How do replicas ensure the consistency or synchronization of their state?

Replicas ensure consistency of the state through replication mechanisms, which can be synchronous or asynchronous. In synchronous replication, the master waits for confirmation from the replica that the data has been written before considering the transaction complete, ensuring that the replica does not lag behind. In asynchronous replication, the master does not wait for confirmation, which increases performance but may lead to some lag in replicas. Transaction logs (WAL in PostgreSQL), which are applied sequentially on replicas, are often used to implement consistency, providing identical data states.