DP-900 Describe core data concepts Practice Question
A database system must ensure that when a transfer of funds between two accounts is processed, if the system crashes after debiting the first account but before crediting the second, the database automatically undoes the debit. This property is best described as:
⚠ Common exam trap
Many exam-takers confuse atomicity with consistency, thinking that maintaining a correct total balance (consistency) is what undoes the debit, but atomicity is the property that specifically handles the rollback of incomplete transactions after a crash.
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. If the system crashes after debiting one account but before crediting the other, the database's transaction log records the partial changes, and during recovery, the database engine (e.g., SQL Server's ARIES recovery model) performs an automatic rollback of the uncommitted transaction, undoing the debit to maintain 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.
- ✓
Atomicity
Why this is correct
Atomicity treats the transfer as a single indivisible unit: both the debit from one account and the credit to another must succeed together. If any operation fails or the system crashes mid-transaction, the database rolls back to the pre-transaction state, so the partial debit is undone. This ensures no orphaned or incomplete financial entry persists.
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Consistency
Why it's wrong here
Consistency guarantees that a transaction transforms the database from one valid state to another, enforcing integrity constraints like balances being non-negative. It does not specify how to recover from a crash; it only checks that the final state satisfies rules. In the transfer scenario, even if consistency is preserved overall, the atomicity failure (partial debit) would have already violated the invariant if the debit remained, but consistency itself does not trigger rollbacks.
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Isolation
Why it's wrong here
Isolation controls how concurrently executing transactions interact, preventing dirty reads or lost updates by locking resources or using multiversion concurrency control. It is relevant when multiple transfers happen simultaneously, but it does not govern the outcome of a single transaction that crashes. The partial debit issue occurs even with only one transaction executing, so isolation is not the solution.
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Durability
Why it's wrong here
Durability ensures that once a transaction commits, its changes survive failures through write-ahead logs and persistent storage. In the transfer, the crash happens before commit, so the changes are not durable; they are rolled back. Durability would only be relevant after a successful commit, but here the transaction is incomplete and must be undone, so durability does not apply.
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Senior Network & Security Engineer · founder of Courseiva
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