System designHardsd-g605

Subject Durability append wal storeLevel Senior–Staff~40 minCommon in Databases & SQL · Storage & CDN · Distributed systems interviewsIndustries Technology

Question

Design a durable append-only / write-ahead-log store that other services use as their commit log (think a Kafka/BookKeeper-style log). Constraints: writes must be durable before ack (no data loss on a single-node power failure), ordered within a partition, high append throughput (hundreds of MB/s per partition), readers tail the log with low latency, and you replicate for fault tolerance. Cover the append path, durability guarantee, and replication.

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.