On-callHardoc-g237

Subject Hot partitionLevel Senior–Staff~35 minCommon in Distributed systems · Algorithms & data structures interviewsIndustries Technology, Software development

Question

Cassandra cluster storing IoT sensor readings, partition key = `(sensorType, day)`, clustering by timestamp. As fleet grew, write p99 climbed and some nodes show high load while others idle. `nodetool` shows a few partitions are enormous (tens of GB) and tombstone/compaction warnings on them; reads of 'latest readings' for popular sensor types time out. Only a handful of `sensorType` values are common. Triage and redesign the data model so this doesn't recur.

What a strong answer looks like

Stop the bleeding first (mitigate), then form hypotheses from real signals. Separate root cause from symptom, communicate status as you go, and close with what prevents a repeat.

Learn the concepts

Diagram & narrate the incident

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Run or narrate your approach, then ask the coach.