Architecture Design

Architecture Design Skill

Design Principles

1. Separation of Concerns

Each module has a single, well-defined responsibility.

2. Dependency Inversion

Depend on abstractions, not concretions.

3. Interface Segregation

Many specific interfaces over one general-purpose interface.

4. Open/Closed

Open for extension, closed for modification.

5. Single Source of Truth

Each piece of data has exactly one authoritative source.

Architecture Patterns

Layered Architecture

┌─────────────────────────────┐
│      Presentation Layer     │
├─────────────────────────────┤
│      Application Layer      │
├─────────────────────────────┤
│       Domain Layer          │
├─────────────────────────────┤
│    Infrastructure Layer     │
└─────────────────────────────┘

Clean Architecture

┌─────────────────────────────────────┐
│            Frameworks               │
│  ┌─────────────────────────────┐   │
│  │      Interface Adapters      │   │
│  │  ┌─────────────────────┐    │   │
│  │  │    Use Cases        │    │   │
│  │  │  ┌─────────────┐    │    │   │
│  │  │  │  Entities   │    │    │   │
│  │  │  └─────────────┘    │    │   │
│  │  └─────────────────────┘    │   │
│  └─────────────────────────────┘   │
└─────────────────────────────────────┘

Hexagonal (Ports & Adapters)

         ┌─────────────┐
    ┌────┤   Adapter   ├────┐
    │    └─────────────┘    │
    │                       │
┌───┴───┐   ┌───────┐   ┌───┴───┐
│ Port  │───│ Core  │───│ Port  │
└───┬───┘   └───────┘   └───┬───┘
    │                       │
    │    ┌─────────────┐    │
    └────┤   Adapter   ├────┘
         └─────────────┘

Decision Framework

When Making Architecture Decisions

1. Identify the problem - What specific issue needs solving?
2. List constraints - Budget, time, team skills, scale requirements
3. Consider options - At least 3 viable approaches
4. Evaluate tradeoffs - Pros/cons of each option
5. Document decision - Record in ADL with rationale

Technology Evaluation Framework

Use this when comparing frameworks, platforms, or core stack choices.

Rubric (Score 1-5)

Recommendation Output Pattern

1. Present 2-4 realistic alternatives.
2. Show rubric scores and one-line rationale per criterion.
3. Recommend one option with explicit context-based reasoning.
4. State when that option is not appropriate.
5. Name a fallback option if core assumptions change.

Neutrality Guardrails

• Start from constraints, not preferred tools.
• Avoid single-framework bias; include a credible alternative.
• Make assumptions explicit when context is incomplete.
• Tie final choice to measurable outcomes (delivery speed, reliability, cost, risk).

Decision Template

### AD-XXX: {Decision Title}

**Context:** {What situation prompted this decision?}

**Options Considered:**
1. {Option A} - {Brief description}
2. {Option B} - {Brief description}
3. {Option C} - {Brief description}

**Decision:** {Which option was chosen}

**Rationale:** {Why this option?}

**Consequences:**
- {Positive consequence}
- {Negative consequence/tradeoff}

**Related:** {Links to related decisions}

Common Patterns

Repository Pattern

Abstracts data access behind a collection-like interface.

Service Layer

Coordinates application operations and transactions.

Factory Pattern

Encapsulates object creation logic.

Observer Pattern

Defines one-to-many dependency for state changes.

Strategy Pattern

Defines family of interchangeable algorithms.

Project Structure

Feature-Based (Recommended)

src/
├── features/
│   ├── auth/
│   │   ├── login.ts
│   │   ├── logout.ts
│   │   └── auth.test.ts
│   └── users/
│       ├── create-user.ts
│       └── users.test.ts
├── shared/
│   ├── database/
│   └── utils/
└── index.ts

Layer-Based

src/
├── controllers/
├── services/
├── repositories/
├── models/
└── utils/

Best Practices

1. Document decisions - Every architectural choice in ADL
2. Start simple - Add complexity only when needed
3. Defer decisions - Make reversible choices when uncertain
4. Vertical slices - Complete features over horizontal layers
5. Test boundaries - Focus tests on public interfaces

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