System designHardsd-g419

Subject EtlLevel Senior–Staff~45 minCommon in Networking & APIs · Distributed systems interviewsIndustries Technology, Software development

Question

Design an exactly-once streaming ETL that consumes ~120k events/sec of financial transactions from Kafka, enriches each with reference data and FX rates, and writes aggregated per-merchant hourly revenue into both an OLAP warehouse (for analytics) and a low-latency KV store (for a live merchant dashboard). A consumer crash, a rebalance, or a redeploy must never double-count or drop revenue, and the dashboard must converge to the same totals the warehouse reports. Reads are heavy on the KV store (10k QPS); the warehouse is append-mostly.

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.