arc42-documentation

# 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

```text
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

```markdown
# 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

```mermaid
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]

```mermaid
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

```mermaid
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

```mermaid
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] |

```text

```

## 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