30 Common MongoDB Interview Questions Every Developer Should Prepare 2027

• By Devraj

• 3rd November 2026

Mastering NoSQL database concepts is essential for modern software engineering roles. Whether you are aiming for a backend position or a full-stack MERN role, clearing technical interviews requires a deep understanding of document design, indexing strategies, and performance optimization. This comprehensive guide covers 30 top MongoDB interview questions ranging from basic CRUD operations to advanced aggregation pipelines, replication, and sharding. Prepare effectively with practical code examples, industry insights, and expert tips designed to help you ace your next technical evaluation and secure your dream developer job with confidence.

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Beginner level

1. What is MongoDB?

MongoDB is an open-source, document-oriented NoSQL database. It stores data as flexible, JSON-like BSON documents instead of rows and columns, which makes it well suited to changing data models and horizontal scaling.

2. How does MongoDB differ from a relational database?

MongoDB is schema-flexible, stores documents in collections, and scales horizontally through sharding. SQL databases use fixed schemas, tables and joins, and traditionally scale vertically.

3. What is BSON?

BSON (Binary JSON) is MongoDB’s storage format. It adds types JSON lacks, such as Date, ObjectId, Decimal128 and binary data, and it is faster to traverse.

4. What is a collection and a document?

A document is a single record made of field-value pairs. A collection is a group of documents, similar to a table, but documents in it need not share the same structure.

5. What is _id?

Every document has a unique _id field that acts as its primary key. If you don’t supply one, MongoDB generates a 12-byte ObjectId containing a timestamp, machine/process identifier and counter.

6. How do you perform CRUD operations?

js
db.users.insertOne({ name: “Ravi”, age: 28 })
db.users.find({ age: { $gt: 25 } })
db.users.updateOne({ name: “Ravi” }, { $set: { age: 29 } })
db.users.deleteOne({ name: “Ravi” })

7. What is the difference between find() and findOne()?

find() returns a cursor over all matching documents. findOne() returns the first matching document, or null.

8. What are the common data types in MongoDB?

String, Number (Int32, Int64, Double, Decimal128), Boolean, Date, ObjectId, Array, Object (embedded document), Null and Binary Data.

9. What are $set, $unset, $inc and $push?

These are update operators. $set sets a field, $unset removes it, $inc increments a number, and $push appends to an array.

10. What is the difference between $and and $or?

$and matches documents that satisfy all conditions. $or matches documents that satisfy at least one.

Intermediate level

11. What are indexes and why are they important?

Indexes are data structures that let MongoDB find documents without scanning the whole collection. Without them, queries do a collection scan (COLLSCAN), which is slow on large data.

12. What types of indexes does MongoDB support?

Single field, compound, multikey (arrays), text, geospatial (2dsphere), hashed, TTL, partial, sparse, unique and wildcard indexes.

13. How do you analyze query performance?

Use explain(“executionStats”). Look for IXSCAN rather than COLLSCAN, and compare totalDocsExamined with nReturned.
js
db.orders.find({ status: “shipped” }).explain(“executionStats”)

14. What is the aggregation framework?

It is a pipeline of stages ($match, $group, $sort, $project, $lookup, $unwind) that transforms and analyzes data.

js
db.orders.aggregate([
{ $match: { status: “paid” } },
{ $group: { _id: “$userId”, total: { $sum: “$amount” } } },
{ $sort: { total: -1 } }
])

15. What is $lookup?

It performs a left outer join between collections inside an aggregation pipeline. It works, but it is costlier than reading embedded data.

16. Embedding vs referencing: when do you use each?

Embed when datais read together, has a one-to-few relationship and is rarely updated independently. Reference when the relationship is one-to-many or many-to-many, the data is large, or it changes often.

17. What is the ESR rule for compound indexes?

Order fields as Equality, Sort, Range. Following it lets MongoDB use the index for filtering and sorting efficiently.

18. What is a covered query?

A query where all requested fields are in the index, so MongoDB never reads the documents. It is very fast.

19. What is a TTL index?

An index that automatically deletes documents after a set time, useful for sessions, OTPs and logs.

js
db.sessions.createIndex({ createdAt: 1 }, { expireAfterSeconds: 3600 })

20. What is the difference between updateOne and replaceOne?

updateOne modifies specific fields using operators. replaceOne replaces the whole document except _id.

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Advanced level

21. What is replication in MongoDB?

A replica set is a group of mongod instances holding the same data: one primary handles writes and secondaries replicate it. It gives high availability and automatic failover through elections.

22. What is sharding?

Sharding distributes data across multiple servers (shards) using a shard key. It enables horizontal scaling, and a mongos router directs queries to the right shard.

23. How do you choose a good shard key?

Pick a key with high cardinality, even distribution and good query isolation. Avoid monotonically increasing keys, which create hotspots. Hashed or compound keys often help.

24. What are write concern and read concern?

Write concern sets how many nodes must acknowledge a write (w: 1, w: “majority”). Read concern sets the consistency level of reads (local, majority, snapshot).

25. Does MongoDB support ACID transactions?

Yes. Single-document operations are always atomic. Multi-document ACID transactions have been supported since 4.0 on replica sets and since 4.2 on sharded clusters. They carry a performance cost, so good schema design should minimize their need.

26. What is the WiredTiger storage engine?

It is MongoDB’s default engine. It provides document-level concurrency, compression (Snappy, zstd) and checkpointing with journaling for durability.

27. What are change streams?

They let applications subscribe to real-time changes on a collection, database or cluster without polling. They are built on the oplog and useful for event-driven systems.

28. What is the oplog?

The operations log is a capped collection on the primary that records all write operations, and secondaries replay it to stay in sync.

29. How do you secure a MongoDB deployment?

Enable authentication and role-based access control, use TLS, restrict network access (bind IP, firewall, VPC), encrypt data at rest, enable auditing, and consider Queryable Encryption for sensitive fields.

30. How would you design a schema for an e-commerce app?

Keep users, products and orders as separate collections. Embed order line items inside the order, store a price snapshot at purchase time, and reference productId and userId. Add compound indexes such as { userId: 1, createdAt: -1 }, and use the Bucket or Extended Reference pattern where it fits.

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Skill Hives is a learning platform for developers who want practical, job-ready skills. We focus on real projects, clear explanations and interview preparation rather than theory alone.

When you prepare with Skill Hives, you get:

• Interview-focused content: questions grouped by Fresher, Intermediate and Advanced levels.
• Hands-on practice: code examples and mini-projects you can build yourself.
• Updated material: content refreshed for current technologies and hiring trends.
• Career guidance: tips on resumes, portfolios and technical rounds.

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FAQs

Q1. How should I prepare for a MongoDB interview in 2027?

Master CRUD, indexing, the aggregation pipeline, schema design, replication and sharding. Practice on a free MongoDB Atlas cluster and be ready to explain your design choices.

Q2. Is MongoDB still in demand in India?

Yes. It is widely used in startups, fintech, e-commerce and MERN stack roles, and Atlas adoption keeps demand steady.

Q3. What MongoDB topics do interviewers ask most?

Indexing and query optimization, aggregation, embedding vs referencing, replication, sharding and transactions.

Q4. Is MongoDB better than MySQL?

Neither is universally better. MongoDB suits flexible, fast-evolving and high-scale document data, while MySQL suits strongly relational data with complex joins.

Q5. How long does it take to learn MongoDB for interviews?

With basic database knowledge, about 2 to 4 weeks of focused practice covers fresher-level and many intermediate questions.