Shopping cart and session management in HLD - Architecture Diagram

System Overview - Shopping cart and session management

This system manages user sessions and shopping carts for an online store. It ensures users can add items to their cart, keep their cart saved during their visit, and retrieve it later. The system must handle many users simultaneously and keep data consistent and fast.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  +-------------------+
  |                   |
Session Service    Cart Service
  |                   |
  v                   v
Session Store      Cache Layer
  |                   |
  v                   v
Database           Database

Components

User

client

Person using the online store through a browser or app

Load Balancer

load_balancer

Distributes incoming user requests evenly to backend services

API Gateway

api_gateway

Routes requests to appropriate services and handles authentication

Session Service

service

Manages user login sessions and session validation

Cart Service

service

Handles shopping cart operations like add, remove, and view items

Session Store

database

Stores session data persistently for user authentication

Cache Layer

cache

Caches shopping cart data for fast access and reduced database load

Database

database

Stores persistent shopping cart data and user information

Request Flow - 15 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Session Service

Session Service → Session Store

Session Store → Session Service

Session Service → API Gateway

API Gateway → Cart Service

Cart Service → Cache Layer

Cache Layer → Cart Service

Cart Service → Database

Database → Cart Service

Cart Service → Cache Layer

Cart Service → API Gateway

API Gateway → Load Balancer

Load Balancer → User

Failure Scenario

Component Fails:Cache Layer

Impact:Cart service cannot retrieve cached cart data, causing slower responses due to direct database queries

Mitigation:System falls back to database queries for cart data; cache is rebuilt on next successful cart operation

Architecture Quiz - 3 Questions

Which component is responsible for validating the user's session token?

A API Gateway

B Cart Service

C Session Service

D Load Balancer

Design Principle

This design separates session management and cart operations into distinct services, improving modularity and scalability. Using a cache layer reduces database load and speeds up cart data access. The API Gateway centralizes routing and authentication, simplifying client interactions.

Practice

1. What is the primary purpose of session management in a shopping cart system?

easy

A. To keep track of user activity and cart contents securely

B. To permanently store all user purchases in the database

C. To display advertisements based on user preferences

D. To manage payment processing and billing

Solution

1. Step 1: Understand session management role

   Sessions keep temporary data about a user's interaction, such as cart contents and login status.

2. Step 2: Identify the main goal in shopping cart context

   The main goal is to track user activity and cart items securely during their visit.

3. Final Answer:

   To keep track of user activity and cart contents securely -> Option A

4. Quick Check:

   Session = Track user and cart data [OK]

Hint: Sessions track temporary user data like cart items [OK]

Common Mistakes:

• Confusing session with permanent storage
• Thinking sessions handle payment processing
• Assuming sessions display ads

2. Which of the following is the correct way to maintain a user's shopping cart session in a web application?

easy

A. Store cart data only in browser cookies without server validation

B. Require user to log in before adding items to cart

C. Save cart data directly in the URL query parameters

D. Use a unique session ID stored in a cookie linked to server-side cart data

Solution

1. Step 1: Review session management best practice

   Best practice is to store a unique session ID in a cookie that references server-side data.

2. Step 2: Evaluate options for security and scalability

   Storing cart data only in cookies or URLs is insecure and limited in size; requiring login is not always needed.

3. Final Answer:

   Use a unique session ID stored in a cookie linked to server-side cart data -> Option D

4. Quick Check:

   Session ID cookie + server data = Correct [OK]

Hint: Session ID cookie links to server cart data securely [OK]

Common Mistakes:

• Storing sensitive data only in cookies
• Putting cart info in URL causing security risks
• Assuming login is mandatory for cart usage

3. Consider this simplified flow for a shopping cart session:

1. User adds item A to cart
2. Server stores cart in session store
3. User adds item B
4. Server updates session
5. User refreshes page
6. Server retrieves session cart

What will the cart contain after step 6?

medium

A. Only item B

B. Only item A

C. Both item A and item B

D. Empty cart

Solution

1. Step 1: Track items added to cart in session

   Item A is added first and stored in session, then item B is added and session updated.

2. Step 2: Understand session retrieval on refresh

   On refresh, server retrieves the session cart which includes both items added previously.

3. Final Answer:

   Both item A and item B -> Option C

4. Quick Check:

   Session stores cumulative cart items [OK]

Hint: Session updates accumulate cart items, not replace [OK]

Common Mistakes:

• Assuming new item replaces old in session
• Thinking refresh clears session data
• Confusing client-side and server-side storage

4. A developer notices that users lose their shopping cart items after closing the browser. What is the most likely cause?

medium

A. Cart data is stored permanently in the database

B. Session cookie is set as a session-only cookie without expiration

C. User authentication is required to save cart

D. Server is caching cart data incorrectly

Solution

1. Step 1: Understand session cookie behavior

   Session cookies without expiration are deleted when browser closes, losing session data.

2. Step 2: Identify cause of cart loss

   Because cart data is linked to session cookie, losing cookie means losing cart info.

3. Final Answer:

   Session cookie is set as a session-only cookie without expiration -> Option B

4. Quick Check:

   Session cookie expires on browser close [OK]

Hint: Session cookies without expiry vanish on browser close [OK]

Common Mistakes:

• Assuming database storage causes cart loss
• Thinking authentication affects session persistence
• Blaming server cache without evidence

5. You are designing a scalable shopping cart system for millions of users. Which approach best balances performance and reliability for session and cart management?

hard

A. Use a distributed in-memory session store with periodic database backups

B. Store cart data in client-side cookies only to reduce server load

C. Save all cart data directly in the main database synchronously on every change

D. Require users to log in before adding items to cart to simplify session handling

Solution

1. Step 1: Evaluate client-side vs server-side storage

   Client-only storage risks security and size limits; main DB sync on every change causes latency.

2. Step 2: Consider scalability and reliability

   Distributed in-memory store (like Redis) offers fast access and can be backed up to DB for durability.

3. Step 3: Assess login requirement impact

   Requiring login reduces friction and scalability; anonymous carts improve UX.

4. Final Answer:

   Use a distributed in-memory session store with periodic database backups -> Option A

5. Quick Check:

   Distributed cache + DB backup = Scalable & reliable [OK]

Hint: Use distributed cache with DB backup for scalable sessions [OK]

Common Mistakes:

• Relying only on client cookies for cart data
• Writing to DB synchronously on every cart update
• Forcing login before cart usage unnecessarily