How to Watch Collection Changes in MongoDB

To watch collection changes in MongoDB, use the watch() method on a collection to open a change stream. This stream provides real-time notifications of inserts, updates, deletes, and other operations on the collection.

Syntax

The watch() method is called on a MongoDB collection to start listening for changes. It returns a change stream cursor that you can iterate or listen to for events.

• collection.watch(pipeline, options)
• pipeline: Optional aggregation pipeline to filter or transform change events.
• options: Optional settings like fullDocument to get the full updated document.

javascript

const changeStream = collection.watch(pipeline, options);

Example

This example shows how to watch changes on a collection and log each change event in real time.

javascript

const { MongoClient } = require('mongodb');

async function watchCollection() {
  const uri = 'mongodb://localhost:27017';
  const client = new MongoClient(uri);

  try {
    await client.connect();
    const db = client.db('testdb');
    const collection = db.collection('testcollection');

    const changeStream = collection.watch();

    console.log('Watching for changes...');

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

    // Keep the process alive to listen
    await new Promise(() => {});
  } finally {
    // client.close(); // Uncomment to close connection when done
  }
}

watchCollection().catch(console.error);

Output

Watching for changes...
Change detected: { _id: ..., operationType: 'insert', fullDocument: {...}, ns: {...}, documentKey: {...} }
Change detected: { _id: ..., operationType: 'update', updateDescription: {...}, ... }

Common Pitfalls

• Not keeping the process alive: The change stream listens asynchronously, so the program must stay running to receive events.
• Not handling errors: Always add error handling on the change stream to catch disconnections.
• Using an unsupported MongoDB version: Change streams require MongoDB 3.6 or later.
• Not specifying fullDocument option when you want the updated document in update events.

javascript

/* Wrong: Not keeping process alive, so no events received */
const changeStream = collection.watch();
changeStream.on('change', (change) => {
  console.log(change);
});
// Program exits immediately

/* Right: Keep process alive with a promise or loop */
await new Promise(() => {});

Quick Reference

Method	Description
collection.watch()	Starts watching changes on the collection
changeStream.on('change', callback)	Listens for change events
pipeline	Optional filter to watch specific changes
options.fullDocument	Returns the full updated document on update events

Key Takeaways

Use collection.watch() to open a change stream for real-time updates.

Keep your program running to continuously receive change events.

Specify options like fullDocument to get detailed update info.

Handle errors and disconnections on the change stream.

Change streams require MongoDB version 3.6 or newer.