Forwards DNS queries to any zone from on-prem DNS (For example) (whether it is a Hosted Zone in your Account, another account in Publically) using the the Route 53 resolver.

These queries are passed back from the resolver back to the inbound endpoint and back to the on-prem DNS. The Internet resolver will resolve anything on the Internet.

Acts a a conditional forwarder for anything else that is defined as a resolver rule to other DNS Servers (Whether these be Hosted in AWS or elsewhere)

Custom patching - applied directly on instance

Custom high availability option

Managed instance - provides the automation - provisioning, monitoring, backup/restore. Custom allows to pause RDS to make customisations and resume automation

EC2 instance launch and EBS are in customer AWS Account. IAM also manageable

Apps and DB can be lifted and shifted to RDS Custom. DMS can be used to migrate. Or RDS Custom instance can be launched as a standby to the on-premise instance. Once replication between on-prem and RDS Custom is complete, it can be failed over to the RDS Custom instance. Close to zero downtime for failover

RDS Custom allows to customise Host, O/S and DB for certain use cases. CAVEAT: RDS CUSTOMISATIONS CAN IMPACT RDS CUSTOM AUTOMATION. IF SO IT IS THE CUSTOMERS RESPONSIBILITY TO BACK OUT CUSTOMISATIONS THEY MADE OR IMPLEMENT IN A DIFFERENT WAY OR REVERT THEM ALTOGETHER.

Keeps primary data in S3 while caching frequently accessed data locally

Used as an extension of the onprem SAN Envionment where storage space could be limited

Still provides low latency access to data

Keeps primary data locally while asynchronosly copying data to S3 (Used like an offsite backup of the Data)

Used for DR scenarios