DP-900 Describe core data concepts Practice Question
A retail company uses a point-of-sale (POS) system that records each sales transaction in a database. Each transaction involves reading the current inventory, updating the stock level, and recording the sale. The database must ensure that concurrent transactions do not interfere with each other, so that one transaction does not see partially updated data from another. Which property of a database transaction ensures this isolation?
⚠ Common exam trap
Test-takers frequently confuse isolation with atomicity, thinking that 'not seeing partially updated data' is about the transaction being all-or-nothing, when in fact it is about preventing interference between concurrent transactions.
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
✓
Isolation
Isolation ensures that concurrent transactions do not interfere with each other, so each transaction sees a consistent snapshot of the database as if it were the only transaction running. In the POS scenario, isolation prevents one transaction from reading partially updated inventory data from another transaction, which could lead to overselling or stock discrepancies. This property is typically implemented through locking mechanisms or multi-version concurrency control (MVCC).
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 guarantees that a multi-step transaction is treated as a single, indivisible unit: either all its operations (like updating inventory, recording payment, and printing a receipt) commit successfully, or none are applied. It prevents partial updates when a failure occurs mid-transaction, but it has nothing to do with how different transactions interact when run at the same time. Interference between concurrent POS transactions is a matter of isolation, not atomicity.
- ✗
Consistency
Why it's wrong here
Consistency ensures that every transaction leaves the database in a valid state that satisfies all defined rules, such as foreign keys, unique constraints, check constraints, and data types. For a POS system, this might mean that a product's quantity on hand can never drop below zero and that a sale must reference an existing customer. This correctness of data relative to schema rules does not control how concurrent transactions observe or interfere with each other's uncommitted changes; that is the role of isolation.
- ✓
Isolation
Why this is correct
Isolation is the ACID property that separates the effects of concurrent transactions so that one transaction cannot see the intermediate, uncommitted state of another. Without isolation, a POS system could read inventory levels that another sale is in the middle of updating, leading to overselling or duplicate charges. Isolation ensures that the concurrent execution produces the same result as some serial order of the transactions, thereby preventing dirty reads, non-repeatable reads, and phantom reads. This directly addresses the retail scenario described.
- ✗
Durability
Why it's wrong here
Durability guarantees that after a transaction is committed, its changes are permanently recorded even if the system immediately crashes, loses power, or experiences a storage failure. In a POS context, this means a completed sale's record and inventory deduction survive a reboot or network outage. However, durability says nothing about the visibility of uncommitted data or the interaction between transactions executing concurrently; those issues are governed by isolation.
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