Updated May 8, 2023
Mongo Database Interview Questions and Answers
Mongo database interview questions allow interviewers to test candidates’ knowledge, especially software developers and database administrators, in the domain of MongoDB DBMS.
With the popularity of “MERN” (MongoDB, Express, ReactJS, Node.js) and “MEAN” (MongoDB, Express, AngularJS, Node.js) stack web development, the popularity of MongoDB has skyrocketed as a requirement for candidates. As such, the average base salary of a software engineer with knowledge of MongoDB is $91,524 per year. Furthermore, senior developers can earn up to $159,000, and senior database administrators earn a base salary of $165,000 annually.
MongoDB helps efficiently store data for content management systems and web applications and supports several programming languages. Hence, several job positions require MongoDB proficiency. Preparing before the interview with Mongo database interview questions can help the candidate to answer well and obtain an exciting job with a lucrative salary.
Part 1 – Mongo Database Interview Questions
1. What is MongoDB?
Answer:
- MongoDB is a popular open-source NoSQL document-oriented database management system.
- MongoDB stores data in flexible, JSON-like documents, allowing for easy data modeling and flexible schema design.
- It supports dynamic schema, meaning fields can be added to documents without defining a rigid structure.
- It has a powerful query language and supports an aggregation framework for advanced data analysis.
- MongoDB is widely used in modern web applications, content management systems, mobile apps, real-time analytics, and many other use cases.
2. Mention unique features of Mongo Database.
Answer:
- Indexing: MongoDB supports generic secondary indexes, allowing a variety of fast queries and providing unique, compound, geospatial, and full-text indexing capabilities.
- Aggregation: MongoDB supports an “aggregation pipeline” that allows you to build complex aggregations from simple pieces and will enable the database to optimize them.
- Special collection types: MongoDB supports time-to-live collections for data that should expire at a particular time, such as sessions. It also helps fixed-size collections and holds recent data, such as logs.
- File storage: MongoDB supports an easy-to-use protocol for storing large files and metadata.
3. What is the command for starting MongoDB?
Answer:
mongodmongod –help for help and startup options
4. How to represent a null value in a variable in MongoDB?
Answer:
{"x" : null}5. Write down the code to connect to MongoDB
Answer:
var connectTo = function(port, dbname)
{
if (!port) {
port = 27017;
}
if (!dbname) {
dbname = "test";
}
db = connect("localhost:"+port+"/"+dbname);
return db;
};6. What is GridFS in MongoDB?
Answer:
- GridFS is a mechanism for storing and retrieving large files in MongoDB.
- It is a file system abstraction built on top of MongoDB.
- GridFS stores files in two collections – one for metadata and another for binary data.
- Files are divided into chunks and stored in 255KB chunks.
- GridFS helps handle large files and distribute them across multiple shards.
- It allows easy management and retrieval of files using the same MongoDB query language and drivers.
7. What are the benefits of Mondo DB?
Answer:
MongoDB has many benefits; some features, such as Using GridFS, can simplify your stack. If you are already using MongoDB, you might be able to use GridFS instead of a separate tool for file storage. GridFS will leverage any existing replication or auto-sharding you’ve set up for MongoDB, so getting failover and scale-out for file storage is more accessible GridFS can alleviate some of the issues specific file systems can exhibit when used to store user uploads. For example, GridFS does not have problems with storing large numbers of files in the same directory.
8. Write down the syntax for string expression in MongoDB?
Answer:
"$substr" : [expr, startOffset, numToReturn]9. What is MapReduce in MongoDB?
Answer:
- MapReduce is a powerful and flexible tool for aggregating data.
- It can solve problems too complex to express using the aggregation framework’s query language.
- MapReduce uses JavaScript as its “query language” to express arbitrarily complex logic.
- MapReduce tends to be slow and is not ideal for real-time data analysis.
Part 2 – Mongo Database Interview Questions (Advanced)
10. Write the difference between normalization and denormalization?
Answer:
- Normalization means dividing up data into multiple collections with references between collections.
- Each piece of data lives in one collection, although multiple documents may reference it.
- Thus, to change the data, only one document must update. However, MongoDB has no joining facilities, so gathering documents from multiple collections will require various queries.
- Denormalization is the opposite of normalization: embedding all the data in a single document.
- Instead of documents containing references to one definitive copy of the data, many documents may have copies of the data.
- This means that multiple documents need to update if the information changes but that a single query can fetch all related data.
11. What is cardinality?
Answer:
Cardinality means how many references a collection has to another collection. Common relationships are one-to-one, one-to-many, or many-to-many.
- One-to-one relationship: A single record in one collection has an association with only one record in another collection.
- One-to-many relationship: A single record in one collection has an association with multiple records in another collection.
- Many-to-many relationship: Multiple records in one collection have associations with multiple records in another collection.
12. What are the limitations of MongoDB?
Answer:
- MongoDB does not offer traditional joins. Instead, it provides “lookups,” which are slower in execution.
- It does not support transactions across multiple clusters.
- Compared to traditional RDBMS, it consumes more memory.
- It has a limit on the number of indexes.
- It has limited support for complex transactions with multiple operations.
13. What is replication in MongoDB?
Answer:
- Replication is a way of keeping identical copies of data on multiple servers.
- It is ideal for all production deployments.
- Replication keeps the application running and its data safe, even if something happens to one or more servers.
- With MongoDB, one can set up replication by creating a replica set.
- A replica set is a group of servers with one primary (the server taking client requests) and multiple secondary servers that keep copies of the primary’s data. If the primary crashes, the secondaries can elect a new primary from amongst themselves.
14. What is the command used to set replication in MongoDB?
Answer:
replicaSet = new ReplSetTest({"nodes" : 3})15. What is rollback? When does rollback fail in MongoDB?
Answer:
- Rollback is the process of undoing changes made to a database.
- Rollback ensures that the database is consistent and does not contain any incomplete or incorrect data.
- It helps to restore data to a previous state when a transaction encounters an error.
- Rollback can fail in MongoDB if a user has already committed a transaction or if there are network or system failures during the rollback process.
- Rollback may also fail if multiple transactions are in progress simultaneously and the user attempts to roll back one while another is still in progress.
- MongoDB decides that the rollback is too significant to undertake. Rollback can fail if there are more than 300 MB of data or about 30 minutes of operations to roll back. In these cases, you must re-sync the node stuck in rollback.
16. What is Sharding in MongoDB?
Answer:
- Sharding refers to splitting data up across machines.
- The term partitioning is also sometimes used to describe this concept.
- Putting a subset of data on each machine makes it possible to store more data and handle more load without requiring more powerful machines, just a larger quantity of less-powerful machines.
17. What is Manual Sharding?
Answer:
- Manual sharding is when an application maintains connections to several different database servers, each of which is entirely independent.
- The application stores different data on different servers and queries against the appropriate server to get data back.
- This approach can work well but becomes difficult to maintain when adding or removing nodes from the cluster or in the face of changing data distributions or load patterns.
Final Thoughts
MongoDB is a powerful and flexible NoSQL database that has become increasingly popular in recent years. As such, developers and database administrators need to be familiar with the ins and outs of MongoDB, from basic querying and indexing to more advanced features like sharding and replication. By preparing for common MongoDB interview questions, you’ll be well-equipped to demonstrate your expertise and secure the job you’re after.
Frequently Asked Questions (FAQs)
1. What are the interview questions for MongoDB?
Answer: MongoDB interview questions may cover various topics, such as data modeling, indexing, query optimization, aggregation, administration, security, and scaling.
2. What type of database is MongoDB?
Answer: MongoDB is a NoSQL document-oriented database.
3. What is MongoDB used for?
Answer: MongoDB is used for storing and retrieving data in a flexible and scalable manner. It is commonly used in web applications, real-time analytics, content management systems, and mobile apps.
4. Why use MongoDB over SQL?
Answer: MongoDB offers several advantages over traditional SQL databases, including flexible data modeling, automatic sharding for scalability, support for complex queries, and easy integration with modern development tools and frameworks. MongoDB’s document-oriented approach can simplify application development and improve performance for specific use cases.
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