DP-900 Describe core data concepts Practice Question
A financial database system ensures that once a transaction is committed, the data changes are permanently stored and will survive any subsequent system failure, such as a power outage or crash. Which property of ACID transactions does this describe?
⚠ Common exam trap
Watch out — candidates often confuse durability with atomicity, thinking 'permanent storage' relates to the all-or-nothing nature of a transaction, but atomicity only guarantees that partial changes are rolled back, not that committed data survives crashes.
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
✓
D: Durability
D is correct because durability guarantees that once a transaction is committed, the changes persist permanently, even in the event of a system failure like a power outage or crash. In SQL Server, this is implemented via the write-ahead log (WAL) and checkpoint processes, ensuring committed data is flushed to disk before acknowledging success.
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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A: Atomicity
Why it's wrong here
Atomicity guarantees that all operations inside a transaction are treated as a single indivisible unit: if any step fails, the entire transaction is rolled back to its original state. However, this property only governs execution until the moment of commit; it does not ensure that a successfully committed transaction's data will persist across a power outage or crash, which is the domain of durability.
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B: Consistency
Why it's wrong here
Consistency ensures that every transaction transitions the database from one valid state to another, enforcing constraints, triggers, and business rules to maintain data integrity. This property is about correctness of data before and after a transaction, not about making committed data persistent. Even if consistency is upheld, a sudden system failure could still lose the effects of a committed transaction unless durability guarantees are in place.
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C: Isolation
Why it's wrong here
Isolation controls the visibility of intermediate, uncommitted changes to concurrent transactions by serializing their execution or using locks/multi-versioning, so they don't interfere with each other. While this is crucial for concurrency control, it has no bearing on the survival of committed data after a hardware failure or crash. The question's focus on post-commit permanence is explicitly handled by durability, not isolation.
- ✓
D: Durability
Why this is correct
Durability is the ACID property that once a transaction has been committed, its changes are permanently recorded and will survive subsequent system failures, such as sudden power loss or crashes. Database engines implement this using mechanisms like write-ahead logging (WAL) or backup/restore strategies to ensure committed transactions can be recovered. This directly matches the scenario in the question: after the 'COMMIT' statement returns success, the data is expected to remain intact.
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