Your question is EC2 Instance Retrieval System. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Design a memory-efficient and fast retrieval system for available EC2 instances from a pool. Focus on how you would represent instance state, retrieve eligible instances quickly, and keep results fresh under changing availability.