Separation of concerns in LLD - System Design Exercise

Design: Separation of Concerns in Software Design

Focus on designing a software system architecture that applies separation of concerns principles. Out of scope are specific programming language implementations or UI design details.

Functional Requirements

FR1: Divide the system into distinct sections, each handling a specific responsibility

FR2: Ensure that changes in one section have minimal impact on others

FR3: Support easy maintenance and scalability

FR4: Allow independent development and testing of each section

Non-Functional Requirements

NFR1: Keep clear boundaries between concerns to avoid overlap

NFR2: Design for modularity and reusability

NFR3: Maintain low coupling and high cohesion

NFR4: Support integration of different modules with well-defined interfaces

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Presentation layer (UI or API)

Business logic layer

Data access layer

Service or integration layer

Common utilities or helpers

Design Patterns

Layered architecture

Model-View-Controller (MVC)

Microservices

Event-driven architecture

Dependency injection

Reference Architecture

  +-------------------+       +-------------------+       +-------------------+
  |   Presentation    | <---> |   Business Logic  | <---> |   Data Access     |
  |    Layer (UI)     |       |      Layer        |       |      Layer        |
  +-------------------+       +-------------------+       +-------------------+
           |                          |                           |
           v                          v                           v
  +-------------------+       +-------------------+       +-------------------+
  |  Service Layer /   |       |  Utilities Layer  |       | External Systems  |
  | Integration Layer  |       |                   |       | (DB, APIs, etc.)  |
  +-------------------+       +-------------------+       +-------------------+

Components

Presentation Layer

Any UI framework or API gateway

Handles user interaction and input/output formatting

Business Logic Layer

Application server or service code

Processes core rules and workflows of the system

Data Access Layer

Database connectors, ORM tools

Manages data storage and retrieval

Service Layer / Integration Layer

REST/gRPC services, message queues

Coordinates communication between modules and external systems

Utilities Layer

Common libraries or helper modules

Provides shared functions like logging, validation, error handling

Request Flow

1. User sends a request to the Presentation Layer

2. Presentation Layer validates input and forwards to Business Logic Layer

3. Business Logic Layer applies rules and calls Data Access Layer for data

4. Data Access Layer queries or updates the database

5. Results flow back through Business Logic Layer to Presentation Layer

6. Presentation Layer formats response and sends back to user

7. Service Layer manages communication if external systems are involved

8. Utilities Layer supports all layers with common functions

Database Schema

Entities depend on system domain but are accessed only via Data Access Layer. Relationships are managed by Business Logic Layer to keep data integrity. Example: User, Order, Product entities with appropriate foreign keys.

Scaling Discussion

Bottlenecks

Tight coupling between layers causing ripple effects on changes

Monolithic design limiting independent scaling of components

Single database becoming a performance bottleneck

Complex inter-module communication slowing down response times

Solutions

Enforce strict interfaces and contracts between layers to reduce coupling

Adopt microservices to separate concerns into independently deployable units

Use database sharding, replication, or caching to improve data layer scalability

Implement asynchronous communication and message queues to decouple modules

Interview Tips

Time: Spend 10 minutes understanding and clarifying requirements, 20 minutes designing the layered architecture with separation of concerns, and 15 minutes discussing scaling and trade-offs.

Explain the importance of dividing responsibilities clearly

Describe how each layer has a focused role and communicates via interfaces

Discuss how separation of concerns improves maintainability and testing

Highlight how the design supports future changes and scaling

Mention common patterns like MVC or microservices as examples