MongoDB Interview Questions

1. What is MongoDB and how does a document database differ from a relational database?

2. What is a document and a collection in MongoDB?

3. What is BSON and how does it differ from JSON in MongoDB?

4. How does indexing work in MongoDB and what types of indexes exist?

5. What is the MongoDB aggregation pipeline and how does it work?

6. What is the difference between embedding and referencing in MongoDB data modeling?

7. How does sharding work in MongoDB and why is it used?

8. What is a replica set in MongoDB and how does it provide high availability?

9. What is the difference between find() and aggregate() in MongoDB?

10. How do write concern and read concern work in MongoDB?

11. What is the _id field in MongoDB and how are ObjectIds generated?

12. How do you model a one-to-many relationship in MongoDB?

13. What are MongoDB transactions and when should you use them?

14. What is the difference between updateOne, updateMany, and replaceOne in MongoDB?

15. How does the MongoDB query optimizer choose an index?

16. What is a compound index and how does the index prefix rule work?

17. What is the difference between a covered query and a normal query in MongoDB?

18. How does MongoDB handle schema validation?

19. What are the trade-offs between normalization and denormalization in MongoDB?

20. What is capped collection in MongoDB and when is it useful?

21. How does the $lookup stage perform joins in MongoDB aggregation?

22. What is the difference between findOneAndUpdate and updateOne in MongoDB?

23. How do TTL indexes work in MongoDB?

24. What is the aggregation $group stage and how does it work?

25. How does MongoDB pagination work and why is skip/limit inefficient at scale?

26. What is the difference between MongoDB and a key-value store like Redis?

27. How do you ensure data consistency in a sharded MongoDB cluster?

28. What is the oplog in MongoDB and what role does it play in replication?

29. What are the different BSON data types in MongoDB?

30. How does the MongoDB aggregation $match stage differ from a find filter?

31. What is MongoDB?

32. What is a Document in MongoDB?

33. What is a Collection in MongoDB?

34. What is BSON?

35. What is an Index in MongoDB?

36. What is Aggregation in MongoDB?

37. What is Sharding in MongoDB?

38. What is Replication in MongoDB?

39. What Is a Capped Collection in MongoDB?

40. What Is WiredTiger in MongoDB?

41. What Is an Upsert in MongoDB?

42. What Is a Compound Index in MongoDB?

43. What Is the _id Field in MongoDB?

44. What Is Schema Design in MongoDB?

45. What Is Mongoose in MongoDB?

46. How Do You Decide Field Order in a MongoDB Compound Index (the ESR Rule)?

47. How Do You Read a MongoDB Explain Plan with executionStats?

48. How Does the MongoDB Plan Cache Work and Why Do Rejected Plans Matter?

49. Why Does Aggregation Stage Order Matter, and What Does the Optimiser Rewrite?

50. What Happens When an Aggregation Stage Exceeds Its Memory Limit?

51. Why Is $lookup Expensive, and What Would You Do Instead?

52. How Do Read Preference, Read Concern and Write Concern Differ?

53. What Is a Causally Consistent Session in MongoDB and When Do You Need One?

54. What Are the Practical Limits of MongoDB Multi-Document Transactions?

55. How Do You Choose a Shard Key, and What Makes a Bad One?

56. What Is a Jumbo Chunk, and What Does Chunk Migration Cost a Live Cluster?

57. How Do Change Streams Work, and What Is a Resume Token For?

58. Why Do TTL-Expired Documents Still Appear, and How Do You Operate TTL Indexes Safely?

59. What Is the Working Set, and How Do You Tell If It No Longer Fits in Memory?

60. How Do You Size a MongoDB Connection Pool, and What Goes Wrong If You Get It Wrong?

61. How Do You Use the Database Profiler to Diagnose Slow Queries in Production?