All stacks · Operational data stores
Wide-column serving stores and the modeling rules that keep them fast.
Cassandra🟢BeginnerALLOW FILTERING timed out the coordinator and a "quick report" full-scanned the serving cluster. Model the query first, extract without a table scan, and keep Cassandra off the warehouse path.
Cassandra (and Scylla) is a wide-column serving store: query-first modeling, partition keys, and no joins. Data engineers meet it as a sink for features or a source that cannot be SQL-scanned like Postgres.
No. Wrong access pattern, wrong cost, no SQL grain. Extract to a lake/warehouse for analytics. Cassandra is for known keys at serving latency.
New table (or materialized view, with eyes open). You do not add a join. If the product adds a query path, the table design changes. That is the interview answer.
Redis for hot, small, volatile keys. Cassandra for larger, durable, query-shaped serving. Many ML platforms use both.
