MongoDB interview questions and answers to help you prepare for MongoDB-related interviews:
What is MongoDB?
MongoDB is a NoSQL, document-oriented database that stores data in JSON-like BSON (Binary JSON) format, offering high scalability and flexibility.
Explain NoSQL databases.
NoSQL databases are non-relational databases designed to store unstructured data and scale easily. They include document, key-value, column, and graph databases.
What are some advantages of MongoDB?
High scalability, flexibility with schema design, fast performance, and support for big data are key advantages of MongoDB.
How does MongoDB differ from traditional SQL databases?
MongoDB stores data as documents (BSON) rather than tables and rows. It is schema-less, supports horizontal scaling, and is ideal for handling large volumes of unstructured data.
What is a BSON?
BSON is Binary JSON, a binary representation of JSON documents. It supports additional data types such as date and binary, making it more efficient for MongoDB storage.
What are collections in MongoDB?
A collection is a group of MongoDB documents, equivalent to tables in relational databases.
Explain the concept of a document in MongoDB.
A document is a basic unit of data in MongoDB, represented in BSON format. It is similar to a row in relational databases, but it can store more complex data types and structures.
What is a primary key in MongoDB?
Every MongoDB document has a unique _id field that acts as the primary key.
What is sharding in MongoDB?
Sharding is a method of distributing data across multiple servers to achieve horizontal scaling and handle large datasets.
What is replication in MongoDB?
Replication involves duplicating data across multiple servers to ensure high availability and data redundancy in case of server failure.
What is a schema in MongoDB?
MongoDB is schema-less, meaning documents can have different fields and structures, but the schema can still be enforced at the application level.
How does MongoDB store data?
MongoDB stores data in BSON documents, which are collections of key-value pairs, where the value can be an array or another document.
Explain embedded/nested documents in MongoDB.
Embedded or nested documents allow storing related data within a single document, improving read efficiency and reducing the need for joins.
What is data normalization and denormalization in MongoDB?
Normalization refers to breaking data into smaller pieces and linking them, while denormalization stores related data in the same document for quicker access at the cost of storage redundancy.
What are indexes in MongoDB?
Indexes improve query performance by creating a data structure that allows MongoDB to quickly find documents matching query criteria.
How do you create an index in MongoDB?
Use the createIndex() method, for example: db.collection.createIndex({name: 1}) creates an index on the name field.
What are compound indexes?
Compound indexes index multiple fields in a document, which can help optimize queries that use multiple fields in the query filter.
What is the capped collection?
A capped collection is a fixed-size collection that automatically overwrites its oldest entries when it reaches its maximum size.
Explain GridFS in MongoDB.
GridFS is a specification for storing large files in MongoDB. It splits files into smaller chunks and stores them across multiple documents.
What are the types of relationships that MongoDB supports?
MongoDB supports one-to-one, one-to-many, and many-to-many relationships, typically implemented through embedded documents or references.
How do you perform CRUD operations in MongoDB?
CRUD stands for Create, Read, Update, Delete. Operations are performed using methods like insert(), find(), update(), and remove().
What is the MongoDB query language?
MongoDB uses a query language based on JSON-like syntax to interact with the database, for example: db.collection.find({age: {$gt: 25}}) to find all documents where age is greater than 25.
Explain the Aggregation Framework in MongoDB.
Aggregation is a way to process data in stages, such as filtering, grouping, and sorting, using the $match, $group, $sort, and $project stages.
What is a pipeline in MongoDB aggregation?
A pipeline is a series of stages through which data is processed, each stage transforming the data in some way (e.g., filtering, grouping, sorting).
How do you sort query results in MongoDB?
Use the sort() method, for example: db.collection.find().sort({name: 1}) to sort documents by name in ascending order.
What is the difference between findOne() and find()?
findOne() returns a single document that matches the query, while find() returns all documents that match the query.
How do you update documents in MongoDB?
Use the updateOne(), updateMany(), or replaceOne() methods. For example, db.collection.updateOne({name: 'John'}, {$set: {age: 30}}) updates the age of the document where the name is 'John'.
Explain the $set operator in MongoDB.
The $set operator is used to update specific fields in a document without affecting the other fields.
How do you delete documents in MongoDB?
Use deleteOne() or deleteMany(). For example, db.collection.deleteOne({name: 'John'}) deletes one document where the name is 'John'.
How does MongoDB handle joins?
MongoDB doesn't support traditional joins like SQL databases. However, related data can be embedded in the same document or referenced through manual joins in the application layer.
What are replica sets in MongoDB?
Replica sets are groups of MongoDB servers that maintain the same data. They provide redundancy and high availability, with one primary node and multiple secondary nodes.
What is a primary node in a replica set?
The primary node is the only server that can accept write operations. It replicates the changes to the secondary nodes.
What is the role of secondary nodes in replication?
Secondary nodes replicate data from the primary node and can serve read operations, providing high availability and load balancing.
How does automatic failover work in MongoDB?
In a replica set, if the primary node fails, MongoDB automatically elects a new primary node from the secondaries, ensuring high availability.
What is a write concern in MongoDB?
Write concern determines the level of acknowledgment required from MongoDB after a write operation. It controls the durability and consistency of writes.
What is a read concern in MongoDB?
Read concern controls the consistency and isolation properties of read operations, specifying whether reads should return data that has been fully committed to disk or just acknowledged by the primary node.
What is journaling in MongoDB?
Journaling ensures data durability by recording write operations in a journal file before they are written to the database, protecting against power failures or crashes.
How do you back up data in MongoDB?
MongoDB provides several backup methods, including the mongodump utility, snapshot backups, and backup services like MongoDB Atlas.
What is the oplog in MongoDB?
The oplog (operations log) is a special capped collection in a replica set that records all changes made to the data on the primary node, allowing secondary nodes to replicate them.
How is MongoDB scalable?
MongoDB supports horizontal scaling through sharding, distributing data across multiple servers to handle large datasets and high traffic loads.
How do you optimize MongoDB performance?
Optimizations include using indexes, proper schema design, sharding, caching, and limiting the number of returned documents in queries.
What are write locks in MongoDB?
Write locks prevent other write operations from being executed until the current write operation completes, ensuring data consistency.
How does MongoDB ensure security?
MongoDB provides authentication, authorization, encryption (TLS/SSL), and IP whitelisting to ensure secure access and data protection.
What is MongoDB Atlas?
MongoDB Atlas is a fully managed cloud database service provided by MongoDB, offering automated backups, scaling, monitoring, and security features.
How do you use MongoDB in a production environment?
Use replica sets for high availability, sharding for scalability, proper indexing, and backup strategies to ensure reliability in production.
What is the purpose of $lookup in MongoDB?
$lookup performs a left outer join with another collection, allowing data from different collections to be combined in the same query result.
Explain change streams in MongoDB.
Change streams allow applications to receive real-time notifications of changes in the data, such as document insertions, updates, or deletions.
What is the Aggregation Pipeline Optimization?
Aggregation Pipeline Optimization refers to techniques MongoDB uses to optimize queries and aggregation pipelines for faster processing, such as reordering pipeline stages and using indexes.
How does MongoDB handle ACID transactions?
MongoDB provides multi-document ACID transactions, which allow atomicity, consistency, isolation, and durability across multiple documents and collections.
How do you monitor MongoDB?
MongoDB provides several tools for monitoring, including mongostat, mongotop, the MongoDB Atlas monitoring dashboard, and third-party tools.
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