Overcoming Overlapping Subnets: Syncing RDS Across AWS Orgs with PrivateLink

When we were tasked with an application rebuild, a unique networking challenge came to light: we needed to sync an existing legacy MySQL database (in our general AWS organization) to a new Postgres database (in a client-specific organization). The Challenge Cross-organization communication usually suggests VPC Peering. However, we hit a major roadblock: the separate VPCs were using the same CIDR blocks. Since VPC peering does not support overlapping subnets, it was out of the question. There was of course the option to change subnets, but with existing infrastructure, this was a volatile option that would lead to resource destruction and rebuilding, increasing the time of the task. After researching ways to tunnel traffic without merging address spaces, I landed on AWS PrivateLink. It was the perfect solution—it provides private connectivity between VPCs even with IP conflicts and integrates seamlessly into Infrastructure as Code (in this case, Terraform). The "Provider" Side (Legacy A