System design interviews for mid-level engineers: what Reddit understates

Junior and mid engineers on Reddit treat system design like a secret syllabus. Interviewers care about **structured thinking under ambiguity**, not reciting Netflix's stack.

**Framework that works** 1. Clarify requirements (scale, latency, consistency). 2. High-level boxes and data flow. 3. Deep dive one bottleneck. 4. Tradeoffs explicitly (CAP, cost, operability).

**Common mistakes** Jumping to Kafka because Reddit said so. Ignoring read/write ratio. No back-of-envelope numbers.

Practice aloud with follow-ups—thin answers get probed exactly like production incidents.