DP-900 Describe core data concepts Practice Question
A banking system processes a money transfer between two accounts. The system is designed so that after the transaction is committed, the results are permanently saved and survive any subsequent system failure, such as a power outage. Which ACID property ensures this behavior?
⚠ Common exam trap
It's easy for candidates to confuse durability with atomicity, thinking that 'surviving failures' means the transaction either completes fully or not at all, but atomicity handles the rollback of partial transactions, not the persistence of committed ones.
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
✓
Durability
Durability ensures that once a transaction is committed, its changes are permanently stored and survive system failures, such as power outages or crashes. In this banking scenario, the money transfer results are written to non-volatile storage (e.g., disk) via a write-ahead log, guaranteeing that the committed state is recoverable even after a restart.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Durability
Why this is correct
Durability is the ACID property that guarantees once a transaction is committed, its changes are permanently recorded in non-volatile storage, even if a crash occurs immediately afterward. In a money transfer scenario, the updated balances in both the source and destination accounts must survive a power loss, disk failure, or system restart to prevent the transferred funds from disappearing. This is implemented via write-ahead logging or journaling (e.g., SQL Server transaction log), where commit records are forced to disk before the transaction is acknowledged as successful.
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Atomicity
Why it's wrong here
Atomicity ensures that a transaction is treated as a single, indivisible unit: all statements within it either succeed fully or roll back completely, with no partial updates visible. In the money transfer example, atomicity ensures that if the debit from the source account succeeds but the credit to the destination fails, the entire transaction is rolled back, leaving both accounts unchanged. Atomicity does not address whether committed data persists after a failure; it only handles the execution phase before the commit point.
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Consistency
Why it's wrong here
Consistency ensures that a transaction transforms the database from one valid state to another valid state, preserving all defined rules, constraints, triggers, and relationships (e.g., foreign keys, check constraints, unique indexes). For a money transfer, consistency would enforce that the total money across both accounts remains the same, and that account balances never become negative if business rules forbid it. Consistency is about logical correctness at the start and end of the transaction, not about the physical persistence of committed data following a system crash.
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Isolation
Why it's wrong here
Isolation governs how uncommitted changes from one transaction are hidden from other concurrently executing transactions, preventing dirty reads, non-repeatable reads, and phantom reads. In a banking transfer, isolation ensures that while the transfer is in progress, other transactions either see the old balances or wait until the transfer commits; they never see an intermediate state where money is debited but not yet credited. Isolation is a concurrency-control concern (implemented via locks, snapshots, or versioning), fundamentally different from durability, which is about surviving failures after commit.
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