7 Interview Questions On MongoDB (With Sample Answers)

Updated 16 September 2022

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MongoDB is a NoSQL database used to manage, store and retrieve information. Developers use MongoDB to build highly scalable and cost-effective applications. If you are interviewing for the position of database developer or a relevant role, learning about commonly asked interview questions and how to answer them can boost your confidence. In this article, we share a few interview questions on MongoDB and their sample answers and go through some additional tips to help you prepare for your interview:

Interview Questions On MongoDB And Example Answers

Interview questions on MongoDB may include a broad range of general, technical and behavioural questions that test your expertise and knowledge of MongoDB and related topics. Interviewers use these questions to assess your understanding of concepts, your professional background and your experience. Here are some examples of responses you can use as a guide:

1. Can you tell me the difference between SQL and NoSQL databases?

Interviewers may ask this question to assess your basic understanding of relational and non-relational databases. When answering this question, start by defining the two database types and list the key differences between them.

Example answer: 'SQL databases have a pre-defined schema. NoSQL databases have a dynamic schema, which makes it easier to store unstructured data. SQL databases are table-based and store data as rows and columns. In contrast, NoSQL databases may store data in various ways, such as document stores, key-value stores, graphs or wide columns.

In SQL databases, vertical scaling can get expensive upon migrating to a larger server with more CPU and RAM capability. NoSQL databases support horizontal scaling, which allows for the addition of nodes and servers in parallel. This helps the application handle more load.

It is preferable to choose SQL databases when the queries are routine and complex. Using a NoSQL database is effective when the data is unstructured or very large and data integrity is not a priority.'

2. Explain the key features of MongoDB?

Interviewers might ask this question to see how well you understand the basic concepts behind MongoDB. In your answer, briefly explain the key features of MongoDB.

Example answer: 'The primary feature of MongoDB is its ability to store data records as documents, grouped into a collection. This collection can contain multiple documents which differ in the number of fields, content and size. MongoDB supports a dynamic schema which allows the storage of unstructured heterogeneous data. It is also easier to add or remove data from the databases without causing any disturbance to the operational processes.

MongoDB supports indexing, which makes it easier to perform queries on large pools of data. It provides horizontal scaling in the form of sharding, which helps distribute data on multiple servers. This improves the read and write operations, storage capacity and availability of data. It also supports replication, where multiple copies of data are present across several servers. In case of failure of one server, you can retrieve the data from another server.'

3. What is an index in MongoDB?

Interviewers may ask this question to assess your understanding of the basic data structures used in MongoDB. Define what an index is and highlight its importance. You can also provide an example to support your answer.

Example answer: 'An index in MongoDB is a special data structure that helps in effective querying of data. Without it, MongoDB would require scanning every document in the database to fetch the relevant data to a query, which would be time-consuming and expensive. The index holds the data of a few fields of each document in a collection, which makes it easier to retrieve the relevant records. You can create an index using the createIndex function based on one or more fields of a document.

For example, consider an employee database with field names such as employee id, employee name, age, job role and nationality. Here, you can create an index on employee id and employee name. This helps in quick retrieval of data records when querying for employee information from a very large database.'

4. What is a document in MongoDB?

Interviewers may ask this question to assess your understanding of MongoDB architecture. You can begin your answer by defining what a document in MongoDB is and provide an example for the same.

Example answer: 'A document in MongoDB is a data unit that stores data as key and value pairs. It stores data in BJSON format, which is a binary representation of the JSON. Each document in a collection may differ in the number of fields, size and content. A group of documents constitute a collection. There is no need for a fixed schema when creating documents and it is easier to add, modify or delete fields when required.'

5. What is sharding?

Hiring managers may ask this question to assess your understanding of data partitioning techniques in a database. In your answer, include the definition of sharding and explain how it works. You can also add a real-world example to illustrate the concept.

Example answer: 'Sharding is the process of splitting a larger database into smaller chunks, also known as data shards and distributing it across multiple machines. This is beneficial when the amount of data is very large to be stored in a single database. Sharding is an example of horizontal partitioning where you split a larger table into multiple smaller tables that have unique rows and the same schema.

Consider the example of a dating application where users who stay in proximity share the same shard. It becomes easier to divide the data into a fixed number of shards based on the radius limit. This technique helps store an enormous amount of user data in an organised manner and matches individuals who have similar personalities and belong to the same shard.'

6. How do you back up and restore a MongoDB database?

Interviewers may ask this question to find out how well you understand the backup and restore features provided by MongoDB. Explain the importance of data backup and restoration and list ways in which you can perform them.

Example answer: 'Backup is necessary to retrieve files from another location in case of a system crash or primary data failure. You can use the mongodump tool to take the backup of a MongoDB database. This dumps all data into the dump directory, also known as BJSON data dumps. You can restore this data by using mongorestore tool. There is an option to either restore the entire data or just a subset of the data.'

7. What are some ways to secure a MongoDB database?

Interviewers may ask this question to find out if you know how you secure a MongoDB database. Explain the importance of database security and list some best practices to adopt.

Example answer: 'Database security is essential to ensuring the confidentiality, availability and integrity of a database. The primary step is to create separate security credentials for users and groups and assign them privileges based on their roles or requirements. It is necessary to limit remote access to the database servers and only allow specific IP addresses to establish connections with the database.

Encrypting data is another way to ensure it is not readable by unauthorised users. MongoDB also supports encryption of data on the client side, which ensures that only parties with the relevant decryption keys can read the data. Companies can also perform audit trails to identify the changes made to the databases and gather information about who performed the change and when.'

Tips To Prepare For A MongoDB Interview

Here are some tips that can help you during interviews:

Research about the company

Learn about the company's history, defining moments and past successes. You can find information about the company on its website, connect with its current employees, discover about their culture and research its competitors. You can learn more about the company by reading their blog, white paper and case studies.

Learn the STAR technique

The STAR technique where STAR stands for situation, task, action and result can help you be prepared to handle real-time situations related to the job. Make sure to read the job description beforehand and make a list of all the challenges you encountered in your previous roles. You can share relevant details, your responsibility during a specific challenge, steps you took to handle the situation and the outcome of the situation while answering behavioural questions.

Revise important concepts

Revising helps identify strong and weak areas and recall the details of the topics you have studied. When revising for a MongoDB interview, focus on important topics such as documents, collections, indexing, querying, sharding, database security and backup and restoring of data. It can also be useful to review real-world use cases of MongoDB that you can talk about during the interview.

Please note that none of the companies, institutions or organisations mentioned in this article are associated with Indeed.