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1. What is an index in MongoDB?

An index makes searches faster by creating shortcuts to find data quickly.

Example:


// Without index - slow search through all documents
db.users.find({ email: "alice@email.com" }); // Scans all documents
 
// With index - fast lookup
db.users.createIndex({ email: 1 });
db.users.find({ email: "alice@email.com" }); // Uses index

2. How do you create an index in MongoDB?

Use createIndex(): db.collection.createIndex({field: 1}) where 1 means ascending order.

Example:


// Single field index
db.users.createIndex({ name: 1 });
 
// Compound index
db.users.createIndex({ name: 1, age: -1 });
 
// Text index for search
db.products.createIndex({ title: "text", description: "text" });

3. What is aggregation in MongoDB?

Aggregation processes data and returns computed results, like grouping and counting.

Example:


// Count users by age group
db.users.aggregate([
  {
    $group: {
      _id: {
        $cond: [{ $gte: ["$age", 30] }, "adult", "young"],
      },
      count: { $sum: 1 },
    },
  },
]);
// Result: [{ "_id": "adult", "count": 5 }, { "_id": "young", "count": 3 }]

4. What is the aggregation pipeline?

The aggregation pipeline is a series of stages that process documents step by step.

Example:


db.orders.aggregate([
  { $match: { status: "completed" } }, // Stage 1: Filter
  {
    $group: {
      // Stage 2: Group
      _id: "$customerId",
      totalSpent: { $sum: "$amount" },
    },
  },
  { $sort: { totalSpent: -1 } }, // Stage 3: Sort
  { $limit: 5 }, // Stage 4: Limit
]);

5. What is sharding in MongoDB?

Sharding splits large databases across multiple servers to handle more data and users.

Example:


// Enable sharding for database
sh.enableSharding("mystore");
 
// Shard a collection based on user_id
sh.shardCollection("mystore.users", { user_id: 1 });
 
// Data automatically distributed:
// Server 1: user_id 1-1000
// Server 2: user_id 1001-2000
// Server 3: user_id 2001-3000

16. What is replication in MongoDB?

Replication creates copies of your database on different servers for backup and reliability.

Example:


// Replica set configuration
{
  "_id": "myReplicaSet",
  "members": [
    { "_id": 0, "host": "server1:27017", "priority": 2 },  // Primary
    { "_id": 1, "host": "server2:27017", "priority": 1 },  // Secondary
    { "_id": 2, "host": "server3:27017", "priority": 1 }   // Secondary
  ]
}

7. What is a replica set?

A replica set is a group of MongoDB servers that keep copies of the same data.

Example:


// Initialize replica set
rs.initiate({
  _id: "myReplicaSet",
  members: [
    { _id: 0, host: "localhost:27017" },
    { _id: 1, host: "localhost:27018" },
    { _id: 2, host: "localhost:27019" },
  ],
});
 
// Check replica set status
rs.status();

8. What are the different types of indexes in MongoDB?

Single field, compound (multiple fields), multikey (arrays), text, and geospatial indexes.

Example:


// Single field index
db.users.createIndex({ name: 1 });
 
// Compound index
db.users.createIndex({ name: 1, age: -1 });
 
// Multikey index (for arrays)
db.users.createIndex({ skills: 1 });
 
// Text index
db.products.createIndex({ title: "text" });
 
// Geospatial index
db.places.createIndex({ location: "2dsphere" });

9. What is the limit() method?

limit() controls how many documents are returned from a query.

Example:


// Get first 5 users
db.users.find().limit(5);
 
// Get 10 users, skip first 20 (pagination)
db.users.find().skip(20).limit(10);
 
// Combined with sort
db.users.find().sort({ age: -1 }).limit(3); // Top 3 oldest users

10. What is the sort() method?

sort() arranges documents in ascending (1) or descending (-1) order by specified fields.

Example:


// Sort by age ascending
db.users.find().sort({ age: 1 });
 
// Sort by age descending
db.users.find().sort({ age: -1 });
 
// Multiple field sort
db.users.find().sort({ age: -1, name: 1 }); // Age desc, then name asc