Change streams on databases in MongoDB - Mini Project: Build & Apply

Monitor Database Changes with MongoDB Change Streams

📖 Scenario: You are managing a MongoDB database for an online bookstore. You want to track any changes made to the books collection in real-time to update your inventory system promptly.

🎯 Goal: Build a MongoDB change stream that listens to changes on the books collection and captures insert, update, and delete events.

📋 What You'll Learn

Create a MongoDB collection named books with sample documents.

Define a change stream on the books collection.

Configure the change stream to listen for insert, update, and delete operations.

Implement the change stream to print the change events.

💡 Why This Matters

🌍 Real World

Change streams help applications react instantly to database changes, such as updating user interfaces or syncing data across services.

💼 Career

Understanding change streams is valuable for backend developers and database administrators who build real-time data-driven applications.

1

Create the books collection with sample data

Create a MongoDB collection called books and insert these exact documents: { _id: 1, title: "The Hobbit", author: "J.R.R. Tolkien" } and { _id: 2, title: "1984", author: "George Orwell" }.

MongoDB

// Insert the sample documents into the books collection
// Your code here

Hint

Use db.books.insertMany() with an array of documents.

2

Define a change stream cursor on the books collection

Create a variable called changeStream that opens a change stream on the books collection using db.books.watch().

MongoDB

db.books.insertMany([
  { _id: 1, title: "The Hobbit", author: "J.R.R. Tolkien" },
  { _id: 2, title: "1984", author: "George Orwell" }
])

// Create the changeStream variable to watch the books collection
// Your code here

Hint

Use db.books.watch() to create the change stream cursor.

3

Configure the change stream to filter for insert, update, and delete events

Modify the changeStream to watch only for insert, update, and delete operation types by passing a pipeline with $match to db.books.watch().

MongoDB

db.books.insertMany([
  { _id: 1, title: "The Hobbit", author: "J.R.R. Tolkien" },
  { _id: 2, title: "1984", author: "George Orwell" }
])

// Update changeStream to filter for insert, update, and delete events
// Your code here

Hint

Pass a pipeline array with { $match: { operationType: { $in: [...] } } } to db.books.watch().

4

Listen for change events and print them

Use changeStream.on('change', callback) to listen for change events and print each event using console.log(change) inside the callback function.

MongoDB

db.books.insertMany([
  { _id: 1, title: "The Hobbit", author: "J.R.R. Tolkien" },
  { _id: 2, title: "1984", author: "George Orwell" }
])

const changeStream = db.books.watch([
  { $match: { operationType: { $in: ["insert", "update", "delete"] } } }
])

// Add an event listener on changeStream to log change events
// Your code here

Hint

Use changeStream.on('change', (change) => { console.log(change) }) to listen and print events.

Practice

1. What is the main purpose of MongoDB change streams on databases?

easy

A. To listen for real-time changes in the database

B. To backup the database automatically

C. To create indexes on collections

D. To run queries faster

Solution

1. Step 1: Understand what change streams do

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

2. Step 2: Compare options with this purpose

   Only To listen for real-time changes in the database describes listening to real-time changes, which matches the main purpose of change streams.

3. Final Answer:

   To listen for real-time changes in the database -> Option A

4. Quick Check:

   Change streams = real-time listening [OK]

Hint: Change streams = real-time database updates listener [OK]

Common Mistakes:

• Confusing change streams with backups
• Thinking change streams create indexes
• Assuming change streams speed up queries

2. Which of the following is the correct syntax to start watching changes on a MongoDB database named mydb?

easy

A. watch.db('mydb')

B. db.watch('mydb')

C. db.mydb.watch()

D. db.getSiblingDB('mydb').watch()

Solution

1. Step 1: Recall how to access a database in MongoDB shell

   To access a database other than the current one, use db.getSiblingDB('mydb').

2. Step 2: Apply the watch() method on the database object

   The watch() method is called on the database object to start change streams.

3. Final Answer:

   db.getSiblingDB('mydb').watch() -> Option D

4. Quick Check:

   Use getSiblingDB to access db, then watch() [OK]

Hint: Use db.getSiblingDB('name').watch() to watch a database [OK]

Common Mistakes:

• Trying to call watch() directly on db with database name
• Using incorrect method like db.watch('name')
• Confusing method order or syntax

3. Given the following code snippet, what will be the output if a document is inserted into any collection in the testdb database?

const changeStream = db.getSiblingDB('testdb').watch();
changeStream.forEach((change) => {
  printjson(change.operationType);
});

medium

A. "update"

B. "delete"

C. "insert"

D. No output

Solution

1. Step 1: Understand what triggers the change event

   The change stream listens to all changes in the database, including inserts, updates, and deletes.

2. Step 2: Identify the operationType for an insert

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

3. Final Answer:

   "insert" -> Option C

4. Quick Check:

   Insert triggers operationType = "insert" [OK]

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

Common Mistakes:

• Expecting 'update' or 'delete' for insert events
• Thinking no output occurs without filters
• Confusing event names with method names

4. Identify the error in this code snippet that tries to watch changes on the sales database:

const changeStream = db.sales.watch();
changeStream.forEach((change) => {
  printjson(change);
});

medium

A. The watch() method cannot be called on a database object

B. The database should be accessed using getSiblingDB('sales') before calling watch()

C. The on() method is not valid for change streams

D. Missing parentheses after watch

Solution

1. Step 1: Check how the database is accessed

   Using db.sales accesses a collection named 'sales' in the current database, not the database itself.

2. Step 2: Correct way to access a different database

   To watch changes on the 'sales' database, use db.getSiblingDB('sales').watch().

3. Final Answer:

   The database should be accessed using getSiblingDB('sales') before calling watch() -> Option B

4. Quick Check:

   Use getSiblingDB to access database, not db.sales [OK]

Hint: Use getSiblingDB('name') to access database, not db.name [OK]

Common Mistakes:

• Assuming db.sales is a database instead of a collection
• Thinking watch() can't be called on database objects
• Confusing event handling methods

5. You want to watch only update operations on the orders database using change streams. Which code snippet correctly applies a filter to achieve this?

hard

A. db.getSiblingDB('orders').watch([{ $match: { operationType: 'update' } }])

B. db.orders.watch({ operationType: 'update' })

C. db.getSiblingDB('orders').watch({ filter: { operationType: 'update' } })

D. db.getSiblingDB('orders').watch([{ $filter: { operationType: 'update' } }])

Solution

1. Step 1: Recall how to filter change streams

   Change streams accept an aggregation pipeline array to filter events, using stages like $match.

2. Step 2: Apply correct syntax for filtering updates

   The correct filter is [{ $match: { operationType: 'update' } }] passed as an array to watch().

3. Final Answer:

   db.getSiblingDB('orders').watch([{ $match: { operationType: 'update' } }]) -> Option A

4. Quick Check:

   Use $match in array to filter change streams [OK]

Hint: Use watch([{ $match: { operationType: 'update' } }]) to filter updates [OK]

Common Mistakes:

• Passing filter as object instead of pipeline array
• Using invalid keys like 'filter' or '$filter'
• Calling watch() on collection instead of database