Writing Maintainable Code

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Stop fighting your own codebase. Learn to design software that stays easy to change.

Almost every codebase starts clean. Then the product succeeds, features pile up, and one day a change that should take an afternoon takes two weeks and breaks something unrelated. The ideas that prevent this — SOLID, design patterns, Clean Architecture, Domain-Driven Design, refactoring — are well known, but usually taught in isolation, with toy examples, or as rules to obey.

This book takes a practical path: one realistic system that grew into a tangled monolith, improved step by step. Every principle and pattern appears where it solves a real problem, with before-and-after code, the costs it adds, and when you should not use it at all.

What's inside

• Coupling, cohesion, and the cost of change — the forces behind every design decision
• Clean code habits, the SOLID principles, composition, and dependency injection without dogma
• Refactoring safely with code smells, characterization tests, and small steps
• Creational patterns: Factory Method, Abstract Factory, Builder, Singleton
• Structural patterns: Adapter, Facade, Decorator, Composite, Proxy, Bridge, plus value objects and Null Object
• Behavioral patterns: Strategy, Template Method, Observer and domain events, Command, Chain of Responsibility, State, Specification, Visitor, Iterator
• Clean, Hexagonal, and Onion Architecture compared; the dependency rule; use cases; ports and adapters; enforcing module boundaries
• Domain-Driven Design: ubiquitous language, bounded contexts, aggregates, repositories, Unit of Work, CQRS, and the outbox pattern
• Testing an architecture, including architecture tests that fail the build when layers are violated
• Migrating legacy code with seams and the Strangler Fig; handling errors, validation, logging, and observability cleanly

One running example, start to finish. You untangle Brewline, a fictional coffee roastery's online platform: pricing rules, subscriptions, shipping carriers, a payment provider, and a legacy ERP.

Language-neutral ideas, readable code. Examples are in TypeScript, which reads naturally for Java, C#, Kotlin, Go, and JavaScript developers. Every chapter includes Tips, Notes, and Warnings, and ends with Key Takeaways.

Format: PDF, 493 pages (A4), linked table of contents and bookmarks. English. First Edition, 2026. Published by thenexbooks.

Table of Contents

Part I — Foundations of Maintainable Code

• Ch 1 · Why Maintainability Matters: Cost of Change, Coupling, and Cohesion
• Ch 2 · Clean Code Essentials: Naming, Functions, and Readable Structure
• Ch 3 · SOLID Principles in Practice
• Ch 4 · Composition, Interfaces, and Dependency Injection
• Ch 5 · Refactoring Safely: Code Smells, Tests as a Safety Net, and Small Steps

Part II — Creational and Structural Patterns

• Ch 6 · Thinking in Patterns: When Patterns Help and When They Hurt
• Ch 7 · Creational Patterns: Factory Method, Abstract Factory, Builder, and Singleton
• Ch 8 · Structural Patterns I: Adapter, Facade, and Decorator
• Ch 9 · Structural Patterns II: Composite, Proxy, and Bridge
• Ch 10 · Value Objects, Immutability, and the Null Object Pattern

Part III — Behavioral Patterns

• Ch 11 · Strategy, Template Method, and Policy Objects
• Ch 12 · Observer, Domain Events, and Publish-Subscribe
• Ch 13 · Command, Chain of Responsibility, and Middleware Pipelines
• Ch 14 · The State Pattern and Explicit State Machines
• Ch 15 · Specification, Visitor, and Iterator

Part IV — Clean Architecture

• Ch 16 · Architecture Fundamentals: Layers, Boundaries, and the Dependency Rule
• Ch 17 · Clean, Hexagonal, and Onion Architecture Compared
• Ch 18 · Entities and Use Cases: Modeling the Core of the Application
• Ch 19 · Ports and Adapters: Controllers, Presenters, Gateways, and Repositories
• Ch 20 · Organizing Code: Package by Feature, Modules, and Enforcing Boundaries

Part V — Domain-Driven Design and Data

• Ch 21 · Domain-Driven Design Essentials: Ubiquitous Language and Bounded Contexts
• Ch 22 · Aggregates, Invariants, and Domain Events
• Ch 23 · Persistence Without Pollution: Repositories, Unit of Work, and Mapping
• Ch 24 · CQRS, Event-Driven Integration, and the Outbox Pattern

Part VI — Sustaining Maintainability

• Ch 25 · Testing an Architecture: Unit, Integration, Contract, and Architecture Tests
• Ch 26 · Refactoring Legacy Code Toward Clean Architecture: Seams and the Strangler Fig
• Ch 27 · Cross-Cutting Concerns: Errors, Validation, Logging, and Observability in a Clean Design
• Ch 28 · Case Study: Evolving Brewline from a Big Ball of Mud to a Clean Architecture

Appendices
A · Patterns and Principles Cheat Sheet
B · Templates and Reference
C · Maintainability Review Checklist
D · Glossary

Book details