System design interviews reward clarity under ambiguity. A copilot helps most when it keeps you on a checklist while you talk through trade-offs out loud.
Start with requirements, not boxes
Clarify functional requirements, non-functional requirements, scale assumptions, and consistency expectations. State what you are optimizing for: latency, cost, operational simplicity, or write throughput.
A simple answer skeleton
- Restate the problem and confirm scope.
- Estimate scale — DAU, QPS, storage growth.
- Draw the high-level flow — clients, API, services, data stores, async paths.
- Deep dive on the bottleneck — sharding, caching, queues, idempotency.
- Cover failures — hot keys, partial outages, backpressure.
- Close with monitoring and rollout plan.
Questions that appear in almost every loop
- Design a URL shortener or rate limiter — classic API + storage trade-offs.
- Design a notification system — fan-out, retries, deduplication.
- Design a feed or search system — indexing, ranking, freshness.
Keep a personal note of two designs you can whiteboard from memory. A copilot fills gaps when the interviewer pivots — for example from storage to caching or from happy path to failure recovery.