Question

Two transactions execute concurrently, but their results are the same as if they were executed one after another. This property is called?

• A. Atomicity
• B. Durability
• C. Consistency
• D. Serializability

Hint:

Think of Serializability as a "Correctness Criterion." If a concurrent execution doesn't match a serial one, it is considered "incorrect" because it has allowed transactions to interfere with each other.

Answer 1

The Correct Option is D

Concept:
This question refers to the "I" in ACID properties: Isolation. The goal of isolation is to ensure that concurrent execution of transactions leaves the database in a state that could have been obtained if the transactions were executed sequentially (one by one). This specific outcome is called "Serializability."

Step 1: The Illusion of Single Access.
Even though the database might be handling hundreds of transactions per second, the serializability property ensures that each transaction feels like it has exclusive access to the data. No transaction should see the "partial work" or intermediate states of another transaction.

Step 2: Comparison with other ACID properties.

• Atomicity: The "all or nothing" property. A transaction either fully completes or is fully rolled back.
• Consistency: Ensures that a transaction takes the database from one valid state to another valid state, following all integrity constraints.
• Durability: Ensures that once a transaction is committed, its changes are permanent, even in the case of a system crash.

Step 3: Why Serializability is the answer.
The question specifically describes the equivalence between concurrent execution and sequential (serial) execution. This is the definition of a serializable schedule. It is the highest level of isolation provided by a database.