DP-900 Practice Question: Identify considerations for relational data on Azure
A banking application processes fund transfers. When a transfer is executed, the system must either successfully debit one account and credit the other, or if any step fails, the entire operation must be rolled back so no partial changes remain. Which ACID property directly enforces this behavior?
⚠ Common exam trap
It's easy for candidates to confuse Consistency with Atomicity, thinking that 'keeping data consistent' means the same as 'all-or-nothing rollback,' but Consistency only enforces rules like constraints and triggers, not the indivisible execution of a multi-step operation.
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
✓
A) Atomicity
Atomicity ensures that a transaction is treated as a single, indivisible unit of work. In this banking scenario, the debit and credit operations are part of one transaction; if either step fails, the entire transaction is rolled back, leaving no partial changes. This is the core property that enforces the 'all-or-nothing' behavior described.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
A) Atomicity
Why this is correct
Atomicity guarantees that a transaction is treated as a single, indivisible unit: either all its operations (e.g., debit and credit in a fund transfer) execute successfully and commit, or none take effect. If any step fails, the database management system rolls back all completed steps to the original state, using undo logs or shadow paging. This all-or-nothing property prevents partial updates, making it the correct ACID property for a multi-step transfer.
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B) Consistency
Why it's wrong here
Consistency ensures that a transaction transforms the database from one valid state to another, preserving invariants such as primary key uniqueness, foreign key references, and check constraints. While this is essential for data integrity, it does not coordinate multiple steps to succeed or fail together; a transfer could leave accounts unbalanced while still satisfying constraints. Atomicity, not consistency, enforces the rollback of a partially executed transfer.
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C) Isolation
Why it's wrong here
Isolation defines when and how changes made by an uncommitted transaction become visible to other concurrent transactions, typically via locking or multi-version concurrency control. It prevents dirty reads, non-repeatable reads, and phantom reads by shielding intermediate states from other sessions. However, isolation does nothing to undo a failed transfer's partial steps; it merely hides them temporarily, whereas atomicity decides whether those steps are permanently applied or discarded.
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D) Durability
Why it's wrong here
Durability guarantees that once the system commits a transaction, its changes persist across subsequent software crashes, power outages, or media failures, typically through write-ahead logging or durable storage. This property applies only after the commit point and does not manage rollback of uncommitted partial work. In a fund transfer that fails before commit, durability would not roll back the debit—atomicity's write-ahead log would ensure that recovery discards or completes the transaction atomically.
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