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:

• Invoke docs-management skill for architecture documentation patterns

• Verify arc42 current version via MCP servers (perplexity)

• 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

1.3 Stakeholders

2. Architecture Constraints

2.1 Technical Constraints

2.2 Organizational Constraints

2.3 Conventions

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.]

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:

- Start with context: Sections 1-3 establish the "what" and "why"

- Document decisions: Section 4 captures strategic choices

- Detail structure: Section 5 shows how it's built

- Show behavior: Section 6 demonstrates how it works

- Map to infrastructure: Section 7 shows where it runs

- Capture patterns: Section 8 documents recurring solutions

- Record decisions: Section 9 links to ADRs

- Define quality: Section 10 sets expectations

- Acknowledge risks: Section 11 shows awareness

- Define terms: Section 12 ensures shared understanding

References

For detailed guidance:

Last Updated: 2025-12-26