System design involves planning the architecture, components, data flow, and interactions required to build a functional system that meets specified requirements. It usually follows these steps:

Requirements Gathering: Understanding functional and non-functional requirements (e.g., scalability, reliability).

High-Level Design (HLD): Defining the system’s core architecture and main components.

Low-Level Design (LLD): Detailing the implementation and design of each component.

High-level design (HLD) and low-level design (LLD) are stages in the software development lifecycle, specifically within the system design phase. These terms come into play after gathering requirements and defining the basic objectives and functionalities of a system. Here’s when and how each term is used:

1. High-Level Design (HLD):

When: HLD is usually the first step after requirements gathering, when the design team aims to create an overall architecture for the system.

Purpose: It provides a bird's-eye view of the system, illustrating the architecture and the major components without diving into deep technical details.

Components of HLD:

System architecture: Identifying main components like servers, databases, APIs, and third-party services.

Modules and layers: Dividing the system into modules or layers (e.g., presentation, business, data layers).

Data flow and interactions: Showing how data flows between components, clients, and external services.

Technology choices: Selecting technologies (programming languages, frameworks, database types) based on system needs.

Non-functional requirements: Addressing scalability, reliability, and performance requirements.

Audience: HLD is often intended for stakeholders, product owners, and architects, providing a shared understanding of the system.

1. Low-Level Design (LLD):

When: LLD is created after the high-level design is completed and approved. It usually takes place in the later stages of the design phase, right before implementation begins.

Purpose: LLD breaks down the high-level components into detailed specifications, covering each module’s inner workings and design choices at a granular level.

Components of LLD:

Detailed module design: Specifying each module, sub-module, and class, including its functions and interfaces.

Database design: Creating detailed schema designs, including tables, fields, relationships, indexes, and constraints.

Algorithms and data structures: Describing algorithms, data structures, and optimization techniques for specific tasks.

Class diagrams and pseudocode: Providing class diagrams, sequence diagrams, and sometimes pseudocode to clarify functionality.

Error handling and logging: Specifying error handling, logging, and debugging mechanisms.

Audience: LLD is mainly intended for developers, providing them with precise instructions for building and coding each component.

In summary:

High-Level Design focuses on the overall structure and the big-picture view of the system.

Low-Level Design provides the technical details needed to implement each part of the system.

Together, HLD and LLD ensure a clear, systematic approach to building complex systems, allowing teams to validate the design before moving into development.