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Mongo DB top 50 interview question and answer

 

1. What is MongoDB?

MongoDB is a NoSQL database that uses a document-oriented data model. It stores data in flexible, JSON-like documents, which allows for rich data structures and dynamic schemas.

2. What are the advantages of MongoDB?

  • Scalability: Horizontal scaling through sharding.
  • Flexibility: Schema-less data model allows easy modification.
  • Performance: Fast read and write operations.
  • Rich Query Language: Supports complex queries and indexing.

3. What are documents in MongoDB?

Documents are individual records stored in MongoDB collections, represented in BSON format (Binary JSON). Each document can have a different structure.

4. What is a collection in MongoDB?

A collection is a grouping of MongoDB documents, similar to a table in relational databases. Collections do not enforce a schema, allowing for varied document structures.

5. How does MongoDB handle data relationships?

MongoDB supports both embedded documents and references to model relationships. Embedded documents are good for one-to-few relationships, while references are suitable for one-to-many or many-to-many.

6. What is BSON?

BSON (Binary JSON) is a binary representation of JSON-like documents. It supports more data types than JSON, including dates and binary data, making it more efficient for storage and processing.

7. Explain the role of an ObjectId in MongoDB.

ObjectId is the default unique identifier for documents in MongoDB. It is a 12-byte identifier that ensures uniqueness across documents and is often used as the primary key.

8. What are the different data types supported by MongoDB?

MongoDB supports various data types, including:

  • String
  • Integer
  • Boolean
  • Double
  • Array
  • Object
  • Null
  • Date
  • Binary data

9. What is the difference between MongoDB and SQL databases?

  • Data Structure: MongoDB uses documents while SQL uses tables.
  • Schema: MongoDB is schema-less; SQL databases have fixed schemas.
  • Scaling: MongoDB offers horizontal scaling; SQL databases typically scale vertically.

10. How do you perform CRUD operations in MongoDB?

  • Create: Use insertOne() or insertMany().
  • Read: Use find() and findOne().
  • Update: Use updateOne(), updateMany(), or replaceOne().
  • Delete: Use deleteOne() or deleteMany().

11. What is an index in MongoDB?

An index improves the speed of data retrieval operations on a database. MongoDB supports various types of indexes, including single field, compound, multi-key, and text indexes.

12. How do you create an index in MongoDB?

You can create an index using the createIndex() method. For example:

db.collection.createIndex({ fieldName: 1 });

13. What is sharding in MongoDB?

Sharding is a method of horizontal scaling that distributes data across multiple servers (shards). Each shard holds a portion of the dataset, enabling the database to scale as needed.

14. What are replica sets in MongoDB?

A replica set is a group of MongoDB servers that maintain the same data set. It provides redundancy and high availability, with one primary and multiple secondary nodes.

15. How do you backup and restore MongoDB data?

You can use mongodump for backup and mongorestore for restoring data. Alternatively, for large data sets, you can use filesystem snapshots.

16. Explain the Aggregation Framework in MongoDB.

The Aggregation Framework allows you to process and transform data. It uses stages (e.g., $match, $group, $sort, $project) to perform operations on documents and return computed results.

17. What is the purpose of the $lookup operator?

The $lookup operator is used for performing left outer joins in the Aggregation Framework, allowing you to combine documents from different collections.

18. How does MongoDB ensure data consistency?

MongoDB uses a combination of journaling and write concerns. Journaling ensures durability, while write concerns define the level of acknowledgment required from the server.

19. What are transactions in MongoDB?

Transactions allow multiple operations to be executed atomically. MongoDB supports multi-document transactions, ensuring that either all operations succeed or none do.

20. What is a pipeline in MongoDB?

A pipeline is a sequence of stages that processes data in the Aggregation Framework. Each stage transforms the data and passes it to the next stage.

21. How can you perform text search in MongoDB?

You can perform text searches using the text index. To create one, use:

db.collection.createIndex({ fieldName: "text" });

Then use the $text operator in your queries.

22. What is the difference between update() and updateOne()?

  • update(): Updates multiple documents that match the criteria.
  • updateOne(): Updates only the first matching document.

23. Explain the difference between find() and findOne().

  • find(): Returns a cursor for all documents that match the query.
  • findOne(): Returns the first document that matches the query.

24. What is the explain() method used for?

The explain() method provides information on the execution plan of a query, helping to optimize performance by showing how MongoDB processes the query.

25. How can you limit the number of documents returned in a query?

Use the limit() method to specify the maximum number of documents to return. For example:

db.collection.find().limit(5);

26. What are the different write concerns in MongoDB?

Write concerns define the level of acknowledgment requested from MongoDB for write operations. Options include:

  • w: 0: No acknowledgment.
  • w: 1: Acknowledgment from the primary.
  • w: "majority": Acknowledgment from the majority of replica set members.

27. How do you handle errors in MongoDB operations?

You can use try-catch blocks in your application code to handle exceptions. MongoDB operations can also return error codes that can be checked programmatically.

28. What is the purpose of the $group operator?

The $group operator is used to group documents by a specified key and perform aggregations like counting, summing, or averaging.

29. How can you sort documents in MongoDB?

You can sort documents using the sort() method. For example:

db.collection.find().sort({ fieldName: 1 }); // Ascending order

30. What are the different levels of data consistency in MongoDB?

MongoDB provides:

  • Strong consistency: Ensures the most recent write is visible.
  • Eventual consistency: Allows for temporary discrepancies between nodes.

31. How do you remove an index in MongoDB?

Use the dropIndex() method. For example:

db.collection.dropIndex("indexName");

32. Explain the concept of a schema in MongoDB.

MongoDB is schema-less, meaning collections can store documents with varying structures. However, you can enforce a schema using Mongoose or JSON Schema validation.

33. What is a MongoDB aggregate function?

Aggregate functions perform operations on multiple documents and return a single result. Examples include $sum, $avg, $max, and $min.

34. How can you prevent duplicate entries in MongoDB?

You can enforce uniqueness by creating unique indexes on fields where duplicates should be avoided. For example:

db.collection.createIndex({ fieldName: 1 }, { unique: true });

35. What is the purpose of the $unwind operator?

The $unwind operator is used to deconstruct an array field in documents, creating a separate document for each element in the array.

36. How do you query for embedded documents in MongoDB?

You can query embedded documents using dot notation. For example:

db.collection.find({ "embedded.fieldName": value });

37. What is a capped collection?

A capped collection is a fixed-size collection that automatically overwrites the oldest documents when it reaches its size limit. It maintains insertion order and is useful for logging.

38. How do you handle schema validation in MongoDB?

You can use the validator option when creating a collection to enforce rules on the documents' structure. For example:

db.createCollection("collectionName", { validator: { $jsonSchema: { /* schema rules */ } } });

39. What is the difference between save() and insert() in MongoDB?

  • save(): Can insert a new document or update an existing one based on the _id field.
  • insert(): Only inserts new documents; it does not update.

40. How do you perform aggregation using the shell?

You can perform aggregation using the aggregate() method. For example:

db.collection.aggregate([{ $match: { /* criteria */ } }, { $group: { /* grouping */ } }]);

41. What is a geospatial index in MongoDB?

A geospatial index allows querying for geographic data using coordinates. It enables spatial queries, such as finding points within a certain radius.

42. How do you find the total number of documents in a collection?

You can use the countDocuments() method:

db.collection.countDocuments();

43. What is the purpose of the $project stage in aggregation?

The $project stage is used to specify which fields to include or exclude from the output documents. It can also be used to add new fields.

44. How do you use environment variables in MongoDB connection strings?

You can define environment variables in your application and reference them in your connection string. For example:

const uri = process.env.MONGODB_URI;

45. What are transactions and how do they work in MongoDB?

Transactions allow multiple operations to be executed as a single atomic operation, ensuring that either all operations succeed or none do. They are used with replica sets and sharded clusters.

46. What is the $facet operator in MongoDB?

The $facet operator allows for multiple aggregation pipelines to be executed on the same input documents. It returns a single document containing the results of each pipeline.

47. How can you optimize query performance in MongoDB?

  • Create appropriate indexes.
  • Use projections to return only necessary fields.
  • Use $limit and $skip for pagination.
  • Analyze queries with the explain() method.

48. What are the default ports used by MongoDB?

MongoDB's default port is 27017. The MongoDB shell connects to this port by default unless specified otherwise.

49. Explain the purpose of the mongos process.

The mongos process is a routing service for sharded clusters, directing client requests to the appropriate shard based on the sharding key.

50. How can you monitor MongoDB performance?

You can monitor MongoDB performance using tools like:

  • MongoDB Atlas: Cloud monitoring service.
  • MongoDB Compass: GUI for visualizing performance.
  • Logs: Check logs for slow queries and other metrics.

This concise guide should help you prepare for a MongoDB interview. For each question, try to expand with examples or deeper explanations based on your experience and understanding!