AWS Cost Optimization for Microservices: ECS vs EKS vs Lambda

The runtime question usually arrives disguised as a cost question: “our AWS bill is too high, should we move to serverless?” Almost always the honest answer is that the runtime is not the biggest line on the invoice. But the runtime decision does determine which levers you get to pull later, so it is worth making deliberately. This is the decision framework I use for Java services, and the levers that have moved real bills.