mongodb

MongoDB

MongoDB is a document database. It stores data in JSON-like documents (BSON). It is the most popular NoSQL database, known for flexibility and scalability.

When to Use

Flexible JSON Document Storage: Modeling evolving, nested domain entities with polymorphic schemas in BSON.
High-Velocity Operational Data Stores: Content management, e-commerce product catalogs, and user personalization platforms.
Rich Aggregation Pipelines: Transforming, joining ($lookup), grouping, and projecting analytical data streams in real time.
Global Sharding & Geospatial Indexing: Scaling collections horizontally across shards and executing 2dsphere geo queries.

Quick Start

// Using Mongoose (Node.js)
const kittySchema = new mongoose.Schema({
  name: String,
});
const Kitten = mongoose.model("Kitten", kittySchema);

const silence = new Kitten({ name: "Silence" }); await silence.save();


## Core Concepts

### BSON Documents & Schema Flexibility

Data is stored as binary JSON (BSON), supporting native dates, 64-bit integers, decimals, and geospatial points:

```javascript
// MongoDB Document in collections/orders
{
  "_id": ObjectId("66f6ab42d1e1c3a628a58a98"),
  "orderNumber": "ORD-2026-901",
  "customer": { "id": "cust_12", "email": "alex@example.com" },
  "items": [
    { "sku": "WIDGET-01", "qty": 2, "price": 49.99 }
  ],
  "shippingAddress": {
    "type": "Point",
    "coordinates": [-73.9851, 40.7488] // 2dsphere GeoJSON
  },
  "status": "PROCESSING",
  "createdAt": ISODate("2026-09-27T10:00:00Z")
}

Powerful Multi-Stage Aggregation Framework

Transforms and aggregates documents through sequential pipeline stages:

db.orders.aggregate([
  {
    $match: { status: "COMPLETED", createdAt: { $gte: ISODate("2026-01-01") } },
  },
  { $unwind: "$items" },
  {
    $group: {
      _id: "$items.sku",
      totalRevenue: { $sum: { $multiply: ["$items.qty", "$items.price"] } },
      unitsSold: { $sum: "$items.qty" },
    },
  },
  { $sort: { totalRevenue: -1 } },
  { $limit: 5 },
]);

Replica Sets & Automated Failover

Ensures high availability through 3-node primary-secondary elections:

[ Primary (Read/Write) ] ──Oplog Replication (Async)──→ [ Secondary (Read Replicas) ]
                                                              │
                                                              ▼
                                                     [ Secondary (Voter) ]

Common Patterns

Aggregation Pipeline with Lookup and Facets

Problem: Normalizing and joining related documents across collections efficiently in a single query.

Solution: Use $lookup with $project and pagination $facet:

db.orders.aggregate([
  {
    $match: { status: "DELIVERED", createdAt: { $gte: ISODate("2025-01-01") } },
  },
  {
    $lookup: {
      from: "users",
      localField: "userId",
      foreignField: "_id",
      as: "customer",
    },
  },
  { $unwind: "$customer" },
  {
    $project: {
      orderId: "$_id",
      totalAmount: 1,
      customerName: "$customer.name",
      customerEmail: "$customer.email",
    },
  },
  { $sort: { totalAmount: -1 } },
  { $limit: 20 },
]);

Best Practices

Do:

Design for Data Access Patterns: Embed data that is read together frequently; reference data when unbounded growth is expected.
Follow the ESR Rule for Indexes: Order compound indexes by Equality first, Sort second, and Range last.
Enable JSON Schema Validation: Enforce required fields and types at the collection level via $jsonSchema.
Use Bulk Operations: Batch multiple write operations using bulkWrite() to minimize network roundtrips.

Don't:

Create unbounded arrays inside documents: Unbounded arrays degrade performance and can hit the 16MB document size limit.
Use $lookup excessively: MongoDB is not a relational database; heavy multi-collection joins destroy throughput.
Perform unindexed queries: Run .explain("executionStats") to verify queries utilize IXSCAN rather than COLLSCAN.

Troubleshooting

Error	Cause	Solution
`Executor error during find command: OperationExceededTimeLimit`	Query performing full collection scan (`COLLSCAN`) without supporting index.	Run `explain('executionStats')` and add compound index matching filter and sort keys.
`WriteConflict error`	Concurrent updates modifying same document concurrently in WiredTiger.	Implement retry logic in application layer or batch modifications.
`BSONObj size exceeds maximum allowed size (16MB)`	Single document exceeded 16MB limit due to unbounded arrays.	Refactor schema: move unbounded child arrays into separate collection with parent references.

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