What are Change Streams in MongoDB? | MongoDB Interview Questions

What You'll Learn

What change streams are
Setting up watchers
Use cases

What are Change Streams?

Change streams allow applications to watch for real-time changes in collections, databases, or deployments.

Basic Usage

code.jsJavaScript

const changeStream = db.collection('orders').watch();

changeStream.on('change', (change) => {
  console.log('Change detected:', change);
});

// With async iterator
for await (const change of changeStream) {
  console.log(change);
}

Change Event Structure

code.jsJavaScript

{
  _id: { ... },  // Resume token
  operationType: 'insert',  // insert, update, delete, replace
  fullDocument: { ... },    // The document (for insert/update)
  ns: { db: 'mydb', coll: 'orders' },
  documentKey: { _id: ObjectId('...') },
  updateDescription: {      // For updates
    updatedFields: { status: 'shipped' },
    removedFields: []
  }
}

Filtering Changes

code.jsJavaScript

const pipeline = [
  { $match: { operationType: { $in: ['insert', 'update'] } } },
  { $match: { 'fullDocument.status': 'completed' } }
];

const changeStream = collection.watch(pipeline);

With Mongoose

code.jsJavaScript

const Order = mongoose.model('Order', orderSchema);

Order.watch().on('change', (change) => {
  if (change.operationType === 'insert') {
    sendNotification(change.fullDocument);
  }
});

Resume After Disconnect

code.jsJavaScript

let resumeToken;

changeStream.on('change', (change) => {
  resumeToken = change._id;
  // Process change
});

// Later, resume from token
const newStream = collection.watch([], { resumeAfter: resumeToken });

Use Cases

Real-time notifications
Data synchronization
Audit logging
Cache invalidation