What specific property of database transactions guarantees that either all operations within a transaction complete successfully, or none of them do?

The specific property of database transactions that guarantees either all operations within a transaction complete successfully, or none of them do, is Atomicity. Atomicity is one of the four key properties of ACID transactions, where ACID stands for Atomicity, Consistency, Isolation, and Durability.

Atomicity treats a database transaction as a single, indivisible logical unit of work. A transaction is a sequence of one or more operations (like reading, inserting, updating, or deleting data) that are performed together to achieve a specific logical outcome. For instance, transferring money from one bank account to another involves two primary operations: debiting the sender's account and crediting the recipient's account. These two operations must either both succeed or both fail together for the transaction to be valid.

If all operations within a transaction complete successfully without any errors or interruptions, the entire transaction is considered complete and its changes are permanently applied to the database. This finalization process is known as a commit. Once a transaction is committed, its changes are irreversible within the context of that transaction.

Conversely, if any operation within the transaction fails for any reason—such as a system crash, a power outage, a database error, or an explicit cancellation (known as an abort)—then the entire transaction is considered to have failed. In such a scenario, Atomicity ensures that none of the partial changes made by the transaction are saved. The database management system automatically reverses all operations that were part of the failed transaction, returning the database to the exact state it was in before the transaction began. This reversal process is called a rollback.

For example, in a bank transfer: if the operation to debit the sender's account completes but the operation to credit the recipient's account fails (perhaps due to a system error), Atomicity ensures that the debit operation is also undone. The sender's account balance is restored to its original value, and the recipient's account remains unchanged. This way, the database maintains its integrity, preventing a situation where money is debited from one account but not credited to another, which would result in lost funds.