Top MongoDB Interview Questions and Answers

This section covers a wide range of MongoDB interview questions, from fundamental NoSQL concepts to more advanced topics like sharding, replication, and performance tuning.

NoSQL Databases and MongoDB

NoSQL databases are non-relational databases designed to handle large volumes of unstructured or semi-structured data. They are often more scalable and flexible than traditional relational databases (RDBMS). MongoDB is a popular example of a NoSQL database.

Languages Supported by MongoDB

MongoDB drivers are available for many programming languages, including:

• C
• C++
• C#
• Java
• Node.js
• Perl
• PHP
• Python
• Ruby
• Scala
• Go
• Erlang

Types of NoSQL Databases

NoSQL databases are categorized into several types:

• Key-value stores: Simple key-value pairs (e.g., Redis).
• Document stores: Store data in JSON-like documents (e.g., MongoDB).
• Column-family stores: Store data in columns (e.g., Cassandra).
• Graph databases: Represent data as nodes and relationships (e.g., Neo4j).

MongoDB vs. Other SQL Databases

MongoDB's advantages include:

• Flexible schema (documents can have different structures).
• High performance (due to efficient indexing and storage).
• Scalability.

MongoDB's Database Type

MongoDB is a document-oriented database management system.

MongoDB vs. MySQL

MongoDB (NoSQL, document-oriented) and MySQL (relational) are fundamentally different. MongoDB is more flexible in schema design and often offers better scalability for certain types of data, but relational databases offer stronger consistency and ACID properties.

Why MongoDB is Popular

MongoDB's popularity stems from its:

• Document-oriented structure.
• Rich query language.
• High performance.
• High availability features.
• Scalability.

Primary and Foreign Key Relationships in MongoDB

MongoDB doesn't directly support relational primary and foreign key constraints like relational databases. However, you can achieve similar functionality through embedding documents or using references.

MongoDB and RAM Usage

MongoDB doesn't require a huge amount of RAM. It dynamically allocates memory based on the application's needs.

MongoDB ObjectID Structure

A MongoDB ObjectID is a 12-byte value composed of:

• 4 bytes: Timestamp (seconds since epoch)
• 3 bytes: Machine identifier
• 2 bytes: Process ID
• 3 bytes: Counter

BSON in MongoDB

Yes, MongoDB uses BSON (Binary JSON) to represent document structures.

Indexes in MongoDB

Indexes improve query performance. Without indexes, MongoDB performs a collection scan. Indexes allow MongoDB to quickly locate documents matching specific criteria.

Default Index in MongoDB

The _id field is automatically indexed in every collection.

Namespaces in MongoDB

A namespace is a combination of the database name and collection name (e.g., databaseName.collectionName).

Journaling and Hot Backups

Enabling journaling in MongoDB allows for creating safe hot backups.

Database Profiler

The MongoDB profiler tracks database operations, helping you identify slow queries and optimize performance.

Removing Object Attributes

Removing an attribute from a MongoDB document requires updating and saving the document.

MongoDB's Programming Language

MongoDB is primarily written in C++.

MongoDB RAM Requirements

MongoDB's RAM usage is dynamic and depends on the data and workload.

Programming Languages with MongoDB Drivers

MongoDB provides drivers for various popular programming languages.

Tables in MongoDB

MongoDB doesn't use tables; it stores data in collections.

Schema in MongoDB

MongoDB has a flexible, schema-less design. You don't need to define a fixed schema beforehand.

Configuring Cache Size in MongoDB

The cache size for the MMAPv1 storage engine is not configurable. The WiredTiger engine allows you to configure its cache size (using the storage.wiredTiger.engineConfig.cacheSizeGB setting).

Transactions and Locking in MongoDB

MongoDB uses a simpler approach to transactions than traditional relational databases, prioritizing performance and avoiding complex locking mechanisms. It supports atomicity for operations on a single document.

32-bit vs. 64-bit MongoDB

64-bit MongoDB is preferred over 32-bit because it doesn't have the 2GB memory limitation imposed by 32-bit systems.

Cleaning Up moveChunk Files

Files in the moveChunk directory are temporary files created during sharding operations. You can safely delete them after the operation completes.

Handling Slow or Down Shards

If a shard is down, queries against that shard will fail unless you use a strategy to handle partial failures. If a shard is slow, the query will wait until a response is received.

Covered Queries

A covered query uses an index to satisfy the query without needing to access the actual documents. This results in faster query execution.

Sharding in MongoDB

Sharding horizontally partitions data across multiple machines for improved scalability.

Replica Sets

Replica sets provide data redundancy and high availability. One node acts as primary; others are secondary replicas.

Primary vs. Secondary in Replica Sets

The primary node handles writes. Secondary nodes are read replicas that replicate data from the primary.

Default Write Target

By default, writes go to the primary node in a replica set.

CRUD Operations

MongoDB supports Create, Read, Update, and Delete (CRUD) operations on documents.

Document Structure in MongoDB

MongoDB uses BSON (Binary JSON) to represent documents.

Removing Documents from MongoDB

Removing a document also removes it from disk.

Large MongoDB Data File Sizes

MongoDB's pre-allocation of storage space can result in large file sizes.

Storage Engines in MongoDB

Storage engines manage how data is stored and accessed on disk (MMAPv1 and WiredTiger are common choices).

Configuring WiredTiger Cache Size

You can configure the WiredTiger cache size using the configuration option storage.wiredTiger.engineConfig.cacheSizeGB.

Concurrency in MongoDB

MongoDB uses reader-writer locks to manage concurrent access to data. Multiple readers can access the data simultaneously, but only one writer can access it at a time.

MongoDB vs. Redis

More details on MongoDB vs. Redis

MongoDB vs. CouchDB

More details on MongoDB vs. CouchDB

MongoDB vs. Cassandra

More details on MongoDB vs. Cassandra

Creating Databases in MongoDB

You don't need to explicitly create a database in MongoDB. A database is created automatically when you first insert data into a collection within that database.