Causal consistency concept in MongoDB - Mini Project: Build & Apply

Understanding Causal Consistency in MongoDB

📖 Scenario: You are working on a simple social media app where users post messages and read others' posts. To keep the app responsive and consistent, you want to use MongoDB's causal consistency feature. This ensures that when a user reads posts, they see all the updates that happened before their last action.

🎯 Goal: Build a MongoDB session with causal consistency enabled, insert a post, and read posts ensuring causal consistency is maintained.

📋 What You'll Learn

Create a MongoDB client connection

Start a session with causal consistency enabled

Insert a document into a collection within the session

Read documents from the collection using the same session

💡 Why This Matters

🌍 Real World

Causal consistency helps apps like social media or chat show users the latest updates in the correct order without delays.

💼 Career

Understanding causal consistency is important for developers working with distributed databases to ensure data correctness and user experience.

1

Create MongoDB client and connect to database

Create a MongoDB client called client connecting to mongodb://localhost:27017. Then get the database called socialApp and assign it to db.

MongoDB

# Create MongoDB client and get database
# Your code here

Need a hint?

Use MongoClient from pymongo to connect and then access the database as an attribute.

2

Start a session with causal consistency enabled

Start a session from client with causal_consistency=True and assign it to session.

MongoDB

from pymongo import MongoClient

client = MongoClient('mongodb://localhost:27017')
db = client.socialApp

# Start a session with causal consistency enabled
# Your code here

Need a hint?

Use start_session method on client with the argument causal_consistency=True.

3

Insert a post document within the session

Insert a document {'user': 'Alice', 'message': 'Hello world!'} into the posts collection of db using the session.

MongoDB

from pymongo import MongoClient

client = MongoClient('mongodb://localhost:27017')
db = client.socialApp

session = client.start_session(causal_consistency=True)

# Insert a post document using the session
# Your code here

Need a hint?

Use insert_one on db.posts with the document and pass session=session.

4

Read posts using the same session to ensure causal consistency

Find all documents in posts collection of db using session and assign the cursor to posts_cursor.

MongoDB

from pymongo import MongoClient

client = MongoClient('mongodb://localhost:27017')
db = client.socialApp

session = client.start_session(causal_consistency=True)

db.posts.insert_one({'user': 'Alice', 'message': 'Hello world!'}, session=session)

# Find all posts using the session
# Your code here

Need a hint?

Use find on db.posts and pass session=session to read with causal consistency.