DP-900 Describe core data concepts Practice Question
A bank processes online fund transfers. Each transaction must ensure that either both the debit from the sender's account and the credit to the receiver's account occur, or if any part fails, the entire transaction is rolled back. Which ACID property does this guarantee?
⚠ Common exam trap
Microsoft often tests atomicity by describing a multi-step operation and asking which ACID property ensures the 'all-or-nothing' behavior, and the trap here is that candidates confuse atomicity with consistency, thinking that consistency alone prevents partial updates, when in fact atomicity is the property that enforces the rollback of incomplete transactions.
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
✓
Atomicity
Atomicity ensures that a transaction is treated as a single, indivisible unit of work. In this fund transfer scenario, atomicity guarantees that both the debit and credit operations either complete successfully together or are fully rolled back if any part fails, preventing partial updates that could leave the system in an inconsistent state.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Atomicity
Why this is correct
Atomicity is the ACID property that treats the entire fund transfer—debit source account and credit destination account—as one indivisible unit. If either SQL statement succeeds while the other fails, the transaction manager issues a rollback, discarding the partial write and restoring the original balances. Without this all-or-nothing guarantee, a bank could lose money or create funds from nothing during a network or application failure.
- ✗
Consistency
Why it's wrong here
Consistency ensures that a transaction transforms the database from one valid state to another by enforcing constraints, triggers, and referential integrity—for example, a negative account balance may be rejected. In the bank scenario, consistency would catch a state where the destination account was credited without a corresponding debit if the database has a rule stating that transfers must balance, but it does not manage the sequencing or rollback of operations that is the essence of atomicity.
When this WOULD be correct
A question that asks: 'A bank requires that after a fund transfer, the total balance across all accounts remains unchanged. Which ACID property ensures this?' would make consistency the correct answer.
- ✗
Isolation
Why it's wrong here
Isolation controls how concurrent transactions see each other's uncommitted changes, governing read phenomena such as lost updates or dirty reads. If two transfers touch the same account, isolation levels (e.g., READ COMMITTED, SERIALIZABLE) determine whether one transaction's intermediate debits are visible to the other. However, isolation does not bundle multiple statements into one unit; it addresses interleaved executions, not the rollback of a failed transfer sequence, so it is not the decisive property here.
When this WOULD be correct
A question asking: 'A database system must prevent two concurrent transfers from reading the same account balance and causing a race condition. Which ACID property is primarily responsible?' would make Isolation correct.
- ✗
Durability
Why it's wrong here
Durability focuses on what happens after a COMMIT is acknowledged, not on preventing partial work during execution. Once the transfer’s commit record is written to the transaction log and the acknowledgment is sent, durability guarantees the updated balances survive subsequent power loss, crash, or restart. In the scenario, if the system fails before commit, durability does not require undoing a partial $500 debit—that responsibility belongs to atomicity, not durability.
When this WOULD be correct
A question that asks: 'After a successful online fund transfer, the bank's system crashes. When the system recovers, the transferred amount is still reflected in both accounts. Which ACID property is demonstrated?' would make durability the correct answer.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The DP-900 exam frequently reuses these exact scenarios with slightly different constraints.
✓AtomicityCorrect answer▾
Why this is correct
Atomicity is the ACID property that treats the entire fund transfer—debit source account and credit destination account—as one indivisible unit. If either SQL statement succeeds while the other fails, the transaction manager issues a rollback, discarding the partial write and restoring the original balances. Without this all-or-nothing guarantee, a bank could lose money or create funds from nothing during a network or application failure.
✗ConsistencyWrong answer — click to see why▾
Why this is wrong here
The question describes the 'all-or-nothing' execution of a transaction, which is the definition of atomicity, not consistency. Consistency ensures that a transaction brings the database from one valid state to another, preserving integrity constraints.
★ When this WOULD be the correct answer
A question that asks: 'A bank requires that after a fund transfer, the total balance across all accounts remains unchanged. Which ACID property ensures this?' would make consistency the correct answer.
Why candidates choose this
Candidates may confuse atomicity with consistency because both deal with transaction correctness. They might think that ensuring no partial updates is about maintaining data integrity, which is actually consistency.
✗IsolationWrong answer — click to see why▾
Why this is wrong here
Isolation ensures that concurrent transactions do not interfere with each other, but the question describes a requirement that a transaction must complete entirely or not at all, which is atomicity, not isolation.
★ When this WOULD be the correct answer
A question asking: 'A database system must prevent two concurrent transfers from reading the same account balance and causing a race condition. Which ACID property is primarily responsible?' would make Isolation correct.
Why candidates choose this
Candidates may confuse the 'all-or-nothing' nature of atomicity with the idea that transactions are isolated from failures, or they may think isolation ensures that partial results are not visible, which is actually a consequence of atomicity.
✗DurabilityWrong answer — click to see why▾
Why this is wrong here
Durability ensures that once a transaction is committed, its changes persist even after a system failure. The question describes a transaction that either fully completes or fully rolls back, which is the definition of atomicity, not durability.
★ When this WOULD be the correct answer
A question that asks: 'After a successful online fund transfer, the bank's system crashes. When the system recovers, the transferred amount is still reflected in both accounts. Which ACID property is demonstrated?' would make durability the correct answer.
Why candidates choose this
Candidates may confuse durability with atomicity because both deal with transaction outcomes, but durability focuses on persistence after commit, while atomicity focuses on all-or-nothing execution.
Analysis generated from the official DP-900blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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