Which type of replication ensures that data is current between primary and backup databases in real-time?

Synchronous replication is the method that guarantees that data is current between primary and backup databases in real-time. In this approach, data written to the primary database is simultaneously written to the backup database. This ensures that both databases maintain identical data at all times, making it highly suitable for applications that require immediate data consistency and minimal data loss in case of failure.

For environments where data integrity and real-time availability are critical, synchronous replication is preferred as it offers an immediate response once the data is acknowledged by both the primary and secondary sites. This characteristic makes it the best choice for disaster recovery scenarios, transactional applications, or any situation where data accuracy and timeliness are paramount.

The other options do not provide the same level of real-time consistency. Asynchronous replication, for example, involves a lag between the primary database and the backup, as data is sent to the backup after it is written to the primary. Volume sync and mirroring also have specific use cases or configurations but do not inherently guarantee real-time data consistency like synchronous replication does.