Apache Cassandra Interview Questions

1. What is Apache Cassandra and what problems does it solve?

2. What is the difference between Cassandra and a relational database?

3. What is the difference between Cassandra and MongoDB?

4. What is a wide-column store and how does Cassandra's data model work?

5. What is a partition key and how does it determine data distribution?

6. What is the difference between a partition key and a clustering key?

7. How does Cassandra's ring architecture and consistent hashing work?

8. What are replication and the replication factor in Cassandra?

9. What are replication strategies (SimpleStrategy vs NetworkTopologyStrategy)?

10. How does tunable consistency work in Cassandra (consistency levels)?

11. What does eventual consistency mean in Cassandra?

12. What is the difference between QUORUM and ONE consistency levels?

13. How does Cassandra handle writes with the commit log and memtable?

14. What are SSTables and how does the write path persist data?

15. What is compaction in Cassandra and why is it important?

16. What is a tombstone in Cassandra and why can tombstones cause problems?

17. What is read repair and how does anti-entropy work in Cassandra?

18. What are hinted handoffs in Cassandra?

19. What is CQL and how does it differ from SQL?

20. How do you design data models in Cassandra (query-driven design)?

21. Why is denormalization encouraged in Cassandra?

22. What are secondary indexes in Cassandra and when should you avoid them?

23. What are materialized views in Cassandra?

24. What is a gossip protocol and how do Cassandra nodes communicate?

25. How does Cassandra achieve high availability and fault tolerance?

26. What is a coordinator node in Cassandra?

27. How do you handle multi-datacenter deployments in Cassandra?

28. What are common Cassandra anti-patterns to avoid?

29. When should you choose Cassandra over other databases?

30. What are Cassandra performance tuning and best practices?

31. A read query fails with TombstoneOverwhelmingException — how do you diagnose and fix it?

32. How do you choose between STCS, LCS and TWCS for a Cassandra table?

33. How do you detect and fix large or hot partitions in Cassandra?

34. What is the difference between incremental and full repair, and how do you run repair safely in production?

35. What happens to reads and writes at each consistency level when replicas are down?

36. How do lightweight transactions work in Cassandra and what do they cost?

37. Nodes show long GC pauses and dropped mutations — how do you investigate?

38. When is a CQL BATCH the right tool, and when does it hurt the cluster?

39. How do you add, replace and decommission Cassandra nodes without data loss?

40. How do you add a new datacentre to a live Cassandra cluster without downtime?

41. How do you configure a Cassandra driver's load-balancing, retry and speculative-execution policies for production?

42. Read latency has degraded at the high percentiles but the cluster looks healthy — how do you triage it?

43. How do you back up and restore a Cassandra keyspace, and what makes a restore actually work?

44. How do you secure a Cassandra cluster with authentication, authorisation and encryption?

45. How do you upgrade a Cassandra cluster to a new major version without downtime?

46. How does driver paging work, and how do you scan an entire Cassandra table safely?

47. How do you apply schema changes safely on a live Cassandra cluster, and what causes schema disagreement?

48. How does TTL work in Cassandra at cell level, and what surprises does it cause in production?

49. How do counter columns work in Cassandra, and why are counter updates not idempotent?

50. Which Cassandra metrics do you monitor and alert on in production, and why those?

51. How do you test an application that uses Cassandra, including its failure behaviour?

52. How do racks and snitches affect replica placement, and what breaks when racks are configured unevenly?

53. How do you size partitions for a time-series table in Cassandra so they never grow unbounded?

54. Why do deletes make Cassandra reads slower, and how do you design around it?

55. How do compaction strategies differ in write amplification and disk headroom, and how does that drive your choice?

56. How do you work out consistency levels across two datacentres so reads see writes without cross-region latency?

57. How do you schedule repair in Cassandra, and what happens if a cycle does not finish in time?

58. Given a new query pattern, how do you choose between a secondary index, a materialised view and a second table?

59. How does clustering key ordering shape which range queries are efficient in Cassandra?

60. What happens to lightweight transactions under contention, and what would you use instead?

61. Walk me through tracing a single slow Cassandra query end to end to find where the time goes