Synchronous replication creates a data copy in real time. Real-time data replication helps to reduce the RPO and increase reliability in the event of a disaster. However, it is expensive as it requires additional resources in the primary system for continuous data replication. 

Asynchronous replication creates copies of data with some lag or as per the defined schedule. However, asynchronous replication is less expensive as it uses fewer resources compared to synchronous replication. You may choose asynchronous replication if your system can work with a longer RPO.

In terms of database technology such as Amazon RDS, synchronous replication is applied if we create RDS with multiple availability zone failover. For read replicas, there is asynchronous replication, and you can use that to serve report and read requests.

As illustrated in the following architecture diagram, in synchronous replication, there is no lag of data replication between the master and standby instance of the database while, in the case of asynchronous replication, there could be some lag while replicating the data between the master and reapplication instance:

Let's explore some methods of data reapplication for the synchronous and asynchronous approaches.