MongoDB Answers

Basic MongoDB Answers

#	Question	Answer	Examples
1	What is MongoDB?	A NoSQL document database	Stores data in JSON-like documents
2	What is a Document?	A unit of data in MongoDB, containing key-value pairs (like JSON)	{ "_id": ObjectId(""), "name": "Alice", "age": 30 }
3	What is a Collection?	A group of related documents (similar to a table in SQL)	A users collection containing user documents
4	What is a Database?	A container for collections	A my_db database containing users and products collections
5	What is BSON?	Binary JSON; the serialization format MongoDB uses for documents	Supports more data types than standard JSON (e.g., Date, ObjectId)
6	How do you connect to MongoDB?	Using the mongosh shell or by connecting via an application driver	mongosh "mongodb://localhost:27017"
7	How do you create a database?	Using the use command (it's created when the first data is inserted)	use my_new_db;
8	How do you create a collection?	Using the db.createCollection() command	db.createCollection("products");
9	How do you insert a document?	Using the insertOne() method	db.users.insertOne({ name: "Bob", age: 25 });
10	How do you insert multiple documents?	Using the insertMany() method	db.users.insertMany([{ name: "Charlie" }, { name: "David" }]);
11	How do you find documents?	Using the find() method	db.users.find({ age: 30 });
12	How do you find a single document?	Using the findOne() method	db.users.findOne({ name: "Alice" });
13	How do you update documents?	Using the updateOne() or updateMany() methods	db.users.updateOne({ name: "Bob" }, { $set: { age: 26 } });
14	How do you delete documents?	Using the deleteOne() or deleteMany() methods	db.users.deleteOne({ name: "Charlie" });
15	What is the _id field?	The primary key for each document, automatically generated if not provided	{ "_id": ObjectId(""), "name": "Alice" }

Intermediate MongoDB Answers

#	Question	Answer	Examples
1	What is embedding?	Storing related data directly within a single document	A user document: { name: "Alice", address: { street: "123 Main St", city: "Anytown" } }
2	What is referencing?	Storing the _id of one document in another document (like a foreign key)	An order document: { order_id: 123, user_id: ObjectId(""), items: [...] } (referencing a user's ID)
3	When should you use embedding?	When related data is accessed together, is small, and has a one-to-one or few relationship	Embedding address details within a user document if they are always accessed together and don't change much.
4	When should you use referencing?	When related data is large, accessed independently, or has a one-to-many relationship	Referencing multiple orders from a user document, or referencing categories from a product document.
5	What is the Aggregation Framework?	A powerful tool for processing data and returning computed results	db.aggregate([ { $match: { age: { $gt: 25 } } }, { $group: { _id: "$city", count: { $sum: 1 } } } ])
6	What is the $match stage in Aggregation?	Filters documents based on specified criteria	{ $match: { status: "active" } }
7	What is the $group stage in Aggregation?	Groups documents by a specified key and performs aggregation functions	{ $group: { _id: "$category", count: { $sum: 1 } } }
8	What is the $project stage in Aggregation?	Reshapes documents, selecting or excluding fields, creating new fields	{ $project: { _id: 0, userName: "$name", userEmail: "$email" } }
9	What is the $sort stage in Aggregation?	Sorts documents based on specified fields	{ $sort: { timestamp: -1 } }
10	What is the $lookup stage in Aggregation?	Performs a left outer join to another collection	{ $lookup: { from: "orders", localField: "\_id", foreignField: "user_id", as: "userOrders" } }
11	How do you create an index?	Using the db.collection.createIndex() method	db.users.createIndex({ email: 1 });
12	What is a unique index?	Ensures the values in the indexed field are unique	db.users.createIndex({ email: 1 }, { unique: true });
13	What is a compound index?	An index on multiple fields	db.products.createIndex({ category: 1, price: -1 });
14	What is the explain() method?	Provides details about how MongoDB executes a query (useful for performance analysis)	db.users.find({ name: "Alice" }).explain("executionStats");
15	What is Write Concern?	Specifies the level of acknowledgment required for a write operation to be considered successful	db.users.insertOne({...}, { writeConcern: { w: 2 } }); (acknowledgment from primary and one secondary if available)

Advanced MongoDB Answers

#	Question	Answer	Examples
1	What is Sharding?	Distributing data across multiple servers (shards) to handle large datasets	Each shard holds a subset of the data based on a shard key.
2	What is a Shard Key?	The field(s) used to determine which shard a document will be stored on	db.shardCollection("orders", { shardKey: { user_id: 1 } });
3	What is a Replica Set?	A group of MongoDB processes that maintain the same data set, providing high availability and redundancy	One primary node and multiple secondary nodes.
4	What is Read Concern?	Specifies the level of consistency required for a read operation	db.users.find({}, { readConcern: { level: 'local' } }); (can return slightly stale data)
5	What is Read Preference?	Specifies which node(s) in a replica set a read operation should target (e.g., primary, secondary)	db.users.find({}, { readPreference: 'secondaryPreferred' }); (reads from secondary if available, otherwise primary)
6	What are Transactions in MongoDB?	Allows atomic, multi-document operations across multiple collections and shards (introduced in 4.0)	const session = db.startSession(); session.startTransaction(); ... session.commitTransaction(); or session.abortTransaction();
7	What is a TTL Index (Time-To-Live)?	Automatically removes documents from a collection after a specified time	db.logs.createIndex({ logTime: 1 }, { expireAfterSeconds: 3600 }); (removes after 1 hour)
8	What is a Text Index?	Enables full-text search on string fields	db.products.createIndex({ description: "text", tags: "text" });
9	What is MongoDB Atlas Search?	A fully managed full-text search service built on Apache Lucene	Provides advanced search capabilities beyond basic text indexes
10	What are Change Streams?	A mechanism to capture real-time data changes (inserts, updates, deletes) in a collection	Allows applications to receive notifications about data changes
11	What is the Query Cache?	Caches aggregated query results to improve read performance	db.coll.aggregate([...]).cache(...)
12	How do you optimize MongoDB performance?	Use appropriate indexes, optimize queries, consider embedding vs. referencing, use sharding/replica sets	Use EXPLAIN, analyze query patterns, design efficient schemas
13	What is the Write Journal?	A mechanism where MongoDB stores data that is being written but not yet flushed to disk	Ensures data durability in case of crashes during a write operation
14	What is Oplog (Operation Log)?	A capped collection in a replica set that records all operations performed on the primary node	Used by secondary nodes to replicate changes from the primary
15	What is a Capped Collection?	A collection with a fixed maximum size or number of documents; older documents are automatically removed	db.createCollection("logs", { capped: true, size: 1000000 }); (1MB size)