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.