Code Room
System designHard
Question
Design a subscription billing system that charges customers on recurring cycles, handles plan changes with proration, and retries failed payments.
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.
Learn the concepts
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Run or narrate your approach, then ask the coach.