System designMediumsd-g016

Subject Notification systemsLevel Mid–Senior~40 minCommon in Distributed systems interviewsIndustries Technology, Software development

Question

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.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.