HLD vs LLD

High-Level Design (HLD) vs Low-Level Design (LLD) Cheatsheet

When developing software systems, High-Level Design (HLD) and Low-Level Design (LLD) are two crucial stages in the design process. They serve different purposes, and understanding the distinction between them is essential for system architecture and implementation.

1. High-Level Design (HLD)

Purpose:
High-Level Design (HLD) provides a broad overview of the system. It defines the architecture and components involved in the system, their relationships, and how they interact. HLD focuses on the “big picture” and outlines the overall structure and workflow of the system.

Key Characteristics of HLD:

  • Scope: System architecture, modules, and their high-level interactions.
  • Audience: Stakeholders, system architects, and developers who need to understand the entire system.
  • Focus: Identifying the major components and how they interact at a high level.
  • Details: No detailed implementation, algorithms, or data structures.

HLD Includes:

  • System Architecture: Describes the overall architecture of the system (e.g., client-server, microservices, monolithic).
  • Component Design: Major system components and modules (e.g., database, user interface, API layers).
  • Technology Stack: The chosen technologies, frameworks, and tools.
  • Data Flow: How data flows between components and systems.
  • Integration: Interactions with third-party systems or APIs.
  • Security: High-level security measures (e.g., encryption, authentication mechanisms).
  • Scalability and Redundancy: High-level strategies for handling load, replication, and failover.

HLD Example:

- System: E-Commerce Application
- Architecture: Microservices-based architecture with a front-end (React.js), back-end (Spring Boot), and database (MySQL).
- Components:
  - Front-end Service (React)
  - User Service (Spring Boot)
  - Product Service (Spring Boot)
  - Order Service (Spring Boot)
  - Payment Service (External API Integration)
  - Database: MySQL for product and order data.
  - Caching: Redis for session management.
- Data Flow: 
  - User requests login via React, which is handled by the User Service.
  - The Product Service handles product search queries.
  - Payment Service integrates with third-party API for processing payments.

2. Low-Level Design (LLD)

Purpose:
Low-Level Design (LLD) dives into the detailed implementation of the system, describing the actual working of the components, classes, methods, and data structures. LLD focuses on the internal workings of the system and defines how the components will be built and implemented.

Key Characteristics of LLD:

  • Scope: Detailed design, classes, methods, data structures, and algorithms.
  • Audience: Developers and technical teams who will be implementing the system.
  • Focus: Detailed design of each module, including methods, parameters, algorithms, and data structures.
  • Details: Includes detailed interactions, algorithms, class structures, and API contracts.

LLD Includes:

  • Class Diagrams: Defines the classes, objects, their relationships, and methods.
  • Data Structures: Detailed data structures, including their types (e.g., arrays, lists, hash maps) and fields.
  • UML Diagrams: Specific UML class diagrams, sequence diagrams, and state diagrams that show the internal system flow and object interactions.
  • Algorithms: Detailed algorithms for specific functions or processes.
  • Database Design: Detailed schema design, normalization, tables, relationships (e.g., entity-relationship diagrams).
  • API Definitions: Detailed API specifications (input parameters, output, error handling).
  • Error Handling: Handling exceptions, edge cases, and other runtime scenarios.
  • Security and Optimization: Low-level implementation of security measures and performance optimizations.

LLD Example:

- System: E-Commerce Application (Login Functionality)
- Class Diagram:
  - User class: 
    - Attributes: `username`, `password`, `email`, `id`
    - Methods: `validatePassword()`, `sendVerificationEmail()`
  - Database Access Object (DAO): 
    - Methods: `getUserByUsername()`, `updatePassword()`, `getUserOrders()`
  - AuthenticationService class:
    - Methods: `authenticate()`, `generateToken()`, `validateToken()`
    - Flow: Calls `getUserByUsername()` from DAO, validates password, and generates JWT token.

- Sequence Diagram:
  - User sends login request with username and password.
  - AuthenticationService validates credentials, generates JWT, and returns it to the user.
  - On each request, token is validated using the `validateToken()` method.

- API Definition:
  - POST `/api/login`:
    - Request Body: `{"username": "john", "password": "securepassword"}`
    - Response: `{"status": "success", "token": "JWT_TOKEN"}`
    - Error Handling: `400 Bad Request` for invalid credentials.

- Database Schema:
  - Table: `users`
    - Columns: `id (PK)`, `username (unique)`, `email`, `password_hash`, `created_at`
  - Table: `orders`
    - Columns: `order_id (PK)`, `user_id (FK)`, `order_date`, `total_amount`

Key Differences Between HLD and LLD

AspectHigh-Level Design (HLD)Low-Level Design (LLD)
PurposeProvides an overall architecture and structure of the system.Focuses on detailed implementation and internal workings.
ScopeSystem-level, including architecture, components, and modules.Component-level, focusing on classes, functions, and methods.
AudienceStakeholders, system architects, and project managers.Developers and technical teams responsible for coding.
Level of DetailBroad and abstract, no implementation details.Detailed and precise, describing specific implementation.
DocumentationDiagrams like block diagrams, system architecture.UML class diagrams, sequence diagrams, database schemas.
TechnologiesHigh-level technology stack and architecture choices.Specific frameworks, libraries, algorithms, and data types.
ExamplesComponent flow, system interactions, technology stack.Classes, methods, API endpoints, database schema, algorithms.
Use CaseProvides a blueprint for developers to understand the system’s flow.Helps developers implement the system by specifying classes, methods, and interactions.

When to Use HLD vs LLD

  • Use HLD when:
  • You need to communicate the overall structure of the system.
  • You are planning the system architecture for scaling, security, and overall design.
  • You are evaluating technology choices.
  • You need to ensure that all components interact cohesively.
  • Use LLD when:
  • You are ready to implement the system.
  • You need to define specific details about how classes and methods will work.
  • You need to define data structures, algorithms, and database schemas.
  • You are preparing for code writing, including API and database design.

Conclusion

  • HLD provides a macro view of the system, focusing on architecture and major components, while LLD drills into the micro-level, detailing how those components will be implemented.
  • Both HLD and LLD are essential for building a successful system: HLD sets the foundation, and LLD ensures the system is implementable and works as expected.
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