RDS DB, in one AZ, replicates data synchronously to Standby Replica (created automatically), in another AZ, in Multi-AZ deployment. RDS DB replicates data asynchronously to Read Replicas .
If the objects from the source bucket in the same region are replicated to another bucket in the same region, it is called same region replication (SRR). If the objects are replicated to a bucket in a different region, it is called cross region replication (CRR).
Use ASG if the EC2 is unable to handle surge of requests. ASG can be integrated with SQS. Based on the length of the queue, the group of EC2 instances can be increased or decreased.