What does the two-phase commit protocol ensure?

The two-phase commit protocol is primarily designed to ensure that a distributed transaction either commits across all involved nodes or rolls back entirely. This is crucial for maintaining data consistency across distributed systems.

In the first phase, known as the "prepare" phase, the coordinator node requests all participating nodes to prepare to commit the transaction. Each participant confirms whether it can commit the transaction based on its local conditions. This waiting for confirmation from each participant is what guarantees that all systems are ready to finalize the operation.

Only when all participants are ready does the coordinator move to the second phase, where it instructs each participant to commit. If any participant cannot commit, the coordinator can trigger a rollback across all participants, ensuring that no partial data changes are made. Hence, the correct answer highlights the importance of waiting for confirmation before finalizing operations, which is a fundamental aspect of the two-phase commit protocol.

Other options do not accurately reflect the protocol’s function; immediate commits are not guaranteed, rollbacks are not the primary purpose but rather a safety measure, and sequential operation execution is not a defining characteristic of this protocol.