arc42 Documentation Skill

When to Use This Skill

Use this skill when:

• Arc42 Documentation tasks - Working on arc42 architecture documentation template and guidance
• Planning or design - Need guidance on Arc42 Documentation approaches
• Best practices - Want to follow established patterns and standards

Overview

Create comprehensive architecture documentation using the arc42 template.

MANDATORY: Documentation-First Approach

Before creating arc42 documentation:

1. Invoke docs-management skill for architecture documentation patterns
2. Verify arc42 current version via MCP servers (perplexity)
3. Base guidance on official arc42 template

arc42 Template Structure

arc42 Template (12 Sections):

┌─────────────────────────────────────────────────────────────────────────────┐
│  1. Introduction and Goals                                                   │
│     Requirements overview, quality goals, stakeholders                       │
├─────────────────────────────────────────────────────────────────────────────┤
│  2. Architecture Constraints                                                 │
│     Technical, organizational, and convention constraints                    │
├─────────────────────────────────────────────────────────────────────────────┤
│  3. System Scope and Context                                                 │
│     Business context, technical context                                      │
├─────────────────────────────────────────────────────────────────────────────┤
│  4. Solution Strategy                                                        │
│     Technology decisions, top-level decomposition, quality approaches        │
├─────────────────────────────────────────────────────────────────────────────┤
│  5. Building Block View                                                      │
│     Static decomposition: whitebox/blackbox at multiple levels               │
├─────────────────────────────────────────────────────────────────────────────┤
│  6. Runtime View                                                             │
│     Important scenarios, interactions, behaviors                             │
├─────────────────────────────────────────────────────────────────────────────┤
│  7. Deployment View                                                          │
│     Technical infrastructure, mapping of building blocks                     │
├─────────────────────────────────────────────────────────────────────────────┤
│  8. Cross-cutting Concepts                                                   │
│     Recurring patterns, approaches, principles                               │
├─────────────────────────────────────────────────────────────────────────────┤
│  9. Architecture Decisions                                                   │
│     Important decisions with rationale (may link to ADRs)                    │
├─────────────────────────────────────────────────────────────────────────────┤
│ 10. Quality Requirements                                                     │
│     Quality tree, quality scenarios                                          │
├─────────────────────────────────────────────────────────────────────────────┤
│ 11. Risks and Technical Debt                                                 │
│     Known risks, technical debt items                                        │
├─────────────────────────────────────────────────────────────────────────────┤
│ 12. Glossary                                                                 │
│     Important domain and technical terms                                     │
└─────────────────────────────────────────────────────────────────────────────┘

Complete arc42 Template

# Architecture Documentation: [System Name]

**Version:** 1.0
**Date:** [Date]
**Status:** Draft | Review | Final

---

## 1. Introduction and Goals

### 1.1 Requirements Overview

[Brief description of the system and its purpose. What business problem
does it solve? Who are the main users?]

**Key Features:**
- [Feature 1]
- [Feature 2]
- [Feature 3]

### 1.2 Quality Goals

| Priority | Quality Goal | Description |
|----------|--------------|-------------|
| 1 | [Goal] | [Description] |
| 2 | [Goal] | [Description] |
| 3 | [Goal] | [Description] |

### 1.3 Stakeholders

| Role | Name/Team | Expectations |
|------|-----------|--------------|
| Product Owner | [Name] | [Expectations] |
| Development Team | [Team] | [Expectations] |
| Operations | [Team] | [Expectations] |
| Security | [Team] | [Expectations] |

---

## 2. Architecture Constraints

### 2.1 Technical Constraints

| Constraint | Description | Background |
|------------|-------------|------------|
| [TC-1] | [Description] | [Why this constraint exists] |
| [TC-2] | [Description] | [Why this constraint exists] |

### 2.2 Organizational Constraints

| Constraint | Description | Background |
|------------|-------------|------------|
| [OC-1] | [Description] | [Why this constraint exists] |
| [OC-2] | [Description] | [Why this constraint exists] |

### 2.3 Conventions

| Convention | Description |
|------------|-------------|
| [CON-1] | [Description] |
| [CON-2] | [Description] |

---

## 3. System Scope and Context

### 3.1 Business Context

[Diagram showing the system in its business environment, with actors
and external systems it interacts with.]

```mermaid
C4Context
    title System Context Diagram

    Person(user, "User", "End user of the system")
    System(system, "System Name", "System description")
    System_Ext(ext1, "External System 1", "Description")
    System_Ext(ext2, "External System 2", "Description")

    Rel(user, system, "Uses")
    Rel(system, ext1, "Calls API")
    Rel(system, ext2, "Sends events")

Actor/System	Description	Communication
[Actor 1]	[Description]	[Protocol/Format]
[External System 1]	[Description]	[Protocol/Format]

3.2 Technical Context

[Technical details of integration: protocols, data formats, interfaces.]

Interface	Technology	Description
[API 1]	REST/JSON	[Description]
[Queue 1]	Kafka	[Description]
[File 1]	SFTP/CSV	[Description]

---

4. Solution Strategy

4.1 Technology Decisions

Decision	Technology	Rationale
Programming Language	C# (.NET 10)	[Rationale]
Database	PostgreSQL	[Rationale]
Message Broker	Kafka	[Rationale]
Cloud Platform	Azure	[Rationale]

4.2 Top-Level Decomposition

[High-level description of how the system is structured.]

Approach: [Microservices / Modular Monolith / etc.]

Key Modules:

4.3 Approaches to Achieve Quality Goals

Quality Goal	Approach
Performance	Caching, async processing, optimized queries
Reliability	Redundancy, circuit breakers, retry patterns
Security	Defense in depth, encryption, audit logging

---

5. Building Block View

5.1 Level 1: Whitebox Overall System

C4Container
    title Container Diagram

    Container(api, "API Gateway", "Kong", "Routes and secures API calls")
    Container(web, "Web Application", "Blazor", "User interface")
    Container(svc1, "Service 1", ".NET", "Business logic")
    Container(svc2, "Service 2", ".NET", "Business logic")
    ContainerDb(db, "Database", "PostgreSQL", "Stores data")
    ContainerQueue(queue, "Message Queue", "Kafka", "Async messaging")

    Rel(web, api, "Calls", "HTTPS")
    Rel(api, svc1, "Forwards", "gRPC")
    Rel(api, svc2, "Forwards", "gRPC")
    Rel(svc1, db, "Reads/Writes")
    Rel(svc1, queue, "Publishes")
    Rel(svc2, queue, "Subscribes")

Contained Building Blocks:

Building Block	Purpose
API Gateway	Request routing, authentication, rate limiting
Web Application	User interface and presentation
Service 1	[Core business function]
Service 2	[Core business function]

5.2 Level 2: [Building Block Name]

[Whitebox description of a specific building block, showing its internal structure.]

Responsibility: [What this block does]

Interfaces:

Internal Structure:

Component	Responsibility
[Component 1]	[Description]
[Component 2]	[Description]

---

6. Runtime View

6.1 Scenario: [User Creates Order]

sequenceDiagram
    participant U as User
    participant W as Web App
    participant A as API Gateway
    participant O as Order Service
    participant I as Inventory Service
    participant D as Database
    participant Q as Message Queue

    U->>W: Submit order
    W->>A: POST /orders
    A->>O: CreateOrder
    O->>I: CheckInventory
    I->>D: Query stock
    D-->>I: Stock levels
    I-->>O: Available
    O->>D: Save order
    O->>Q: Publish OrderCreated
    O-->>A: Order confirmed
    A-->>W: 201 Created
    W-->>U: Order confirmation

Description: [Explanation of the scenario and any important details]

6.2 Scenario: [Another Important Scenario]

[Similar structure...]

---

7. Deployment View

7.1 Infrastructure Level 1

C4Deployment
    title Deployment Diagram

    Deployment_Node(cloud, "Azure", "Cloud Platform") {
        Deployment_Node(aks, "AKS Cluster", "Kubernetes") {
            Container(api, "API Pods", "3 replicas")
            Container(svc, "Service Pods", "3 replicas")
        }
        Deployment_Node(data, "Data Tier") {
            ContainerDb(db, "Azure PostgreSQL", "Managed database")
            ContainerDb(redis, "Azure Redis", "Caching")
        }
    }

7.2 Infrastructure Elements

Element	Technology	Description
Kubernetes Cluster	AKS	Container orchestration
Load Balancer	Azure LB	Traffic distribution
Database	Azure PostgreSQL	Persistent storage
Cache	Azure Redis	In-memory caching

---

8. Cross-cutting Concepts

8.1 Domain Model

[Core domain entities and their relationships]

8.2 Security

Authentication: OAuth 2.0 / OpenID Connect Authorization: Role-based access control (RBAC) Data Protection: Encryption at rest (AES-256), in transit (TLS 1.3)

8.3 Error Handling

Strategy: Structured error responses with correlation IDs Logging: Structured logging with severity levels Monitoring: Distributed tracing with OpenTelemetry

8.4 Testability

Unit Testing: xUnit, Moq, FluentAssertions Integration Testing: TestContainers E2E Testing: Playwright

---

9. Architecture Decisions

ID	Decision	Status	Date
ADR-001	[Decision title]	Accepted	[Date]
ADR-002	[Decision title]	Accepted	[Date]

[Link to detailed ADR documents]

---

10. Quality Requirements

10.1 Quality Tree

mindmap
    root((Quality))
        Performance
            Response Time
            Throughput
        Reliability
            Availability
            Recoverability
        Security
            Confidentiality
            Integrity
        Maintainability
            Modifiability
            Testability

10.2 Quality Scenarios

ID	Scenario	Attribute	Target
QS-1	API response under load	Performance	P95 < 200ms
QS-2	System availability	Reliability	99.9%
QS-3	Add new payment provider	Modifiability	< 5 days

---

11. Risks and Technical Debt

11.1 Risks

ID	Risk	Impact	Probability	Mitigation
R-1	[Risk description]	High	Medium	[Mitigation]
R-2	[Risk description]	Medium	Low	[Mitigation]

11.2 Technical Debt

ID	Debt Item	Impact	Priority
TD-1	[Debt description]	[Impact]	High
TD-2	[Debt description]	[Impact]	Medium

---

12. Glossary

Term	Definition
[Term 1]	[Definition]
[Term 2]	[Definition]
[Term 3]	[Definition]

Section Guidelines

When to Include Each Section

Section	Always Include	Include If...
1. Introduction	✓	-
2. Constraints	✓	-
3. Context	✓	-
4. Solution Strategy	✓	-
5. Building Blocks	✓	-
6. Runtime		Important scenarios exist
7. Deployment	✓	-
8. Cross-cutting	✓	-
9. Decisions	✓	-
10. Quality		Quality requirements defined
11. Risks		Known risks exist
12. Glossary		Domain-specific terms

Workflow

When creating arc42 documentation:

1. Start with context: Sections 1-3 establish the "what" and "why"
2. Document decisions: Section 4 captures strategic choices
3. Detail structure: Section 5 shows how it's built
4. Show behavior: Section 6 demonstrates how it works
5. Map to infrastructure: Section 7 shows where it runs
6. Capture patterns: Section 8 documents recurring solutions
7. Record decisions: Section 9 links to ADRs
8. Define quality: Section 10 sets expectations
9. Acknowledge risks: Section 11 shows awareness
10. Define terms: Section 12 ensures shared understanding

References

For detailed guidance:

---

Last Updated: 2025-12-26