Sequence diagrams in LLD - System Design Exercise

Design: Sequence Diagram for User Login Flow

Focus on the login process only, excluding registration or password reset flows

Functional Requirements

FR1: Show the interaction between user, frontend, backend, and database during login

FR2: Include authentication and error handling steps

FR3: Demonstrate synchronous and asynchronous message flows

Non-Functional Requirements

NFR1: Diagram must be clear and easy to understand for beginners

NFR2: Must represent real-world request-response timing

NFR3: Should cover up to 10 steps in the sequence

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

Key Components

User interface (frontend)

Backend authentication service

Database for user credentials

Session or token management

Design Patterns

Request-response pattern

Error handling in sequence diagrams

Synchronous vs asynchronous messages

Activation bars to show processing time

Reference Architecture

User          Frontend          Backend          Database
  |               |                |                 |
  |---Login Req-->|                |                 |
  |               |---Auth Req---->|                 |
  |               |                |---Query User--->|
  |               |                |<--User Data-----|
  |               |<--Auth Resp----|                 |
  |<--Login Resp--|                |                 |
  |               |                |                 |

Components

User

Human interaction

Initiates login request

Frontend

Web or mobile client

Sends login request and displays response

Backend

Authentication server

Processes login, verifies credentials

Database

Relational or NoSQL DB

Stores user credentials and data

Request Flow

1. User enters credentials and sends login request to Frontend

2. Frontend forwards authentication request to Backend

3. Backend queries Database for user data

4. Database returns user data to Backend

5. Backend verifies password and authentication rules

6. Backend sends authentication result back to Frontend

7. Frontend displays login success or failure to User

Database Schema

Entities: - User(id PK, username, password_hash, salt, email) Relationships: - None needed for login flow Notes: - Password stored securely as hash with salt - User table supports authentication queries

Scaling Discussion

Bottlenecks

Backend authentication server CPU limits under high login volume

Database read latency for user credential queries

Frontend handling many simultaneous login requests

Solutions

Use load balancer and multiple backend instances for horizontal scaling

Cache user credentials or session tokens in memory (e.g., Redis) to reduce DB hits

Implement rate limiting and throttling on frontend and backend to prevent overload

Interview Tips

Time: Spend 10 minutes clarifying requirements and participants, 20 minutes drawing and explaining the sequence diagram, 15 minutes discussing scaling and error handling

Clearly identify all actors and their roles

Explain synchronous vs asynchronous messages in the flow

Highlight error handling and alternative flows

Discuss how the design supports scalability and reliability