Change streams on collections in MongoDB

Introduction

Change streams let you watch for changes in a collection so you can react to them right away without checking the data again and again.

You want to update a user interface immediately when data changes.

You need to keep data in sync between different parts of your app.

You want to log or audit changes made to your data.

You want to trigger actions when specific data changes happen.

You want to build real-time notifications based on database updates.

Syntax

MongoDB

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

The 'pipeline' is optional and lets you filter which changes to watch.

The 'options' can control things like full document return or resume tokens.

Examples

Watch all changes on the 'orders' collection.

MongoDB

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

Watch only insert operations on the 'orders' collection.

MongoDB

const pipeline = [ { $match: { operationType: 'insert' } } ];
const changeStream = db.collection('orders').watch(pipeline);

Watch all changes and get the full updated document after update operations.

MongoDB

const changeStream = db.collection('orders').watch([], { fullDocument: 'updateLookup' });

Sample Program

This code listens for any change on the 'products' collection and prints the change details as they happen.

MongoDB

const changeStream = db.collection('products').watch();
changeStream.on('change', (change) => {
  printjson(change);
});

Important Notes

Change streams require a replica set or sharded cluster; they do not work on standalone MongoDB servers.

You can use change streams to build reactive apps that respond instantly to data changes.

Remember to close the change stream when you no longer need it to free resources.

Summary

Change streams let you watch for real-time changes in a collection.

You can filter which changes to watch using a pipeline.

Use change streams to build live updates, notifications, or audit logs.

Practice

1. What is the main purpose of using change streams on a MongoDB collection?

easy

A. To create indexes on the collection

B. To backup the collection data automatically

C. To watch for real-time changes happening in the collection

D. To delete documents based on a condition

Solution

1. Step 1: Understand what change streams do

   Change streams allow applications to listen for real-time changes like inserts, updates, or deletes in a collection.

2. Step 2: Compare options with this purpose

   Only To watch for real-time changes happening in the collection describes watching for real-time changes, which matches the purpose of change streams.

3. Final Answer:

   To watch for real-time changes happening in the collection -> Option C

4. Quick Check:

   Change streams = watch real-time changes [OK]

Hint: Change streams = real-time watch on collection changes [OK]

Common Mistakes:

• Thinking change streams backup data
• Confusing change streams with indexing
• Assuming change streams delete documents

2. Which of the following is the correct way to start a change stream on a collection named orders in MongoDB?

easy

A. db.orders.changeStream()

B. db.orders.watch()

C. db.orders.listen()

D. db.orders.observe()

Solution

1. Step 1: Recall the MongoDB method for change streams

   The correct method to open a change stream on a collection is watch().

2. Step 2: Match the method with the collection name

   Using db.orders.watch() correctly starts the change stream on the orders collection.

3. Final Answer:

   db.orders.watch() -> Option B

4. Quick Check:

   Change stream method = watch() [OK]

Hint: Use .watch() to start change streams on collections [OK]

Common Mistakes:

• Using non-existent methods like changeStream()
• Confusing with listen() or observe() which don't exist
• Misspelling the method name

3. Given the following code snippet, what will be the output when a new document is inserted into the products collection?

const changeStream = db.products.watch();
changeStream.on('change', (change) => {
  console.log(change.operationType);
});

medium

A. 'insert'

B. 'update'

C. 'delete'

D. 'replace'

Solution

1. Step 1: Understand the event listener on change stream

   The code listens for any change and logs the operationType of that change.

2. Step 2: Identify the operation type for inserting a document

   When a new document is inserted, the operationType is insert.

3. Final Answer:

   'insert' -> Option A

4. Quick Check:

   Insert operation logs 'insert' [OK]

Hint: Insert triggers operationType 'insert' in change stream [OK]

Common Mistakes:

• Confusing insert with update or delete
• Expecting full document instead of operationType
• Misreading event listener output

4. You wrote this code to watch changes on the users collection but it throws an error:

const changeStream = db.users.watch({ operationType: 'insert' });

What is the mistake?

medium

A. The operationType should be capitalized as 'Insert'

B. The collection name should be user not users

C. You must call start() after watch()

D. The watch method does not accept a filter object directly

Solution

1. Step 1: Check the argument to watch()

   The watch() method expects an array pipeline for filtering, not a direct object.

2. Step 2: Understand correct filtering syntax

   To filter by operationType, you must pass a pipeline like [{ $match: { operationType: 'insert' } }].

3. Final Answer:

   The watch method does not accept a filter object directly -> Option D

4. Quick Check:

   Filter must be pipeline array, not object [OK]

Hint: Use pipeline array for filters in watch() [OK]

Common Mistakes:

• Passing filter as object instead of pipeline array
• Wrong collection name assumption
• Expecting a start() method
• Capitalizing operationType values incorrectly

5. You want to build a live notification system that only alerts when a document in the tasks collection is updated and the status field changes to 'completed'. Which change stream pipeline correctly filters these changes?

hard

A. [{ $match: { operationType: 'update', 'updateDescription.updatedFields.status': 'completed' } }]

B. [{ $match: { operationType: 'insert', 'fullDocument.status': 'completed' } }]

C. [{ $match: { 'updateDescription.updatedFields.status': 'completed' } }]

D. [{ $match: { operationType: 'delete', 'updateDescription.updatedFields.status': 'completed' } }]

Solution

1. Step 1: Identify the operation type needed

   We want to watch only updates, so operationType must be 'update'.

2. Step 2: Filter for status field changed to 'completed'

   The updateDescription.updatedFields.status contains changed fields; we check if it equals 'completed'.

3. Step 3: Combine filters in a pipeline

   The correct pipeline is [{ $match: { operationType: 'update', 'updateDescription.updatedFields.status': 'completed' } }].

4. Final Answer:

   [{ $match: { operationType: 'update', 'updateDescription.updatedFields.status': 'completed' } }] -> Option A

5. Quick Check:

   Filter updates with status='completed' = [{ $match: { operationType: 'update', 'updateDescription.updatedFields.status': 'completed' } }] [OK]

Hint: Filter update and updatedFields.status in $match pipeline [OK]

Common Mistakes:

• Filtering inserts or deletes instead of updates
• Checking fullDocument instead of updateDescription
• Missing operationType filter
• Using wrong field path in filter