Architecture & Design Principles Skill

Transform codebases into maintainable, testable, and scalable architectures using Clean Architecture principles, SOLID design patterns, and industry best practices from the Essential Developer methodology.

Overview

This skill guides you through a structured 5-step process to:

1. Analyze Requirements → Identify components, responsibilities, and boundaries
2. Apply SOLID Principles → Design with SRP, OCP, LSP, ISP, DIP
3. Define Clean Architecture → Establish layers, boundaries, and dependency rules
4. Design Testing Strategy → Plan unit tests, integration tests, and test boundaries
5. Document Decisions → Create Architecture Decision Records (ADRs)

Core Philosophy

"Good architecture is a byproduct of good team processes and communication"

This skill follows these principles:

• Framework Independence - Business logic doesn't depend on frameworks
• Testability - Architecture enables easy testing
• UI Independence - UI can change without affecting business rules
• Database Independence - Business rules don't know about the database
• External Agency Independence - Business rules don't depend on external services

The 5-Step Process

Step 1: Analyze Requirements

Objective: Identify components, responsibilities, and architectural boundaries

Actions:

1. Break down feature into distinct responsibilities
2. Identify core business logic vs infrastructure concerns
3. Recognize cross-cutting concerns (logging, analytics, caching)
4. Map data flow through the system
5. Identify potential architectural boundaries

Key Questions to Ask:

• What is the core business logic?
• What are the external dependencies (network, database, UI)?
• What needs to be testable in isolation?
• What components might change independently?
• What are the inputs and outputs of each component?

Output: Component diagram with clear responsibilities

Reference: See references/modular_design.md for component identification patterns

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Step 2: Apply SOLID Principles

Objective: Design components following SOLID principles

S - Single Responsibility Principle (SRP)

Each class/module has one reason to change. Separate business logic from infrastructure.

• ❌ A class that loads data AND presents it
• ✅ Separate UseCase for business logic; separate Presenter for view logic

O - Open/Closed Principle (OCP)

Open for extension via protocols/interfaces, closed for modification:

// Open for extension via protocol (Swift example — same idea in any language)
protocol ItemLoader {
    func load() async throws -> [Item]
}

// Closed for modification — implementations extend behavior
final class RemoteItemLoader: ItemLoader { ... }
final class CachedItemLoader: ItemLoader { ... }
final class FallbackItemLoader: ItemLoader { ... }

L - Liskov Substitution Principle (LSP)

Subtypes must be substitutable for base types. Contracts must be honored by all implementations.

I - Interface Segregation Principle (ISP)

Clients shouldn't depend on interfaces they don't use. Create focused, specific protocols.

• ❌ protocol DataStore { func save(_:); func load(); func delete(); func migrate(); func backup() }
• ✅ protocol ItemReader { func retrieve() throws -> [Item]? } + separate ItemCache

D - Dependency Inversion Principle (DIP)

High-level modules depend on abstractions, not concretions.

class ItemListViewController {
    private let loader: ItemLoader  // depends on abstraction, not concrete type
    init(loader: ItemLoader) { self.loader = loader }
}

Output: SOLID-compliant component design

Reference: See references/solid_principles.md for detailed patterns and anti-patterns

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Step 3: Define Clean Architecture

Objective: Establish clear architectural layers with proper dependency flow

The Clean Architecture Layers:

┌─────────────────────────────────────────┐
│          Presentation Layer             │
│    (UI, ViewModels, Presenters)        │
└─────────────────┬───────────────────────┘
                  │ depends on
┌─────────────────▼───────────────────────┐
│         Domain/Business Layer           │
│      (Use Cases, Entities, Rules)      │
└─────────────────┬───────────────────────┘
                  │ depends on
┌─────────────────▼───────────────────────┐
│         Infrastructure Layer            │
│   (Network, Database, Framework Code)  │
└─────────────────────────────────────────┘

Dependency Rule: Source code dependencies point inward only

Key Patterns:

1. Use Cases — contain business rules, orchestrate data flow, independent of UI and frameworks
2. Boundaries (Protocols) — define contracts between layers, enable testability
3. Adapters — convert data between layers, implement boundary protocols
4. Composition Root — wire dependencies together, configure the object graph

Output: Layered architecture with clear boundaries

Reference: See references/clean_architecture.md for detailed layer definitions; examples/generic/layered_architecture.md for a language-agnostic example; examples/swift/ for Swift/iOS real implementations

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Step 4: Design Testing Strategy

Objective: Plan comprehensive testing at all architectural layers

Testing Pyramid:

        ┌──────────┐
        │    UI    │ Few - End to End
        ├──────────┤
        │Integration│ Some - Integration
        ├──────────┤
        │   Unit   │ Many - Fast & Isolated
        └──────────┘

Testing Boundaries:

1. Domain Layer (Unit Tests) — test use cases in isolation, mock all dependencies
2. Infrastructure Layer (Integration Tests) — test adapters with real dependencies
3. Presentation Layer (Unit Tests) — test presenters/view models, mock use cases

Test Double Vocabulary:

• Stubs: Provide canned answers
• Spies: Record calls for verification
• Mocks: Verify behavior expectations
• Fakes: Working implementations for testing

Testing Strategies:

• Test behavior, not implementation details
• Use native async test primitives; avoid manual callback synchronization when the language supports await
• Factory helper pattern: create the system under test (SUT) and its dependencies together in a makeSUT() helper to keep individual test methods clean
• Arrange, Act, Assert structure; extract helpers for clarity

Output: Comprehensive testing strategy document

Reference: See references/testing_strategies.md for patterns and best practices

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Step 5: Document Decisions

Objective: Create clear architectural documentation and decision records

ADR Format:

# ADR-NNN: <Decision Title>
## Status: Accepted | Superseded | Deprecated
## Context: Why this decision was needed
## Decision: What was decided
## Consequences: Positive and negative outcomes
## Alternatives Considered: What was rejected and why

See examples/generic/ for a complete ADR example.

Documentation Requirements:

1. Architecture Overview — system context diagram, component diagram, layer relationships
2. Component Documentation — purpose, dependencies, usage examples
3. Design Patterns Used — which patterns and why, trade-offs
4. Testing Strategy — what gets tested and how

Output: Complete architectural documentation

Reference: See references/ for detailed documentation patterns

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Best Practices

DO ✅

• Separate business logic from infrastructure
• Depend on abstractions, not concretions
• Make dependencies explicit through initializer injection
• Write tests for all business logic
• Use async IO protocols at network/network boundaries; synchronous protocols for in-process storage
• Isolate presentation types to the main/UI thread using the language's concurrency primitives
• Design for testability from the start

DON'T ❌

• Let business logic depend on frameworks
• Use singletons for dependency management
• Mix presentation and business logic
• Create god classes with multiple responsibilities
• Use manual callback/completion-handler patterns where the language's native async/await is available
• Dispatch to the UI thread manually — use structured concurrency instead

Swift/iOS: Use async throws at network boundaries, sync throws at cache boundaries, @MainActor for presentation types, and Sendable for value types crossing actor boundaries. See the Swift Concurrency section below.

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Common Architectural Patterns

1. Clean Architecture (Recommended) — clear separation of concerns, dependency rule: inward only
2. Hexagonal Architecture (Ports & Adapters) — business logic at the center, ports define boundaries
3. MVVM — separation of UI and logic, testable view models
4. MVC — traditional separation, controller as composition root

Reference: See examples/generic/layered_architecture.md for a language-agnostic layered example; examples/swift/ for MVVM+Coordinator, Composition Root, and Two-Layer View in Swift/iOS

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Language-Specific Guidance

Swift/iOS

• Use async throws protocols for network boundaries
• Use sync throws for cache/store boundaries; bridge to async via Scheduler protocol
• Apply @MainActor to presentation types (ResourceView, LoadResourcePresenter)
• Mark domain value types Sendable (e.g. FeedImage: Hashable, Sendable)
• Use LoadResourcePresenter<Resource, View> generic presenter — one type for all features
• Use WeakRefVirtualProxy<T> to break retain cycles without per-type boilerplate
• Apply Composition Root pattern in SceneDelegate; extract infrastructure into @MainActor service objects (see examples/swift/composition_root.md)

Reference: See examples/swift/ for Swift-specific patterns

Generic/Agnostic

• Apply SOLID principles universally
• Use interfaces/traits/protocols depending on language
• Adapt patterns to language features
• Maintain Clean Architecture layers

Reference: See examples/generic/ for language-agnostic examples

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Swift Concurrency (Swift/iOS only)

The concepts in this section — async IO boundaries, main-thread isolation, value-type thread safety, generic presenter, memory-safe proxies — apply universally. The syntax and APIs below are Swift-specific. For the detailed implementations, see references/concurrency_patterns.md.

Async Protocol Boundaries

Network protocols use async throws; store protocols use sync throws:

// Network boundary — async because IO is inherently async
public protocol HTTPClient {
    func get(from url: URL) async throws -> (Data, HTTPURLResponse)
}

// Cache/store boundary — sync because the store runs on its own queue (bridged via Scheduler)
// Apply this pattern to any persistent store: CoreData, SQLite, in-memory, Realm, etc.
public protocol ItemStore {
    func delete() throws
    func insert(_ items: [LocalItem], timestamp: Date) throws
    func retrieve() throws -> CachedItems?
}
// Essential Feed uses FeedStore with identical structure for FeedImage caching

@MainActor Isolation

Presentation layer types are @MainActor — no manual thread dispatching:

@MainActor public protocol ResourceView {
    associatedtype ResourceViewModel
    func display(_ viewModel: ResourceViewModel)
}

@MainActor public final class LoadResourcePresenter<Resource, View: ResourceView> {
    public typealias Mapper = (Resource) throws -> View.ResourceViewModel
    public init(resourceView: View, loadingView: ResourceLoadingView,
                errorView: ResourceErrorView, mapper: @escaping Mapper)
    public init(resourceView: View, loadingView: ResourceLoadingView,
                errorView: ResourceErrorView) where Resource == View.ResourceViewModel
    public func didStartLoading()
    public func didFinishLoading(with resource: Resource)
    public func didFinishLoading(with error: Error)
}

Sendable Value Types

Domain entities crossing actor boundaries must be Sendable. All stored properties must themselves be Sendable; the compiler verifies this:

// Generic pattern — apply to any domain entity
public struct Item: Hashable, Sendable {
    public let id: UUID
    public let title: String
    // String, UUID, URL are all Sendable — compiler is satisfied
}

// Concrete example from Essential Feed:
// public struct FeedImage: Hashable, Sendable { ... }

WeakRefVirtualProxy

Prevents retain cycles via a single generic type with conditional conformances:

final class WeakRefVirtualProxy<T: AnyObject> {
    private weak var object: T?
    init(_ object: T) { self.object = object }
}

extension WeakRefVirtualProxy: ResourceErrorView where T: ResourceErrorView {
    func display(_ viewModel: ResourceErrorViewModel) { object?.display(viewModel) }
}

extension WeakRefVirtualProxy: ResourceLoadingView where T: ResourceLoadingView {
    func display(_ viewModel: ResourceLoadingViewModel) { object?.display(viewModel) }
}

Scheduler Protocol

Bridges sync FeedStore into async contexts without making the store async:

protocol Scheduler {
    @MainActor
    func schedule<T>(_ action: @escaping @Sendable () throws -> T) async rethrows -> T
}

Reference: See references/concurrency_patterns.md for full implementations and migration guide

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Integration with Requirements Engineering

1. Start with requirements → Use Cases → BDD scenarios
2. Apply this skill → Architecture → Component design
3. Implement → Following architectural patterns
4. Test → Using defined testing strategy

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References

Inside references/:

• clean_architecture.md - Clean Architecture layers and rules
• solid_principles.md - Detailed SOLID explanations with examples
• design_patterns.md - Common patterns (Adapter, Decorator, Composite, Null Object, etc.)
• null_object_pattern.md - Null Object Pattern with testing examples
• command_query_separation.md - CQS principle for cache design
• dependency_management.md - DI patterns and strategies
• testing_strategies.md - Testing patterns and best practices
• modular_design.md - Module organization and boundaries
• concurrency_patterns.md - Swift Concurrency: async/await, @MainActor, Sendable, Scheduler

Inside examples/:

• swift/ - Real implementations from Essential Feed (Swift 6)
• generic/ - Language-agnostic examples

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Output Format

When applying this skill, provide:

1. Component Analysis - Identified components and responsibilities
2. SOLID Review - Applied principles with rationale
3. Architecture Diagram - Layers and dependencies (Mermaid)
4. Testing Strategy - Test structure and coverage plan
5. ADRs - Key decisions documented
6. Implementation Guide - Step-by-step refactoring or implementation plan

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Credits

Based on the Essential Developer's proven architecture methodology:

• Essential Feed Case Study
• Essential Developer Resources

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Version History

• 2.1.0 - Generalized for multi-project use: domain-neutral SOLID examples, separated Swift-specific guidance, added scope labels, created generic ADR template and updated generic architecture example
• 2.0.0 - Swift 6 migration: async/await, @MainActor, Sendable, LoadResourcePresenter, Scheduler, WeakRefVirtualProxy
• 1.0.0 - Initial release with 5-step process