SMS carrier routing gateway
Design a carrier-grade SMS routing gateway (an SMPP-based messaging hub) that routes 50,000 messages/sec between application senders and dozens of mobile carriers, each carrier with its own throughput limits, encoding rules, and delivery-receipt behavior. You need least-cost/quality routing across carriers, reliable delivery-receipt correlation, and to never exceed a carrier's bind/throughput cap. How do you bind to carriers, route, and reconcile receipts?
What a strong answer looks like
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.
Clarify5:00 left
Estimate5:00 planned
Design15:00 planned
Deep dive12:00 planned
Failure8:00 planned
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.