In the world of software engineering, terms like system, application, service, microservice, module, and class are often used interchangeably—yet each plays a very different role in the software hierarchy. Understanding how these layers fit together is essential for architects, developers, and testers alike.
From the high-level system that integrates multiple applications, down to the class that represents the smallest unit of code in object-oriented programming, every layer has its own purpose and responsibilities. This hierarchy not only helps in designing scalable software but also in managing complexity, improving maintainability, and fostering collaboration across teams.
In this article, we’ll explore the structure step by step—illustrating how systems, applications, services, microservices, modules, and classes connect to form the backbone of modern software architecture.
1. System
A system is the complete ecosystem that delivers a business solution. It typically consists of multiple software applications working together, supported by infrastructure, databases, and integrations.
Example: A banking system includes mobile apps, web portals, ATMs, and backend services.
2. Software / Application
A software application is a standalone product designed to solve a specific problem or provide value to users. It can run on a desktop, mobile device, or in the cloud.
Example: The customer-facing mobile banking app that allows users to transfer money and check balances.
3. Service
A service represents a distinct business capability or functionality provided within a software application. Services are often exposed via APIs or internal components.
Example: A payment service, an authentication service, or an email notification service.
4. Microservice
A microservice is a small, independently deployable unit of a larger service. Microservices architecture allows teams to build, scale, and deploy components independently, increasing flexibility and resilience.
Example: Fraud Detection Microservice inside payment service that Runs fraud checks and risk scoring, or Refund Microservice that Handles partial/full refunds independently.
also Login Microservice inside authentication service that Manages username/password login.
5. Module
A module is a logical grouping of related functionality inside a service or application. It helps organize code, making the system more maintainable and testable.
Example: A "Credentials Validation Module" inside Login Microservice that Checks if username/email and password format are valid.
also "Refund Request Module" inside Refund Microservice that Accepts refund requests from users or other services.
6. Class
A class is the smallest building block in object-oriented programming (OOP). It encapsulates data (attributes) and behavior (methods), serving as the foundation for modules and higher layers
Example: A "User" class with properties like username and methods like validatePassword(), "PasswordPolicy" class that enforces password complexity rules in Credentials Validation Module
also "RefundRequestHandler" Class: main entry point for handling new refund requests.
