The following MongoDB Interview Questions have been prepared to give you an understanding of the type of questions you may encounter during your interview on the subject of MongoDB. Generally, good interviewers do not plan to ask a specific question during your interview; questions begin with a basic idea of the subject and then proceed as the discussion and your answers unfold.

MongoDB tutorials can make complex tasks easy and working with information straightforward.

MongoDB Interview Questions and Answers

1. What is Mongo shell?

Answer: Mongo Shell is a JavaScript interface to MongoDB that can query and update data. Additionally, it can be used to perform administrative functions. To start the shell, run the mongo executable:

$ mongod
$ mongo
MongoDB shell version: 4.2.0
connecting to: test
>

2. What are some of the advantages of MongoDB?

Answer: MongoDB offers the following advantages:

• Primary and secondary indexes can be created on any field in MongoDB.
• Rather than procedures, MongoDB uses JavaScript objects.
• Database schemas in MongoDB are dynamic.
• You can easily scale up and down MongoDB.
• Data partitioning (sharding) is built into MongoDB.
• MongoDB supports field, range-based, and string pattern matching queries for searching the database.

3. How does MongoDB store data?

Answer: As a document-based database, MongoDB stores documents in BSON notation, which is a binary encoding of JSON.

4. Describe the key features of MongoDB.

Answer: MongoDB has the following important features:

• Document-based and cross-platform.
• No complex joins
• High availability and automatic failover.
• A schema-less database
• Faster access to data since the working set (internal memory) is present.
• Aggregation, sharding, and replication make this a user-friendly solution.

5. Which are the different languages supported by MongoDB?

Answer: Official MongoDB drivers are available for C, C++, C#, Java, Node.js, Perl, PHP, Python, Ruby, Scala, Go, and Erlang. Each of these languages can use MongoDB.

MongoDB provides some community-supported drivers as well, but the above-mentioned ones are the official ones.

6. What are NoSQL databases? What are the different types of NoSQL databases?

Answer: NoSQL databases provide a mechanism for storing and retrieving data that is different from that of relational databases (such as SQL, Oracle, etc.). Types of NoSQL databases:

• Key-Value
• Graph
• Document Oriented
• Column Oriented

7. How is MongoDB better than other SQL databases?

Answer: MongoDB provides a highly flexible and scalable document structure. As an example, one data document in MongoDB can have five columns, while another in the same collection can have ten columns. In addition, MongoDB databases are faster than SQL databases due to efficient indexing and storage techniques.

8. What type of DBMS is MongoDB?

Answer: MongoDB is a cross-platform document-oriented DBMS (Database Management System).

9. What are the different index types in MongoDB??

Answer:

• Default – this is the _id that MongoDB builds.
• Single field – for sorting and indexing on a single field.
• Compound – for multiple fields
• Multi-key – for indexing array data
• Hashed – indexes the hashes of a field value.
• Geospatial – to query geospatial(location) data.

10. Does MongoDB support primary-key, foreign-key relationship?

Answer: No. MongoDB does not support the primary key-foreign key relationship by default.

11. What are Indexes in MongoDB?

Answer: By using indexes, MongoDB can execute queries efficiently. As a result of the absence of an index, MongoDB performs a collection scan, which means selecting every document in a collection to match the query. MongoDB can limit the number of documents MongoDB inspects if it finds an appropriate index for a query.

12. Which index is created by MongoDB by default for every collection?

Answer: The_id collection is created for every collection by MongoDB by default.

13. MongoDB is referred to as a schema-less database. Is that true? How to create the schema in MongoDB?

Answer: Since MongoDB relies on JSON, a schema-free data structure, it is better to refer to it as having dynamically typed schemas. A schema can be created by creating and inserting a document. Upon insertion of a document, the database will create a corresponding collection.

14. How are constraints managed in MongoDB?

Answer: Document validators can be added to collections starting with MongoDB 3.2. Unique indexes can also be created with db.collection.createIndex({“key” : 1} , );

15. What is BSON?

Answer: BSON, or binary JSON, is the binary encoding of JSON. This library extends JSON and adds new data types and fields.

16. What is a namespace in MongoDB?

A namespace is a result of concatenating both the database name and the collection name. Example – Students.the subject, where students build the database and the subject makes up the collection.

17. Explain the structure of ObjectID in MongoDB.

Answer: ObjectID is a 12-byte BSON type. The following are:

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

18. What are the differences between MySQL and MongoDB?

Answer:

19. How do you perform CRUD operations in MongoDB?

Answer: CRUD operations stands for Create, Read, Update, and Delete documents. To perform CRUD operations in MongoDB:

• C – Create – db.collection.insert();
• R – Read – db.collection.find();
• U – Update – db.collection.update();
• D – Delete – db.collection.remove({“fieldname” : ”value”});

20. How does MongoDB do indexing?

Answer: Indexing in MongoDB is done using the ensureIndex() method.

Syntax:

db.COLLECTION_NAME.ensureIndex()

21. What is a covered query and why is it important?

Answer: In a covered query, all the fields used in the query are indexed. Similarly, the results returned can also be indexed. MongoDB retrieves the results without viewing the documents, which saves time and increases efficiency.

22. Name the 2 storage engines using MongoDB?

Answer: MongoDB uses two storage engines, MMAPv1 and WireTiger.

23. What is sharding in MongoDB?

Answer: MongoDB handles data growth through Sharding, which involves sorting data records across multiple machines. Every shard is a set of replicas.

24. What is replication and what are the primary and secondary replica sets?

Answer: Synchronizing data across multiple servers is known as replication. Increasing data availability is one of its benefits. If one server fails, data remains on the other servers.

• Primary replica set – MongoDB only writes data to the primary replica set.
• Secondary replica set – Secondary or Slave nodes can only accept reads. From the primary, they replicate.

25. What is journaling and how does it work?

Answer: MongoDB ensures data integrity by creating an on-disk journal for every write operation. In the event of a server crash, a journal can be used to track copies that did not make it to disk or data files.

26. In what ways is MongoDB better than MySQL? Elaborate.

Answer: MongoDB provides a flexible data model, allowing the schema to be expanded according to business requirements. Additionally, MongoDB offers greater scalability and can easily handle more types of data for managing real-time applications compared to MySQL.

27. What is an ACID transaction? Does MongoDB support it?

Answer: ACID is an acronym for Atomicity, Consistency, Isolation, and Durability. This is ensured by the transaction manager. MongoDB version 4.0 does support ACID.

28. How does MongoDB handle transactions and locks?

Answer: MongoDB uses multi-granular locking, where operations can be locked at the global, database, or collection level. The storage engines decide the level of concurrency. In WiredTiger, for example, it is at the document level. There is a shared locking mode for reads, while there is an exclusive locking mode for writes.

29. Explain the role of the profiler in MongoDB.

Answer: Profilers are in-built tools that analyze queries to figure out how slow they are and which resources they consume. As a DBA, you can analyze the queries. The profiler collects all data under the system.profile collection.

30. How does MongoDB do text search?

Answer: Text search in MongoDB can be done using text index. For example:

db.collection_name.ensureIndex();

31. Explain the concept of pipeline in the MongoDB aggregation framework.

Answer: Each stage of an aggregation pipeline is a data processing unit. It receives a stream of input documents, processes them one by one, and produces a stream of output documents one by one.

32. How do you Delete a Document in MongoDB?

Answer: MongoDB's CRUD API provides deleteOne and deleteMany for this purpose. The first parameter in both of these methods is a filter document. When removing documents, the filter specifies a set of criteria to match against. For example:

> db.books.deleteOne({"_id" : 3})

33. How is querying done in MongoDB?

Answer: In MongoDB, the find method is used to perform queries. Querying returns either none of the documents in the collection or all of them. To determine which documents are returned, the first argument to find must specify the query criteria. For example: If we want to match a string, such as a "username" key with the value "alice", we use that key/value pair instead:

> db.users.find({"username" : "alice"})

34. What are Geospatial Indexes in MongoDB?

Answer: There are two types of geospatial indexes in MongoDB: 2dsphere and 2d. In 2dsphere indexes, we work with spherical geometries based on the WGS84 datum, which models the earth's surface. It assumes that the earth's surface is an oblate spheroid, which means that the poles are somewhat flattened. Therefore, using 2sphere indexes means that distance estimation between, for example, two cities can be more accurate than using 2D indexes because the earth's shape is considered. For points stored on a two-dimensional plane, use 2d indexes.

35. What are some utilities for backup and restore in MongoDB?

Answer: Mongo shell does not include functions for exporting, importing, backing up, or restoring. However, MongoDB has developed methods for accomplishing this, which eliminates the need for scripting or complicated GUIs. To facilitate this, several utility scripts can be used to get data into or out of the database in bulk. Among these utility scripts are:

• mongoimport
• mongoexport
• mongodump
• mongorestore

36. What is a Replica Set in MongoDB?

Answer: Replication is the process of storing identical copies of your data on multiple servers. For all production deployments, it is recommended. Even if one or more of your servers fail, your application will continue to run and your data will remain safe.

A replica set with MongoDB can be used to create such replication. In a replica set, the primary server takes writes and the secondary servers maintain copies of the primary's data. The secondary can elect a new primary if the primary crashes.

37. What are MongoDB Charts?

Answer: MongoDB Charts are the best way to visualize data from a MongoDB database. By using this tool, users can visualize data without having to write any code or learn any programming languages.

These are the two different MongoDB Chart implementations:

• MongoDB Charts Server
• MongoDB Charts PaaS (Platform as a Service)

38. What is the Aggregation Framework in MongoDB?

Answer: Aggregation frameworks are analytical tools within MongoDB that allow you to analyze documents in one or more collections. It is built on the concept of pipelines. An aggregation pipeline takes input from a MongoDB collection and passes the documents from that collection through one or more stages, each of which performs a different operation on its input. Every stage takes as input whatever the stage before it produces as output. Throughout all stages, documents are the input and output.

39. What do you mean by Transactions?

Answer: In a database, a transaction is a logical unit of processing that includes one or more database operations, either reads or writes. Transactions are an important feature of MongoDB to ensure consistency.

MongoDB provides two APIs for using transactions.

• Core API: It is a similar syntax to relational databases (e.g., start_transaction and commit_transaction)
• Call-back API: This is the recommended method for using transactions. It initiates a transaction, executes the specified operations, and commits (or aborts on an error). Additionally, it also automatically incorporates error handling logic for "TransientTransactionError" and "UnknownTransactionCommitResult".

40. How should MongoDB be used?

Answer: When building internet or business applications that need to evolve quickly and scale gracefully, MongoDB is the right choice. Developers of all kinds are using MongoDB to build scalable applications using agile methodologies.

The following are just a few of the reasons to use MongoDB:

• It supports rapid iterative development.
• Make your data repository as large as possible.
• Change the deployment type as the business changes.
• Organize, manage, and search data with text, geospatial, or time-series dimensions.
• The MongoDB database supports horizontal scaling through Sharding, distributing data across multiple machines, and facilitating high-performance operations on large sets of data.

41. How does concurrency affect the primary replica set?

Answer: Whenever the collection changes are written to primary, MongoDB also writes the changes to a special collection in the local database known as the primary's oplog. Thus, both the collection's database and the local database are locked.

42. What have capped collections?

Answer: A capped collection is a fixed-size collection that inserts and retrieves data based on insertion order. When the collection's space is full, the oldest documents will be overwritten by the newest ones.

43. List some of the data types supported by MongoDB.

Answer: Some data types supported by MongoDB are Numbers, String, Array, Binary data, Boolean, Date, ObjectId, Regular expressions, etc.

44. Explain the purpose of the map-reduce command in Mongo-DB.

Answer: A method of performing aggregation is map-reduce.

• Key-value pairs are emitted by the map function.
• By combining key-value pairs, the Reduce function returns the aggregate result.

45. What are the commands to create a backup of the data and restore the data?

Answer: You can use the command mongodump to create a backup. Meanwhile, to restore the data, you can use the command mongorestore [backup_path].

46. How can applications access real-time data changes in MongoDB?

Answer: Applications can access real-time data changes using Change streams, which act as subscribers to all the collection operations such as insert, delete, and update.

47. Is it possible to run multiple Javascript operations in a MongoDB instance?

Answer: It's possible to run multiple JS operations with a MongoDB instance. By using the mongo shell instance, we can specify the name of the JavaScript file to be executed on the server. You can have as many JS operations in the file as you like.

48. Can MongoDB Support foreign key constraints?

Answer: MongoDB does not support foreign key constraints. MongoDB provides flexibility in defining relationship definitions because of its document structure.

49. Explain GridFS in MongoDB?

Answer: Large files such as images, audio, and video files are stored and retrieved by GridFS. Although the maximum size for a file is 16MB, GridFS can store files that are larger than that. GridFS breaks the file into chunks and stores each chunk as a separate document with a maximum size of 255k. It uses two collections, fs.chunks, and fs.files for storing chunks and metadata, individually.

50. What is a Storage Engine in MongoDB?

Answer: This is a component of the database that helps manage the storage of data in both memory and on disk. MongoDB supports multiple storage engines to improve performance for different workloads. WiredTiger (MongoDB3.2) is the default storage engine, which is well-suited for most workloads.

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