Question 1

What is MongoDB and how does it differ from traditional relational databases?

Accepted Answer

MongoDB is a document-oriented NoSQL database that stores data in flexible, JSON-like documents instead of tables with fixed schemas. Unlike relational databases that use SQL and require predefined schemas, MongoDB allows you to store semi-structured data with varying fields, making it ideal for applications with evolving data models.

Question 2

What is BSON and how does it relate to JSON?

Accepted Answer

BSON (Binary JSON) is a binary-encoded serialization format that MongoDB uses internally to store documents and transmit data. While JSON is text-based and human-readable, BSON extends JSON with additional data types like Date, Binary data, and ObjectId, and is optimized for fast encoding, decoding, and traversal.

Question 3

What is a MongoDB document and what is its maximum size?

Accepted Answer

A MongoDB document is a BSON object consisting of field-value pairs, similar to a JSON object. Documents can contain nested documents, arrays, and various data types, with a maximum size limit of 16 megabytes to ensure efficient performance and prevent excessive RAM usage.

Question 4

What is a collection in MongoDB?

Accepted Answer

A collection is a group of MongoDB documents, analogous to a table in relational databases. Collections don't enforce a schema, so documents within the same collection can have different fields and structures, though they typically store related data with similar structure for organizational purposes.

Question 5

What is the _id field and ObjectId in MongoDB?

Accepted Answer

The _id field is the primary key for every MongoDB document, automatically created if not provided. ObjectId is the default type for _id - a 12-byte unique identifier that includes timestamp, machine identifier, process ID, and counter components, guaranteeing uniqueness across distributed systems without coordination.

Question 6

What are indexes in MongoDB and why are they important?

Accepted Answer

Indexes are special data structures that store a small portion of the collection's data in an easy-to-traverse form, dramatically improving query performance. Without indexes, MongoDB must perform a collection scan examining every document, while indexed queries can locate documents in logarithmic time using B-tree structures.

Question 7

What is a covered query in MongoDB?

Accepted Answer

A covered query is one where all queried fields exist in the index, allowing MongoDB to return results directly from the index without examining any documents. This is extremely fast because indexes are stored in memory and are much smaller than full documents, reducing query execution time by 10x or more.

Question 8

What is the aggregation framework in MongoDB?

Accepted Answer

The aggregation framework is MongoDB's powerful data processing pipeline that transforms and analyzes documents through a sequence of stages like $match, $group, and $project. It's similar to Unix pipes for data, allowing you to filter, reshape, and compute results for analytics, reporting, and complex queries beyond simple find() operations.

Question 9

What is embedding vs referencing in MongoDB data modeling?

Accepted Answer

Embedding stores related data within a single document as nested objects or arrays, providing excellent read performance. Referencing stores relationships using document IDs that point to data in other collections, keeping documents smaller and avoiding duplication but requiring multiple queries or $lookup operations to retrieve related data.

Question 10

What is a replica set in MongoDB?

Accepted Answer

A replica set is MongoDB's built-in replication mechanism consisting of multiple MongoDB instances that maintain the same data set. It provides redundancy and high availability with automatic failover - if the primary node fails, the replica set automatically elects a new primary from the secondary nodes.

Question 11

What is the oplog in MongoDB?

Accepted Answer

The oplog (operations log) is a special capped collection that records all write operations that modify data in a MongoDB replica set. Secondaries read from the primary's oplog and apply these operations to maintain data consistency - it's the mechanism that powers replication.

Question 12

What is sharding in MongoDB and when would you use it?

Accepted Answer

Sharding is MongoDB's horizontal scaling strategy that distributes data across multiple servers based on a shard key. You use sharding when your dataset exceeds a single server's storage capacity, when you need to distribute read/write operations for better performance, or when you need geographic data distribution for compliance or latency optimization.

Question 13

What is a shard key and how do you choose one?

Accepted Answer

A shard key is the indexed field or fields that MongoDB uses to partition data across shards. A good shard key has high cardinality (many unique values), even distribution to avoid hotspots, and alignment with your most common query patterns to minimize cross-shard queries.

Question 14

What are multi-document transactions in MongoDB?

Accepted Answer

Multi-document transactions allow you to execute multiple operations across multiple documents and collections as a single atomic unit. If any operation fails, the entire transaction is rolled back, ensuring data consistency with ACID guarantees similar to traditional RDBMS transactions.

Question 15

What is the difference between MongoDB and SQL databases?

Accepted Answer

MongoDB uses a flexible document model with JSON-like documents and dynamic schemas, while SQL databases use rigid table structures with predefined schemas. MongoDB encourages denormalization and embedding data for read performance, whereas SQL databases normalize data across tables and use JOINs, making MongoDB better suited for applications with evolving data models and horizontal scaling needs.

MongoDB Fundamentals(6 questions)

What is MongoDB and how does it differ from traditional relational databases?

What is BSON and how does it relate to JSON?

Explain the structure of a MongoDB document and its maximum size limit.

What is a collection in MongoDB?

What is the _id field and ObjectId in MongoDB?

What is the difference between MongoDB and other NoSQL databases like Cassandra or Redis?

Security(11 questions)

What are the authentication mechanisms in MongoDB?

Explain role-based access control (RBAC) in MongoDB.

What is field-level encryption in MongoDB?

How do you enable authentication in MongoDB?

What are the built-in roles in MongoDB?

How do you create custom roles?

What is TLS/SSL encryption in MongoDB?

What is auditing in MongoDB?

How do you implement IP whitelisting?

What is Queryable Encryption in MongoDB 7.0+?

What security best practices should you follow for MongoDB deployments?

MongoDB Drivers and Applications(6 questions)

What are the different MongoDB drivers and how do you connect from Node.js?

What is Mongoose and how does it differ from the native MongoDB driver?

How do you handle connection pooling in MongoDB?

How do you implement retry logic for transient failures?

What is the difference between callback, promise, and async/await patterns with MongoDB?

How do you handle schema versioning in applications?

MongoDB Atlas and Cloud(6 questions)

What is MongoDB Atlas?

What is Atlas Search and how does it work?

What are Atlas triggers and how do you use them?

What is Atlas Data Lake?

How do you configure backup and restore in Atlas?

What is Atlas App Services (formerly Realm)?

Advanced Topics(15 questions)

What are change streams in MongoDB?

What is time series data support in MongoDB?

What are capped collections and when would you use them?

How does MongoDB handle concurrency and locking?

What are GridFS and when would you use it?

What is MongoDB Compass?

What are views in MongoDB?

What is the $expr operator and when would you use it?

How do you use MongoDB with Docker?

What is MongoDB Realm Sync?

How do you perform database migrations in MongoDB?

What are collations and how do they affect string comparisons?

What is the difference between mongod and mongos?

How do you backup and restore MongoDB databases?

What are TTL indexes and when would you use them?

CRUD Operations(9 questions)

How do you insert documents in MongoDB? Explain insertOne() and insertMany().

Explain the different query operators in MongoDB (comparison, logical, element, array operators).

How do you update documents in MongoDB? What is the difference between updateOne(), updateMany(), and replaceOne()?

What are the different update operators ($set, $unset, $inc, $push, $pull, $addToSet)?

How do you delete documents in MongoDB?

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

What is the difference between remove() and deleteOne()/deleteMany()?

How do you perform bulk write operations in MongoDB?

What is an upsert operation and when would you use it?

Indexing(16 questions)

What are indexes in MongoDB and why are they important?

What are the different types of indexes in MongoDB?

What is a compound index and how does index order matter?

What is a covered query in MongoDB?

How do you analyze query performance using explain()?

What is the ESR (Equality, Sort, Range) rule for compound indexes?

What are multikey indexes and when are they automatically created?

What is a text index and how do you perform text searches?

What are geospatial indexes and what types does MongoDB support?

What is a hashed index and when would you use it?

What are wildcard indexes and when are they useful?

How do you list, create, and drop indexes in MongoDB?

What is index intersection in MongoDB?

What are partial indexes and when would you use them?

What are sparse indexes and how do they differ from partial indexes?

What is the impact of indexes on write performance?

Aggregation Framework(14 questions)

What is the aggregation framework in MongoDB?

Explain the $match stage in aggregation.

Explain the $group stage and its accumulator operators.