Top 15 Most Popular Mongo Database Interview Question and Answers

Mongo Database Interview Question and Answers

The team at GoApti has prepared this list of 15 Mongo database interview question and answers useful for a wide variety of interviews. This is because MongoDB powers the full-stack setup in various use cases and is the backbone of several verticals relying on databases.

MongoDB is a cross-platform document-oriented database program. Classified as a NoSQL database program, MongoDB uses JSON-like documents with optional schemas. MongoDB is developed by MongoDB Inc. and licensed under the Server Side Public License (SSPL). Source: Wikipedia

1. What is so special about MongoDB?

Mongo Database is a document-oriented database, not a relational one. The primary reason for moving away from the relational model is to make scaling out easier. A document-oriented database replaces the concept of a “row” with a more flexible model: the “document”. This is done by allowing embedded documents and arrays.

The document-oriented approach makes it possible to represent complex hierarchical relationships with a single record. This fits naturally into the way developers program in modern object-oriented languages.

2. What are some features of Mongo Database?

  • Indexing: MongoDB supports generic secondary indexes, allowing a variety of fast queries and provides unique, compound, geospatial, and full-text indexing capabilities as well.
  • Aggregation: MongoDB supports an “aggregation pipeline” that allows you to build complex aggregations from simple pieces and allow the database to optimize it.
  • Special collection types: Mongo database supports time-to-live collections for data that should expire at a certain time, such as sessions. It also supports fixed-size collections which are useful for holding recent data such as logs.
  • File storage: Mongo DB supports an easy-to-use protocol for storing large files and file metadata.

Some features common to relational databases are not present in MongoDB; joins and complex multi-row transactions are absent in MongoDB. Omitting them was an architectural decision to allow for greater scalability as both of those features are difficult to provide efficiently in a distributed system.

3. What is the command for getting started with MongoDB?

mongod –help

The second command is for help and startup options

4. How do you represent the null value in a variable in MongoDB?

{“x” : null}

5. Write down the code to connect to MongoDB.

var connectTo = function(port, dbname)
if (!port) {
port = 27017;
if (!dbname) {
dbname = “test”;
db = connect(“localhost:”+port+”/”+dbname);
return db;

Also Read: Top 10 and Best Linux Interview Questions And Answers

6. What is GridFS in MongoDB?

GridFS is a mechanism for storing large binary files in MongoDB.

7. What are the benefits of MongoDB?

There are many benefits of MongoDB. Using GridFS can simplify your stack. If you’re 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 that you’ve set up for MongoDB, so getting failover and scale-out for file storage is easier.

GridFS can alleviate some of the issues that certain file-systems can exhibit when being used to store user uploads. For example, GridFS does not have issues with storing large numbers of files in the same directory.

8. Write down the syntax for string expression in MongoDB.

“$substr” : [expr, startOffset, numToReturn]

9. What is MapReduce in MongoDB?

MapReduce is a powerful and flexible tool for aggregating data. It can solve some problems that are too complex to express using the aggregation framework’s query language.

MapReduce uses JavaScript as its “query language” so it can express arbitrarily complex logic. MapReduce tends to be fairly slow and should not be used for real-time data analysis.

10. What is the difference between Normalization and Denormalization?

Normalization is 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 be updated. However, MongoDB has no joining facilities, so gathering documents from multiple collections will require multiple queries.
Denormalization is the opposite of normalization: embedding all of 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 be updated if the information changes but all related data can be fetched with a single query.

For official documentation and training ,use the Official Website of MongoDB.

11. What is Cardinality?

Cardinality is how many references a collection has to another collection. Common relationships are one-to-one, one-to-many, or many-to-many.

Q12. When should you not use MongoDB?

MongoDB interview questions often consist a couple of queries on the disadvantage of this database. It does not support transactions, so systems that require transactions should use another data store. There are a couple of ways to hack in simple transaction-like semantics, particularly on a single document, but there is no database enforcement.

13. What is replication in MongoDB?

Replication is a way of keeping identical copies of your data on multiple servers and is recommended for all production deployments. Replication keeps your application running and your data safe even if something happens to one or more of your servers.

With MongoDB, you set up replication by creating a replica set. A replica set is a group of servers with one primary server which takes client requests and multiple secondary servers that keep copies of the primary server’s data. If the primary crashes, the secondary servers can elect a new primary amongst themselves.

14. What is the command used to set replication in MongoDB?

replicaSet = new ReplSetTest({“nodes” : 3})

15. When does Rollback fail in MongoDB?

MongoDB decides that the rollback is too large to undertake. Rollback can fail if there is more than 300 MB of data or about 30 minutes of operations to roll back. In these cases, you must re-sync the node that is stuck in rollback.

What were the most difficult Mongodb interview questions that you faced? Are you aware of MongoDB certifications that can help others? What about MongoDB tutorials? Do comment.

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