DP-900 Describe core data concepts Practice Question
A company updates a customer's address in a database. The update must ensure that all existing orders still reference a valid customer ID. The database checks the foreign key constraint and rejects the update if it would violate referential integrity. Which ACID property does this enforcement represent?
⚠ Common exam trap
A common mix-up: candidates confuse consistency with atomicity, thinking that rejecting an invalid update is about 'all-or-nothing' behavior, when in fact consistency is specifically about maintaining data integrity rules and constraints.
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
✓
Consistency
Consistency ensures that any database transaction brings the database from one valid state to another, preserving all defined rules, including constraints like foreign keys. In this scenario, the foreign key constraint enforcement prevents an update that would leave orphaned order records, directly upholding the consistency property by rejecting the transaction if it violates referential integrity.
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 it's wrong here
Atomicity means a transaction is treated as an all-or-nothing unit: if any part of it fails, all changes are rolled back. A foreign-key violation causes the update statement to be rejected before it writes data, so no partial change exists to roll back. Atomicity alone does not validate data; it would still allow an invalid row to remain if the statement completed, so this failure is not about the A property.
- ✓
Consistency
Why this is correct
Consistency guarantees that a transaction will not leave the database in a state that violates any declared integrity constraint. Here, changing a customer's address to a value that does not exist in the referenced table would orphan related records, so the database rejects the update to preserve referential integrity. This is the 'C' in ACID: each transaction must take the database from one valid state to another, regardless of how many rows are affected.
- ✗
Isolation
Why it's wrong here
Isolation controls how concurrent transactions interact, preventing dirty reads, lost updates, and write skew via locks or snapshot mechanisms. In this scenario, the update is invalid even if it is the only transaction executing; no concurrency or interleaving is involved. The rejection results from checking a database constraint, not from any isolation level or locking behavior, so this is not an I property issue.
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Durability
Why it's wrong here
Durability ensures that once a transaction commits, its changes persist even after a system crash, typically through write-ahead logging or similar techniques. Because the foreign-key check fails before the update is committed, the transaction never reaches the commit point, so there is no successful change to be made durable. Durability cannot retrospectively validate data; it only preserves the state of already-committed transactions.
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