Design Patterns & Architecture
Design patterns are the vocabulary of experienced software engineers. When a senior engineer says 'use the observer pattern here' or 'this is a classic factory method problem,' they're communicating a solution in one phrase that would otherwise take five minutes to explain. Vibe coders who've never studied patterns produce code that accidentally implements patterns badly, or solves pattern-shaped problems with ad-hoc solutions that are harder to understand and maintain. This module doesn't treat design patterns as a list of templates to memorize — it treats them as a vocabulary that emerges from recurring problems. You'll encounter each pattern in the context of the problem it solves, implement it from scratch, and — critically — study the pattern trap: the anti-pattern of applying patterns where they add complexity without value. Premature abstraction is the most common misapplication of design pattern knowledge, and recognizing it is as important as knowing the patterns themselves. The module culminates in architecture patterns — layered, hexagonal, clean, and event-driven — and domain-driven design essentials. These are the patterns that operate at the system level, shaping how an entire codebase is organized and how changes propagate through it. By the end, you'll have the vocabulary to participate in architecture discussions, read codebases organized around these patterns, and make informed decisions about how to structure new systems.
What You'll Learn
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1
Creational Patterns — Factory, Abstract Factory, Builder, Singleton, Prototype
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2
Structural Patterns — Adapter, Decorator, Proxy, Facade, Composite
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3
Behavioral Patterns — Observer, Strategy, Command, State machine, Iterator
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4
The Pattern Trap — When NOT to use patterns, premature abstraction
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5
SOLID Principles Deep Dive — Not rules to memorize but consequences of good design
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6
Dependency Injection — Constructor vs property injection, DI containers, manual wiring
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7
Architecture Patterns Compared — Layered, hexagonal, clean, event-driven
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8
Domain-Driven Design Essentials — Bounded contexts, ubiquitous language, aggregates
Capstone Project: Refactor a Pattern-Free Codebase — Identify, Implement, and Document
Take a working but poorly structured codebase with no intentional architecture and refactor it by identifying which design problems each pattern solves, implementing the appropriate patterns incrementally with tests verifying no regressions, and documenting each refactoring with the problem it solved and the tradeoffs involved. The final codebase must have clear separation of concerns, be testable without live dependencies, and use a consistent architectural style throughout.
Why This Matters for Your Career
Codebases without intentional architecture accrete complexity over time. What starts as a reasonable flat structure becomes an unmaintainable tangle when every component knows about every other component and changes ripple unpredictably. Architectural patterns — especially hexagonal architecture's dependency inversion principle — are the engineering discipline that prevents this accrual from being inevitable. SOLID principles deserve their reputation as the most practically useful object-oriented design guidance ever written. The single responsibility principle prevents the 'god class' that accumulates responsibilities until it's impossible to understand. The open-closed principle enables extension without modification. The dependency inversion principle makes components testable without their real dependencies. Engineers who apply these principles consistently produce codebases that stay maintainable as they grow. Domain-driven design — bounded contexts, ubiquitous language, aggregates — is increasingly the standard approach for modeling complex business domains. It's the conceptual foundation of many microservice decompositions and the vocabulary that bridges the gap between business requirements and technical implementation. Engineers who understand DDD can participate in domain modeling conversations and produce implementations that genuinely reflect the business domain.