MongoDB is a free open-source cross-platform document-oriented and leading NoSQL database program. It uses JSON-like documents with schemas and provides high performance, high availability and high scalability.

As one of the most popular tools in its field, MongoDB offers a lot of career opportunities for expert professionals. You can enrol in a MongoDB certification course to enhance your career.

Here are a list of some of the commonly-asked MongoDB interview questions (with answers) that will help you to improve your chances of getting a job:

Q1. What is “Namespace” in MongoDB?

Ans. A Namespace in MongoDB provides access to the running code that are present in different databases. There is a file ‘CACHE.DAT’ in a database that provides a collection of namespaces and databases to the user. The concatenation of the collection name and database name is called a namespace.

Namespace has a maximum length of 120 bytes which includes the collection name, database name, and the dot (.) separator.

Q2. What are the advantages of MongoDB?

Ans. Following are the advantages of MongoDB over other databases:

  • It is easy to set up and install
  • It does not require conversion/mapping of application objects to the database
  • It can derive a document-based data model
  • It supports document query language, essential to prevent dynamic queries

Q3. In which language MongoDB is written?

Ans. MongoDB supports all popular programming languages such as C++, C, C#, Node.js, Python, Erlang, JavaScript, and more.

Q4. Do MongoDB databases have tables?

Ans. No. MongoDB stores its data in collections that include BSON documents, i.e. data records.

 

Q5. What is sharding in MongoDB?

Ans. It is the process of distributing data across various machines. MongoDB uses these approaches to meet the demand for data growth. If there is an increase in the size of data, then a single machine is not able to store the data and also not able to provide read and write throughput. It will create horizontal scaling to solve the issue.xx`

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Q6. How does sharding work with replication?

Ans. Every shard is a collection of data records. The shard could incorporate a single server or a group of replicas. By Using a replica set for every shard is helpful for the data collection.

Q7. Explain what a replica set is?

Ans. A replica set is a cluster of MongoDB database servers that possess the master-slave replication. It provides redundancy and helps to increase data availability with various copies of data on different servers.

Convert standalone MongoDB instance into a replica set:

  • Shutdown the running MongoDB server.
  • Then, start the MongoDB server by specifying the ‘replSet’ option.
  • Syntax of replSet

mongod –port “PORT” –dbpath “YOUR_DB_DATA_PATH” –replSet “REPLICA_SET_INSTANCE_NAME”

Q8. What are the points you need to remember while creating Schema in MongoDB?

Ans.  Following are the points you need to remember:

  • Design your schema according to user demands
  • Combine the objects into one document, if you are using it together or separate them
  • Do join at the time of writing not at the time of reading
  • Optimize your schema for most frequent use cases
  • Perform complex aggregation in the schema

Q9. What is the role of profiler in MongoDB?

Ans. The database profiler collects fine-grained data about MongoDB write operations, cursors, and database commands on a running MongoDB instance.

Q10. What is Journaling in MongoDB?

Ans. Journaling is the process in which the right operation is performed in MongoDB. It provide crash resiliency and create changes in the private view of the database. It will look like the first block is memory and the second block is the ‘my disc’.

Q11. What is the role of profiler in MongoDB?

Ans. The database profiler in MongoDB collects detailed information to write operations and database commands on a running mongod instance. The profiler writes all the data to the ‘system.profile’ collection.

In easy words, the MongoDB database profilers determine the performance of each operation characteristics against the database.

The syntax for profile command:

{

profile: <level>,

slowms: <threshold>,

sampleRate: <rate>

}

Q12. Why are data files so large in MongoDB?

Ans. MongoDB does aggressive pre-allocation of reserved space to avoid file system fragmentation, which is the reason why data files are so large.

Q13. Why is MongoDB not chosen for a 32-bit system?

Ans. MongoDB is not examined as a 32-bit system because 32-bit has limited data of 2 GB. MongoDB storage engine uses memory-mapped files for the performance. Because of not supporting 32-bit, the code is much easier and simpler to work on. It includes various benefits such as less number of bugs, reduces the product release time.

Q14. Explain what are indexes in MongoDB?

Ans. Indexes are special structures in MongoDB, which stores a small portion of the data set in an easy to traverse form.

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Q15. What are the indexes in MongoDB?

Ans. In MongoDB, an index is a special data structure that consists of the data of a few fields of documents and used to execute queries systematically. It helps to improve the speed of search operations.

Syntax used to create Index in MongoDB:

db.collection_name.createIndex({field_name: 1 or -1})

Here, the value 1 is for ascending order and -1 for descending order

For example, there is a collection of student data that includes student_name, student_id and student_age

If you want to create the index for student_name field in ascending order:

db.studentdata.createIndex({student_name: 1})

Output:

{

“createdCollectionAutomatically” : false,

“numIndexesBefore” : 1,

“numIndexesAfter” : 2,

“ok” : 1

}

Q16. Explain what is the use of GridFS in MongoDB?

Ans. It is used for storing and retrieving large files such as images, video files, and audio files.

Q17. What is GridFS in MongoDB?

Ans. GridFS is one of the features of MongoDB, which is used to store and retrieve large files that exceed the BSON document size of 16MB, such as video files, audio files, images, etc. The data of GridFS is stored within a MongoDB collection.

Sample code for GridFS:

{

“filename”: “test.txt”,

“chunkSize”: NumberInt(261120),

“uploadDate”: ISODate(“2014-04-13T11:32:33.557Z”),

“md5”: “7b762939321e146569b07f72c62cca4f”,

“length”: NumberInt(646)

}

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Q18. What is CRUD?

Ans. MongoDB provides CRUD operations that are Create, Read, Update, and Delete.

Q19. What is ObjectID? What are the components ObjectID composed of?

Ans. An ObjectID class is a default primary key for a MongoDB. The syntax used to denote ObjectID is “_id.”

{

“_id”: ObjectId(“54759eb3c090d83494e2d804”)

}

ObjectID is composed of:

  • Timestamp
  • Client machine ID
  • Client process ID
  • 3-byte increment counter

Q20. Which syntax is used to create a collection in MongoDB?

Ans. db.collection.drop() is used to drop collection in mongodb.

Q21. What is vertical scaling?

Ans. Vertical scaling adds more CPU and storage resources to increase capacity.

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Q22. Where do we use MongoDB?

Ans. MongoDB is used instead of RDBMS in various scenarios:

  • Distributed data

MongoDB makes the task easy by the recovery of data because there are multiple copies of data that are collected over multiple servers. It makes the recovery of data safe even if there is a failure in the hardware.

  • Big Data

MongoDB is highly featured with storing a large amount of data. It has a built-in solution for partitioning and sharding of data.

  • Unstable Schema

MongoDB is Schema-less, and it is easy to add a column using MongoDB. Adding a new field to the document does not affect the old document, and it is easy to use.

Q23. Define horizontal scaling.

Ans. It divides the data set and distributes the data over multiple servers or shards.

Q24. Which command is used to restore the backup?

Ans. mongorestore command is used to restore the backup.

Q25. Define auditing.

Ans. Auditing provides administrators with the ability to verify that the implemented security policies are controlling activity in the system.

These are some of the popular questions asked in a MongoDB interview. Always be prepared to answer all types of questions — technical skills, interpersonal, leadership, or methodology. If you are someone who has recently started your career in MongoDB, you can always get certified in the various technique and skills needed to be an expert in the field.

Q26. If you remove an object attribute, is it deleted from the database?

Ans. Yes, it is deleted.

Q27. Does MongoDB support SQL?

Ans. No. However, MongoDB does support a rich query language of its own.

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Q28. What is the structure of ObjectID in MongoDB?

Ans. An ObjectID is a 12-byte BSON type having the following structure:

  • The first 4-byte value representing the seconds
  • The next 3-byte is the machine identifier
  • The next 2-byte contains the process ID
  • The last 3-bytes are a counter value

Q29. How to move old files in the moveChunk directory?

Ans. You can move old files in the moveChunk directory by using Mongod. It will create backup files at the time of normal shard balancing operations and deleted them after the operation is done.

 

Q30. Explain Aggregation in MongoDB?

Ans.  Aggregation is collecting a group of data from various documents and operate them to provide combined results. It is used to process the data that provide computer result.

These are some of the popular questions asked in a MongoDB interview. Always be prepared to answer all types of questions — technical skills, interpersonal, leadership or methodology. If you are someone who has recently started your career in MongoDB, you can always get certified in the various technique and skills needed to be an expert in the field.