Activity-digest aggregation
Design the activity-digest engine for a collaboration product (think GitHub/Jira) that, instead of sending one notification per event, intelligently aggregates many events into batched in-app and email digests: '12 people commented on issues you follow.' Scale: 50M users, millions of events/min. Requirements: collapse bursts (50 comments on one thread = one line, not 50), respect each user's chosen digest cadence (instant / hourly / daily), and don't lose an event that arrives at a window boundary.
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.
Clarify4:30 left
Estimate4:30 planned
Design13:30 planned
Deep dive10:30 planned
Failure7:00 planned
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.