. Without atomicity, a system failure could leave a half-applied transaction, corrupting financial records.
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Consistency
Why it's wrong here
Consistency ensures that a transaction brings the database from one valid state to another, but it does not govern the reversal of partial operations after a crash. The automatic rollback of the debit is enforced by the **Atomicity** property, which guarantees that all steps of a transaction complete or none are applied. Consistency is tempting because it also concerns data correctness, but it would be the correct choice only if the scenario involved violating a business rule—such as allowing the debit to create a negative balance when a constraint forbids it.
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Isolation
Why it's wrong here
Isolation is wrong because it governs how transactions interact with each other when they run concurrently, not how a single transaction behaves after a crash. It ensures that uncommitted changes from one transaction are invisible to other transactions and prevents issues like dirty reads or lost updates. In this scenario, there is no competing transaction interfering with the debit, and the problem is that the transaction itself crashed mid-execution. Since the rollback of the debit is required regardless of any other concurrent activity, isolation is not the ACID property that explains this outcome.
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Durability
Why it's wrong here
Durability does not apply because it only guarantees that once a transaction is committed, its changes will survive subsequent crashes or power failures. Here, the transaction never reached a commit point—the debit was written but the credit was not, and the transaction aborted. Durability would require the debit to remain permanently applied only if a COMMIT had occurred, but no commit happened. The automatic reversal of the uncommitted debit is a rollback, which is the opposite of a durability guarantee.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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