Arc42 Architecture Design Generator

Transform high-level design documentation into comprehensive Arc42 architecture documentation through systematic analysis, validation, and generation.

Overview

Arc42 is a pragmatic, structured template for documenting software architectures with 12 sections covering everything from goals to glossary. This skill guides the process of:

•Analyzing existing repositories and documentation
•Extracting architectural patterns and components
•Validating assumptions through structured user questions
•Generating complete Arc42-compliant documentation

Documentation Structure Best Practices

Progressive Disclosure for AI Agents

Arc42 documentation should be structured for progressive disclosure—enabling AI agents to load only the context they need:

code

docs/architecture/arc42/
├── README.md                    # Index (~200 tokens) - Load first
├── 01-introduction-goals.md     # Section 1 (~500-1000 tokens)
├── 02-constraints.md            # Section 2 (~300-500 tokens)
├── 03-context-scope.md          # Section 3 (~500-800 tokens)
├── 04-solution-strategy.md      # Section 4 (~600-1000 tokens)
├── 05-building-blocks.md        # Section 5 (~800-1500 tokens)
├── 06-runtime-view.md           # Section 6 (~600-1000 tokens)
├── 07-deployment-view.md        # Section 7 (~500-800 tokens)
├── 08-crosscutting-concepts.md  # Section 8 (~600-1000 tokens)
├── 09-architecture-decisions.md # Section 9 (~300 tokens, links to ADRs)
├── 10-quality-requirements.md   # Section 10 (~500-800 tokens)
├── 11-risks-technical-debt.md   # Section 11 (~400-600 tokens)
└── 12-glossary.md               # Section 12 (~300-500 tokens)

Key Principles for AI-Friendly Documentation

•Lightweight Index: README.md provides navigation with 1-2 sentence summaries per section
•Self-Contained Sections: Each document is complete without requiring other sections
•Token Budget: Keep individual documents under 1500 tokens where possible
•Cross-References: Use relative links [Section 5](05-building-blocks.md) for navigation
•Frontmatter Metadata: Include section number, title, and last-updated date
•Diagrams First: Lead with visual representations; details follow

Document Template

Each section document should follow this structure:

markdown

# Section N: Title

> One-sentence summary for index extraction.

**Last Updated**: YYYY-MM-DD
**Related Sections**: [Link1](file.md), [Link2](file.md)

---

[Content organized for quick comprehension]

Arc42 Sections Summary

Section	Purpose	Key Content
1. Introduction & Goals	Business context	Requirements overview, quality goals (top 3-5), stakeholders
2. Constraints	Boundaries	Technical, organizational, and convention constraints
3. Context & Scope	System boundaries	Business context diagram, technical context diagram
4. Solution Strategy	Core decisions	Technology choices, architecture patterns, key approaches
5. Building Block View	Static structure	Hierarchical whitebox/blackbox decomposition (Level 1, 2, 3+)
6. Runtime View	Dynamic behavior	Key scenarios, interaction sequences, workflows
7. Deployment View	Infrastructure	Hardware, environments, deployment mapping
8. Crosscutting Concepts	Shared patterns	Security, logging, error handling, domain models
9. Architecture Decisions	ADRs	Important decisions with rationale (use ADR format)
10. Quality Requirements	Quality tree	Quality scenarios, quality tree structure
11. Risks & Technical Debt	Known issues	Prioritized risks, technical debt items
12. Glossary	Terminology	Domain and technical terms with definitions

Workflow

Execute these phases in order:

Phase 1: Repository Analysis

Analyze the codebase to understand the existing architecture:

code

1. Examine project structure
   - Directory layout and organization patterns
   - Configuration files (package.json, pom.xml, Dockerfile, etc.)
   - Entry points and main modules

2. Identify key documentation
   - README files, ARCHITECTURE.md, design docs
   - API specifications (OpenAPI, GraphQL schemas)
   - Database schemas and migrations

3. Map technical stack
   - Languages, frameworks, libraries
   - External services and integrations
   - Infrastructure as code (Terraform, CloudFormation)

4. Trace component relationships
   - Import/dependency graphs
   - Service communication patterns
   - Data flow paths

Phase 2: Context Validation

CRITICAL: Validate ALL assumptions with user before proceeding.

Use the structured question approach in references/validation-questions.md to gather:

•Business context and goals
•Stakeholder expectations
•Quality priorities
•Constraint clarifications
•Missing architectural context

Ask questions in batches of 3-5 to avoid overwhelming the user. Start with highest-priority gaps.

Phase 3: Architecture Extraction

Extract architectural elements from analyzed sources:

Building Blocks (Section 5)

•Level 1: Top-level system decomposition
•Level 2: Component internals for critical blocks
•Level 3: Only for complex/risky components

Runtime Scenarios (Section 6)

•Primary use cases (happy path)
•Error handling flows
•Integration touchpoints

Deployment (Section 7)

•Production environment topology
•Development/staging differences if significant

Phase 4: Documentation Generation

Generate Arc42 documentation following references/arc42-template.md.

Output Structure (Default - Progressive Disclosure):

code

docs/architecture/arc42/
├── README.md                    # Lightweight index with navigation
├── 01-introduction-goals.md     # Requirements, quality goals, stakeholders
├── 02-constraints.md            # Technical, organizational, conventions
├── 03-context-scope.md          # Business and technical context diagrams
├── 04-solution-strategy.md      # Technology choices, patterns
├── 05-building-blocks.md        # System decomposition (Level 1, 2)
├── 06-runtime-view.md           # Key scenarios and workflows
├── 07-deployment-view.md        # Infrastructure and installation
├── 08-crosscutting-concepts.md  # Security, logging, error handling
├── 09-architecture-decisions.md # Links to ADRs with summaries
├── 10-quality-requirements.md   # Quality tree and scenarios
├── 11-risks-technical-debt.md   # Known risks and debt items
└── 12-glossary.md               # Domain and technical terms

Document Guidelines:

•README.md: <200 tokens, summaries only, navigation links
•Individual sections: 500-1500 tokens each
•Self-contained: Each section readable without others
•Diagrams first: Lead with visuals, follow with details

Quality Checks:

•Verify all 12 sections addressed (sections may be marked "not applicable" with justification)
•Ensure diagrams are described or created
•Cross-reference between sections using relative links
•Validate consistency of terminology with glossary
•Check token budget compliance for each document

Key Principles

Document What Matters

Arc42 is flexible—not every section needs exhaustive content. Focus on:

•Sections most relevant to stakeholders
•Areas with highest risk or complexity
•Information not obvious from code

Quality Goals Drive Architecture

The top 3-5 quality goals (Section 1.2) should influence:

•Solution strategy choices (Section 4)
•Crosscutting concepts (Section 8)
•Architecture decisions (Section 9)
•Quality scenarios (Section 10)

Building Block Hierarchy

code

Level 0: Context (System as blackbox)
    ↓
Level 1: Whitebox of entire system (major components as blackboxes)
    ↓
Level 2: Whitebox of selected Level-1 components
    ↓
Level 3+: Only for complex/risky areas

Blackbox vs Whitebox

•Blackbox: Responsibility + interfaces (what it does)
•Whitebox: Internal structure + contained blackboxes (how it's built)

Diagram Conventions

When describing diagrams, use clear textual notation:

Context Diagram:

code

[External System A] --> [Our System] --> [External System B]
         |                    |
         v                    v
    [Database]           [Message Queue]

Building Block Diagram:

code

+------------------+
|    Our System    |
|  +------------+  |
|  | Component A|--|---> [External API]
|  +------------+  |
|       |          |
|       v          |
|  +------------+  |
|  | Component B|  |
|  +------------+  |
+------------------+

Use Mermaid, PlantUML, or ASCII art based on user preference.

References

•Validation Questions: references/validation-questions.md
•Arc42 Template: references/arc42-template.md
•ADR Template: references/adr-template.md
•Quality Scenarios: references/quality-scenarios.md