System Design: 7 Interview Patterns

System design questions look infinite. They are not. Most of them collapse into seven patterns. Recognize the pattern in the first two minutes and you have already earned half the signal.

The seven patterns

1. Rate limiting: token bucket vs. leaky bucket, where to place limits, how to fail open.
2. Feed and timeline: fan-out on write vs. read, hot users, cache invalidation.
3. Chat and messaging: long polling vs. WebSockets, message ordering, delivery guarantees.
4. Search and ranking: inverted index, sharding, freshness vs. relevance.
5. Uploads and media: presigned URLs, chunking, CDN edge caching, transcoding pipelines.
6. Payments and idempotency: idempotency keys, sagas, exactly-once side effects.
7. Notifications and jobs: queues, at-least-once vs. at-most-once, dead letter queues.

How to answer any of them

1. Clarify scope. Users, QPS, read vs. write ratio.
2. Sketch the happy path in three boxes before touching scale.
3. Name the pattern out loud. 'This is a fan-out on write problem.'
4. Introduce the bottleneck, then a specific fix.
5. Close with the trade-off you accepted and why.

What interviewers actually grade

Not the diagram. They grade whether you noticed the hard part, whether your trade-off is coherent, and whether you can explain why the simple version fails at 10x scale. Miss the hard part and no amount of boxes-and-arrows will save you.