Consistency

Synchronous writes

Primary nodes are strongly consistent. Successful write operations are durably stored in the Multi-AZ transactional log before returning to clients. During normal operations, read operations on primaries always return the most up-to-date data reflecting the effects from all prior successful write operations. Such strong consistency is preserved across primary failovers. Replica nodes are eventually consistent. Read operations from replicas (using READONLY command) might not always reflect the effects of the most recent successful write operations, with lag metrics published to CloudWatch. The read operations from a single replica are sequentially consistent. Successful write operations take effect on each replica in the same order they were executed on the primary.

Asynchronous writes

During normal operation, asynchronous writes provide the same consistency behavior as synchronous writes. However, as the write operations are returned to clients before being durably stored in the Multi-AZ transactional log, strong consistency is not preserved across primary failovers. In the event of a failure, up to 10 seconds of acknowledged write operations may be lost, meaning that after a failover, read operations on the new primary may not reflect all previously acknowledged writes. Asynchronous writes is not recommended for workloads that require strong consistency guarantees.