Your question is API for Millions Daily. 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).
How would you design an API to handle millions of transactions per day?
Explain the API contract, request validation, idempotency, authentication, storage, asynchronous processing, and horizontal scaling strategy. Include an optional low-latency ML decision path, batch training and evaluation, monitoring, and failure handling. State assumptions where requirements such as peak traffic, consistency, latency, and transaction semantics are unspecified.
