Architecture Styles, Core Principles, and Unit Testing Best Practices
Lokoko John | 2026
Software Engineering • System Architecture & Quality Assurance
System design and software testing are two sides of the same coin — one shapes how a system is built, the other proves it actually works. This post walks through the core ideas behind both, from architecture styles to unit testing best practices.
System design is the process of planning a system's architecture, components, and interfaces so it meets end-user requirements — whether you're building a small app or a large-scale platform.
Ensures systems grow and handle increased demand seamlessly without catastrophic downtime.
Optimizes CPU, memory, database IO, and network allocation for lightning-fast, responsive applications.
Enables systems to evolve alongside changing business requirements, reducing long-term technical debt.
Builds strong mastery across microservices, monolithic, event-driven, and distributed architectures.
System design typically happens in two critical stages:
Defines the overall architecture of a system and how the main components interact — a big-picture view of structure, services, and data flow. It identifies major modules, services, and interactions, focusing on system architecture and macro-level design decisions.
Focuses on the internal implementation details of each component — a detailed blueprint for how modules, classes, and functions are built. It describes internal logic, class hierarchies, methods, data structures, and converts the HLD into a concrete implementation roadmap.
Organizes all application components within a single codebase, deployed as one unified unit.
Breaks an application into independent services that can be developed, deployed, and scaled separately.
Components communicate by producing and reacting to events, staying loosely coupled and real-time.
Unit testing is a software testing method that verifies individual units — functions, methods, or classes — work correctly in isolation.
Pick function or class.
Cover happy & edge paths.
Run via test runner.
Review failures & causes.
Resolve bug in code.
Confirm clean pass.
Check branch coverage.
Automate on every commit.
The Arrange–Act–Assert (AAA) pattern is the gold standard structure for writing clear, maintainable unit tests:
Prepare test data, dependencies, inputs, and configure mock expectations.
Invoke the specific method or function being validated.
Verify that the returned outcome matches the exact expected state.
Tests inputs vs outputs with zero internal code knowledge.
Tests internal logic, branches, and execution paths directly.
Combines input validation with partial knowledge of database/internal state.
Should_ReturnTrue_When_InputIsValid).Good system design gives you an architecture that can scale and adapt; disciplined unit testing gives you the confidence to keep changing that architecture without breaking it. Together, they're the foundation of software that's built to last — not just to ship.