Rules of thumb
Partition < 100MB (softer) to 1GB (hard). Rows per partition ≤ 100k typical. Even distribution across cluster.
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Bucketing pattern
Time-series 'sensor_readings by sensor_id + day'. Prevents unbounded growth. Time bucket = day/hour/minute depending on rate.
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Composite partition key
PRIMARY KEY ((user_id, region), created_at, event_id)