1. What is MongoDB?

MongoDB is a source-available, cross-platform, document-oriented database program. Classified as a NoSQL database, MongoDB uses JSON-like documents with optional schemas.

2. What are the key features of MongoDB?

Document-oriented
High performance
High availability
Horizontal scalability
Ad hoc queries
Aggregation framework

3. What is a "Collections" in MongoDB?

A collection is a group of MongoDB documents. It's the equivalent of a table in a relational database. Collections do not enforce a fixed schema.

4. What is a "Document" in MongoDB?

A record in a MongoDB collection and the basic unit of data in MongoDB. Documents are analogous to JSON objects but exist in a binary-encoded format called BSON.

5. What is BSON?

BSON (Binary JSON) is a binary serialization format used by MongoDB to store documents. It extends JSON to include additional data types like Date, int, long, and Binary.

6. What is `_id` field in MongoDB?

A mandatory field for every document in a collection. If not provided by the application, MongoDB automatically generates a unique ObjectId for it.

7. What is an "ObjectId"?

A 12-byte identifier used as the default value for the _id field. It contains a timestamp, machine ID, process ID, and a counter.

8. What is "NoSQL"?

A term used to describe non-relational databases that provide a mechanism for storage and retrieval of data that is modeled in means other than the tabular relations used in relational databases.

9. What is "Dynamic Schema"?

The ability of MongoDB to store documents with different structures within the same collection. You don't need to define a table's columns before inserting data.

10. How do you insert a document in MongoDB?

Using the insertOne() or insertMany() methods.

11. How do you query a document in MongoDB?

Using the find() method. You can specify a query filter to narrow down the results (e.g., db.collection.find({ age: { $gt: 18 } })).

12. What are "Query Operators"?

Keywords starting with $ that allow you to specify complex query conditions (e.g., $eq, $gt, $in, $and, $or).

13. What is "Projection" in MongoDB?

The process of specifying which fields should be returned in a query's result (e.g., { name: 1, _id: 0 } to return only the name).

14. What is "Index" in MongoDB?

A data structure that improves the speed of data retrieval operations on a database table at the cost of additional storage and slower writes.

15. What are the different types of Indexes in MongoDB?

Single Field
Compound
Multikey (for arrays)
Text
Geospatial
Hashed

16. What is a "Compound Index"?

An index that contains references to multiple fields within a collection's documents. The order of fields in a compound index is important.

17. What is "Explain Plan" in MongoDB?

A command (explain()) that provides information on how MongoDB executes a query, including whether it uses an index.

18. What is the "Aggregation Framework"?

A powerful tool for processing records and returning computed results. It uses a pipeline-based approach to transform data.

19. What are the stages in an "Aggregation Pipeline"?

$match: Filters documents.
$group: Groups documents by a field and performs calculations.
$project: reshapes each document.
$sort: Sorts documents.
$limit and $skip: Paginate results.

20. What is "Replication" in MongoDB?

The process of synchronizing data across multiple servers (a replica set) to provide high availability and data redundancy.

21. What is a "Replica Set"?

A group of MongoDB processes that maintain the same data set. It consists of one primary node and multiple secondary nodes.

22. What is the difference between "Primary" and "Secondary" nodes?

Primary: Receives all write operations and records them in the oplog.
Secondary: Replicates data from the primary's oplog and handles read operations (if configured).

23. What is "Sharding"?

A method for distributing data across multiple machines (shards) to support very large data sets and high throughput operations.

24. What are the components of a "Sharded Cluster"?

Shard: Each shard contains a subset of the sharded data.
Config Server: Stores metadata and configuration settings for the cluster.
Query Router (mongos): Directs client operations to the appropriate shards.

25. What is a "Shard Key"?

A field or fields that MongoDB uses to distribute documents across shards. Choosing the right shard key is critical for cluster performance.

26. What is "GridFS"?

A specification for storing and retrieving large files (larger than the 16MB BSON document limit) in MongoDB.

27. What is "Capped Collection"?

A fixed-size collection that supports high-throughput operations that insert and retrieve documents based on insertion order.

28. What is a "Mongoose" in the context of Node/Express?

A popular Object Data Modeling (ODM) library for MongoDB and Node.js. It provides a schema-based solution to model your application data.

29. What is the difference between "Embedded" and "Referenced" documents?

Embedded: Nesting a document within another for related data (denormalization).
Referenced: Storing the _id of another document (normalization).

30. When should you use "Embedding" over "Referencing"?

Embedding: Use for "contains" relationships, data that is rarely changed, and improved read performance.
Referencing: Use for "many-to-many" relationships, large sub-documents, and data that is frequently updated.

31. What is "DBRef" in MongoDB?

A data structure used to represent a reference from one document to another.

32. What is "Journaling" in MongoDB?

A technique used by MongoDB to provide crash consistency by writing operations to an on-disk journal before applying them to the data files.

33. What is "Storage Engine"?

The component of a database responsible for managing how data is stored, both in memory and on disk. MongoDB supports WiredTiger and In-Memory.

34. What is the default Storage Engine in MongoDB?

WiredTiger (since MongoDB 3.2).

35. What is "TTL" (Time To Live) index?

A special type of single-field index that allows MongoDB to automatically remove documents from a collection after a certain amount of time or at a specific clock time.

36. What is "Covered Query"?

A query that can be satisfied entirely by an index without having to examine any documents in the collection.

37. What is "Write Concern"?

Describes the level of acknowledgment requested from MongoDB for write operations.

38. What is "Read Preference"?

Describes how MongoDB clients route read operations to the members of a replica set (e.g., primary, secondary, nearest).

39. What is the MongoDB "Oplog"?

The operations log (oplog.rs) is a special capped collection that keeps a rolling record of all data-modifying operations on the primary member of a replica set.

40. What is "Change Streams"?

A feature that allows applications to access real-time data changes in a collection, database, or entire deployment.

41. What is the "Database Profiler"?

A tool that collects detailed information about database commands executed against a running mongod instance.

42. What is the maximum BSON document size?

16 Megabytes.

43. What is "Data Modeling" in MongoDB?

The process of determining how to structure data based on the application's requirements, access patterns, and MongoDB's capabilities.

44. What is "Dot Notation" in MongoDB?

Used to access the elements of an array or the fields of an embedded document in a query (e.g., "address.city").

45. What is `upsert` in an update operation?

A boolean option that, if set to true, tells MongoDB to create a new document if no document matches the query filter.

46. What is the difference between `update()` and `save()`?

update(): Modifies an existing document.
save(): Replaces the entire document with the new one (upserts if _id is missing).

47. What is "Multikey Index"?

An index on an array field that creates a separate index entry for every element in the array.

48. What is "Atlas" in MongoDB?

The fully managed cloud database service provided by MongoDB, Inc.

49. What is the purpose of `mongoimport` and `mongoexport`?

Utilities used to import/export data from/to JSON, CSV, or TSV formats.

50. What is the "Aggregation stage $lookup"?

Performs a left outer join to a collection in the same database to filter in documents from the "joined" collection for processing.

MongoDB Interview Questions

Table of Contents

What is MongoDB?

What are the key features of MongoDB?

What is a "Collections" in MongoDB?

What is a "Document" in MongoDB?

What is BSON?

What is <code>_id</code> field in MongoDB?

What is an "ObjectId"?

What is "NoSQL"?

What is "Dynamic Schema"?

How do you insert a document in MongoDB?

How do you query a document in MongoDB?

What are "Query Operators"?

What is "Projection" in MongoDB?

What is "Index" in MongoDB?

What are the different types of Indexes in MongoDB?

What is a "Compound Index"?

What is "Explain Plan" in MongoDB?

What is the "Aggregation Framework"?

What are the stages in an "Aggregation Pipeline"?

What is "Replication" in MongoDB?

What is a "Replica Set"?

What is the difference between "Primary" and "Secondary" nodes?

What is "Sharding"?

What are the components of a "Sharded Cluster"?

What is a "Shard Key"?

What is "GridFS"?

What is "Capped Collection"?

What is a "Mongoose" in the context of Node/Express?

What is the difference between "Embedded" and "Referenced" documents?

When should you use "Embedding" over "Referencing"?

What is "DBRef" in MongoDB?

What is "Journaling" in MongoDB?

What is "Storage Engine"?

What is the default Storage Engine in MongoDB?

What is "TTL" (Time To Live) index?

What is "Covered Query"?

What is "Write Concern"?

What is "Read Preference"?

What is the MongoDB "Oplog"?

What is "Change Streams"?

What is the "Database Profiler"?

What is the maximum BSON document size?

What is "Data Modeling" in MongoDB?

What is "Dot Notation" in MongoDB?

What is <code>upsert</code> in an update operation?

What is the difference between <code>update()</code> and <code>save()</code>?

What is "Multikey Index"?

What is "Atlas" in MongoDB?

What is the purpose of <code>mongoimport</code> and <code>mongoexport</code>?

What is the "Aggregation stage $lookup"?