ORIGINAL SYSTEM DESIGN QUESTION
Personalized activity feed
Build a feed of followed creators with private-account permissions and muted topics.
Original exercise · advanced. Editorial role relevance does not imply a company has asked this question.
Requirements and constraints
- Two million daily readers; 100,000 new posts/day; a creator may have one million followers.
- New posts should appear within 30 seconds; deleted content must disappear.
- State assumptions and justify security, cost, operational ownership and data retention.
Pressure round
A celebrity post causes the fan-out queue to lag for 20 minutes.
Build and explain the design
Requirements & scope
Who uses it? What must it do? What is explicitly out of scope? State latency, durability and privacy goals.
Self-review guidance
Define the actors, required operations and out-of-scope behavior. Explicit requirement: New posts should appear within 30 seconds; deleted content must disappear.
Capacity estimates
State assumptions, peak requests/second, data volume and retention. Show your arithmetic and units.
Self-review guidance
Use this workload: Two million daily readers; 100,000 new posts/day; a creator may have one million followers. Convert rates and payload sizes into peak compute/storage assumptions; show units, headroom and what remains unmeasured.
APIs & data model
Describe endpoints/events, entity keys, indexes, authorization, and idempotency.
Self-review guidance
Define post/follow/mute records, pagination cursors and visibility checks.
Architecture & data flow
Describe components and request/event paths. Use plain text arrows to sketch your design; explain each responsibility.
Self-review guidance
Contrast fan-out on write with fan-out on read for celebrity creators.
Failures & observability
What happens on overload, duplicate events, partial failures or lost dependencies? Discuss monitoring and recovery.
Self-review guidance
Pressure-test: A celebrity post causes the fan-out queue to lag for 20 minutes. Describe bounded recovery, deduplication, observability and a user-visible failure state.
Tradeoffs & alternatives
Compare two plausible approaches. State the costs, consistency model and what would make you change the design.
Self-review guidance
Explain hybrid fan-out, freshness and the read amplification it creates.
Design guidance describes assumptions and tradeoffs; no architecture has been simulated or benchmarked.
Practice this question
Save your work privately, explain decisions and return to your revision queue.
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