Top 60 MongoDB Interview Questions and Answers in 2021

MongoDB is very popular among new developers due to its ease of use and flexibility.

What is Mongo DB? MongoDB is defined as a source-available cross-platform document-oriented database program. It is basically classified as a NoSQL database program.

 MongoDB makes use of JSON-like documents with optional schemas. 

We have noted the most frequently asked MongoDB Interview Questions. Please go through our entire blog so that you will not miss any of the MongoDB interview questions.

Table of Contents

Top MongoDB Interview Questions 

1. Explain the concept of Namespace in MongoDB?

MongoDB stores Binary Interchange and Structure Object Notation(BSON ) objects in the collection; the concatenation of the collection name and the database name is known as a namespace.

2. Name the various languages supported by MongoDB?

The various languages supported by MongoDB are:

  1. C
  2. C++
  3.  C#
  4. Java
  5. Node
  6. Js
  7. Perl
  8.  PHP
  9.  Python
  10. Ruby
  11. Scala
  12. Erlang

3. Explain the concept of sharding in MongoDB?

Sharding is a technique for distributing data across multiple machines. MongoDB mainly uses sharding to support deployments with huge data sets and high throughput operations. 

4. Can you name the different types of NoSQL databases?

The different types of NoSQL databases are:

  1. Key-value store NoSQL database
  2. Document store NoSQL database
  3. Column store NoSQL database
  4. Graph-based NoSQL database

5. What is a replica set in MongoDB?

A replica set in MongoDB can be defined as a group of mongod methods that maintain the same data set. Replica sets aim to provide redundancy and high availability. They are the basics for all production deployments.

MongoDB Interview Questions - Replica Set in MongoDB

6. Why is MongoDB better than other SQL databases?

  1. MongoDB is best suitable for hierarchical data storage.
  2. MongoDB supports JSON query language along with SQL.
  3. MongoDB is 100 times faster than a traditional database system like RDBMS.
  4. It allows a very highly flexible and scalable document structure.

7. Mention the points to be taken While creating Schema in MongoDB?

The points are listed below:

  1. Design the schema according to the user specifications
  2. Combine objects into one document if you want to use them together. or else separate them
  3. Do joins while write, and not when it is on reading
  4. Optimize your schema.
  5. Perform complex aggregation in the schema

8. Can you tell us What type of DBMS is MongoDB?

MongoDB is a document-oriented DBMS

9. Write the syntax to create a collection and to drop a collection in MongoDB?

  1.  db.createCollection(name,options) is the syntax used to create a collection
  2. db.collection.drop() is the syntax used to drop a collection.

10. Differentiate between MongoDB and MySQL?

MongoDBMySQL
It represents data as JSON documents.It represents data in tables and rows.
It supports c, c++.It supports c,c++, and javascript.
It uses JavaScript as a query languageIt uses the Structured Query Language (SQL).
It doesn’t support join operationsSupports join operations.
If your services are mainly based on cloud-based, then  MongoDB is the best-suited option for you.If data security is your main priority, then MYSQL is the best option for you.

MongoDB Interview Questions and Answers

11. What is a profiler in MongoDB?

The database profiler is used to collect detailed information about the Database Commands that are executed against a running mongod instance. The profiler will write all the data it collects into the system.

12. Are primary-key, foreign-key relationships supported by MongoDB?

The primary-key, foreign-key relationships are not supported by MongoDB.

13. What feature of MongoDB is used to do safe backups?

The feature in MongoDB that you can use to do safe backups is Journaling.

14. How to achieve primary key – foreign key relationships in MongoDB?

By embedding one document inside another, we can achieve primary key and foreign key relationships in MongoDB.

Example: The address document can be embedded inside the employee document.

15. What is Objecld composed of?

The objectId is basically composed of:

  1. Timestamp
  2. Client machine ID
  3. Client process ID
  4. 3 byte incremented counter

16. Can you explain the structure of ObjectID in MongoDB?

ObjectID is a 12-byte BSON type

  1. 4 bytes value representing seconds
  2. 3-byte machine identifier
  3. 2-byte process id
  4. 3 byte counter

17. Write the command syntax for inserting a document?

Syntax: database.collection.insert (document).

18. What are Indexes in MongoDB?

Indexes are special data structures in MongoDB that can store a small portion of the data set collection. The index stores the value of a particular field or group of fields, ordered by the value of the field.

19. What happens if you remove an object attribute?

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

20. How do we inspect the source code of a function?

The function has to be invoked to inspect the source code.

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MongoDB Interview Questions and Answers

21. Name the language in which  MongoDB is written?

C++ is the language that MongoDB is written.

22.  Name the command syntax that tells you whether you are on the master server or not?

Db.isMaster() tells us whether we are on the master server or not.

23. Does the MongoDB database use tables for storing records?

MongoDB makes use of  “Collections” to store data. They don’t have tables.

24. What is the basic syntax to use the index in MongoDB?

Syntax: >db.COLLECTION_NAME.ensureIndex ( {KEY:1} ).

Here the key represents the name of the COLUMN, which is present in the document

25. Is there a method to configure the cache size in MongoDB?

You cannot configure MongoDB’s cache. Actually, MongoDB makes use of all the free spaces available on the system automatically by way of memory-mapped files.

26. Name few alternatives to MongoDB?

Few alternatives to Mongo DB are listed below:

  1. PostgreSQL
  2. DynamoDB
  3. CouchDB

27. How to perform Transaction/locking in MongoDB?

MongoDB does not use traditional locking or transaction with Rollback. It keeps the transactions support simply to enhance performance.

28. Name the syntax that is used to view Mongo is using the link?

db._adminCommand(“connPoolStats.”)

29. Can we remove old files in the moveChunk directory?

You can remove the old files. These files are deleted once the operations are done because these files backups during normal shard balancing operation. It is a manual cleanup process that is necessary to free up space.

30. How important is the covered query?

Covered queries mainly help us in querying the data faster. It is achieved by ensuring the index that is created contains all the necessary fields required by the query. It doesn’t require any examining any documents apart from the indexed ones. We need to make sure that all the fields in the query, and the results returned, are a part of the index.

31. Mention few important features of MongoDB?

important features of MongoDB
  1. It supports ad hoc queries.
  2. It lets you index any field in a document.
  3. Replication. 
  4. Duplication of data. 
  5. Load balancing. 
  6. It supports map reduction and aggregation tools.
  7. It uses JavaScript instead of Procedures.
  8. It is a schema-less database written in C++.

MongoDB Interview Questions and Answers

32. What will you do if a shard is down or slow and you do a query?

If a shard is slow, then Mongos will wait for it till response.

33. Why don’t we prefer the 32-bit version of MongoDB?

MongoDB makes use of memory-mapped files; hence when you run a 32-bit build of MongoDB, the total storage size of the server that is used is 2 GB. But when we run a 64-bit build of MongoDB, it provides virtually unlimited storage size. Hence 64-bit is preferable over 32-bit.

34. Are ACID transaction management and locking functionalities supported by MongoDB?

The answer is NO.

35. Does MongoDB require a lot of RAM?

The answer is NO. MongoDB can run on a small amount of RAM.

36. If a document is removed from the database, does MongoDB remove it from the disk also?

Yes, if a document is removed from the database, then it is also removed from the disk.

37. How many indexes can MongoDB create by default for a new collection?

-id collection is created for every collection by default.

38. Differentiate MongoDB and Cassandra?

MongoDBCassandra
It stores records in JSON format.It is a column-oriented database.
It uses a Mongo Master, Mongo Shard, and Mongo Config server to replicate the data.There is no master-slave relationship.

39. How MongoDB provides consistency?

MongoDB makes use of the reader-writer locks by allowing simultaneous readers to access any supply like a collection or a database, but it always offers private access to single writes.

MongoDB Interview Questions and Answers

40. How to isolate the cursors from intervening with the write operations?

We use the snapshot() method to isolate the cursors from intervening with writes. This method negotiates the index by making sure that each query comes to any article only once.

41. What is CRUD?

CRUD operations are referred to as the basic Insert, Read, Update and Delete operations. 

CRUD

42. Differentiate between the save and insert commands in MongoDB?

SaveInsert
The save command can insert or update a document.Insert only performs the insertion.
db.users.save({username:”google”,password:”google567″})db.users.insert({username:”google”,password:”google567″})

43. Differentiate between the $all operator and the ‘$in’ operator?

$all$in
It specifies a minimum set of elements that must be matched.It retrieves all the documents which contain the subset of the values that we pass.

44. Explain Aggregation in MongoDB?

Aggregation in MongoDB cab be defined as an operation that is used to process the data that returns the computed results. Aggregation mainly groups the data from various documents and operates in many ways on the clustered data to return one combined result.

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45. Write the command used for inserting a document in MongoDB?

database.collection.insert (document)

46. Write the command to see a connection?

db_adminCommand (“connPoolStats”)

47. Explain BSON in MongoDB?

MongoDB usually stores documents or objects in a format called BSON. BSON can be defined as a binary serialization of JSON-like documents. BSON means r “Binary JSON,” For example, BSON can have a  Date data type and BinData type.

48. What is the primary Replica set?

The primary replica set in MongoDB accepts all write operations from clients.

49. What are secondary Replica sets?

The secondaries usually replicate the primary replica set’s oplog, and they apply those operations to their datasets so that the secondaries’ datasets reflect the primary’s dataset.

50. What are Embedded documents in MongoDB?

An embedded or nested MongoDB Document is usually a normal document that is nested inside another document within the  MongoDB collection. Embedded documents are useful when a one-to-many relationship exists between the documents.

51. How MongoDB stores its data?

MongoDB stores data usually in the form of documents that are JSON-like fields and value pairs.

52. What is Vertical Scaling?

Vertical scaling can be referred to as adding more resources to the server to handle to load. Like all other database systems, MongoDB also prefers more RAM and IO capacity. Vertical scaling of the MongoDB database mainly includes increasing the CPU capacity or disk capacity and increasing the I/O operations.

53. What is storage encryption in MongoDB?

Storage encryption basically encrypts all the MongoDB data on storage or on operating systems ensuring that only authorized processes can access the secured data.

54. Explain Some of the features of BSON?

  1. It is designed to be traversed easily.
  2. It is highly efficient. Encoding the data to BSON and decoding from the BSON can be performed quickly in most languages.
  3. It is lightweight in nature which is an important feature for any data representation format, especially when it is used over the network.

55. Explain the application-level Encryption?

Applications can encrypt the fields in documents before transmitting the data over the wire to the server. Only applications that have access to the correct encryption keys are able to decrypt and read the protected data.

56. What is Horizontal Scaling?

Horizontal Scaling is involved in dividing the systems dataset and load over various servers; adding additional servers is required to increase the capacity.

57. Name the components of Sharded cluster?

The sharded cluster has the below-mentioned components:

  1. Shards
  2. Query routers
  3. Config servers

58. Define Auditing in MongoDB?

MongoDB Enterprise has included the auditing capability for mongod and mongos instances. The auditing facility allows the administrators and the users to track the system activity for deployments with various users and applications.

59. Explain the concept of Splitting in MongoDB?

Splitting is a technique that keeps the chunks from growing too large. When a chunk is grown beyond a specified chunk size, or if the documents number in the chunk has exceeded the Maximum Document numbers  Per Chunk to Migrate, MongoDB splits the chunk based on shard key values the chunk represents.

60. Why do companies and development teams use MongoDB?

Companies and development teams  use MongoDB because:

  1. The document data model is a very powerful way to store and retrieve the data that allows developers to move fastly.
  2. The horizontal, scale-out architecture of MongoDB supports huge volumes of data and traffic.
  3. It provides a  great user experience for developers who can install and start writing code immediately.
  4. It can be used everywhere by anyone.
  5. It can be used in any of the major public clouds through MongoDB Atlas.
  6. It has developed a huge and mature platform ecosystem.

Good luck with your MongoDB interview, and we hope our MongoDB Interview Questions and Answers were of some help to you. You can also check our SQL Interview Questions and MS Access Database Interview Questions which might be of some help to you.

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