System designMediumsd-g713

Subject Storage engineLevel Mid–Senior~40 minCommon in Storage & CDN interviewsIndustries Technology, Software development

Question

Design an append-only event store for an event-sourced system: events are immutable facts grouped into per-aggregate streams, writers append to a stream with an expected-version for optimistic concurrency, and readers replay a stream from the beginning to rebuild state or subscribe to a global feed of new events. Expect millions of streams, ~100K appends/sec, and a guarantee that appends to one stream are strictly ordered. How do you store and serve this?

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.