DynamoDB vs RDS at 10K, 100K, and 1M RPS: a pre-deployment simulation comparison

I have made this mistake exactly once. About three years into my AWS career, I inherited a Lambda-based API with DynamoDB on the backend and was tasked with migrating it to Aurora PostgreSQL - the data model had grown relational and the team wanted proper foreign key constraints. The migration went smoothly in UAT. We promoted to production on a Tuesday night. By Thursday morning, Lambda concurrency was exhausted, Aurora was throwing connection pool errors, and I was sitting in a war room with t