System designEasysd-g535

Subject Todo backendLevel Entry–Mid~25 minCommon in Networking & APIs interviewsIndustries Software development, Technology

Question

Design the backend for a multi-user to-do app. Each user has lists, and each list has tasks with a title, a done flag, an optional due date, and an ordering within the list. Users sign in and only see their own data. The app is used from web and mobile and may go briefly offline. Cover the API and data model, how you scope data to a user, and how you keep task ordering stable.

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

Narrate your design

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Run or narrate your approach, then ask the coach.