Tell me about a system you built or ran on AWS. What services did you use and why?

Note: Naming services is easy; justifying them is what interviewers listen for. For every service you mention, be ready to say what you considered instead.

Cover:

• The workload and its scale. Traffic pattern, data volume, and availability requirement. A spiky consumer application and a steady internal service lead to different architectures.
• The services and the reasoning. "We used Fargate rather than EKS because we had four services and no one to run a cluster" is a real answer. "We used Lambda, S3, and DynamoDB" is a list.
• The hard part. Strong candidates: getting VPC networking and private connectivity right, a database migration with minimal downtime, cost growing faster than usage, or cold starts and concurrency limits under load.
• Outcome — cost, latency, availability, or deployment frequency, with numbers.

If you inherited the architecture rather than designing it, say so and describe what you changed.