C4 Component

C4 Component Level: [Component Name]

Use this skill when

• Working on c4 component level: [component name] tasks or workflows
• Needing guidance, best practices, or checklists for c4 component level: [component name]

Do not use this skill when

• The task is unrelated to c4 component level: [component name]
• You need a different domain or tool outside this scope

Instructions

• Clarify goals, constraints, and required inputs.
• Apply relevant best practices and validate outcomes.
• Provide actionable steps and verification.
• If detailed examples are required, open resources/implementation-playbook.md.

Overview

• Name: [Component name]
• Description: [Short description of component purpose]
• Type: [Component type: Application, Service, Library, etc.]
• Technology: [Primary technologies used]

Purpose

[Detailed description of what this component does and what problems it solves]

Software Features

Code Elements

This component contains the following code-level elements:

• c4-code-file-1.md - [Description]
• c4-code-file-2.md - [Description]

Interfaces

[Interface Name]

• Protocol: [REST/GraphQL/gRPC/Events/etc.]
• Description: [What this interface provides]
• Operations:
  • operationName(params): ReturnType - [Description]

Dependencies

Components Used

• [Component Name]: [How it's used]

External Systems

• [External System]: [How it's used]

Component Diagram

Use proper Mermaid C4Component syntax. Component diagrams show components within a single container:

C4Component
    title Component Diagram for [Container Name]

    Container_Boundary(container, "Container Name") {
        Component(component1, "Component 1", "Type", "Description")
        Component(component2, "Component 2", "Type", "Description")
        ComponentDb(component3, "Component 3", "Database", "Description")
    }
    Container_Ext(externalContainer, "External Container", "Description")
    System_Ext(externalSystem, "External System", "Description")

    Rel(component1, component2, "Uses")
    Rel(component2, component3, "Reads from and writes to")
    Rel(component1, externalContainer, "Uses", "API")
    Rel(component2, externalSystem, "Uses", "API")

**Key Principles** (from [c4model.com](https://c4model.com/diagrams/component)):

- Show components **within a single container** (zoom into one container)
- Focus on **logical components** and their responsibilities
- Show **component interfaces** (what they expose)
- Show how components **interact** with each other
- Include **external dependencies** (other containers, external systems)

Master Component Index Template

# C4 Component Level: System Overview

## System Components

### [Component 1]
- **Name**: [Component name]
- **Description**: [Short description]
- **Documentation**: c4-component-name-1.md

### [Component 2]
- **Name**: [Component name]
- **Description**: [Short description]
- **Documentation**: c4-component-name-2.md

## Component Relationships
[Mermaid diagram showing all components and their relationships]

Example Interactions

• "Synthesize all c4-code-*.md files into logical components"
• "Define component boundaries for the authentication and authorization code"
• "Create component-level documentation for the API layer"
• "Identify component interfaces and create component diagrams"
• "Group database access code into components and document their relationships"

Key Distinctions

• vs C4-Code agent: Synthesizes multiple code files into components; Code agent documents individual code elements
• vs C4-Container agent: Focuses on logical grouping; Container agent maps components to deployment units
• vs C4-Context agent: Provides component-level detail; Context agent creates high-level system diagrams

Output Examples

When synthesizing components, provide:

• Clear component boundaries with rationale
• Descriptive component names and purposes
• Comprehensive feature lists for each component
• Complete interface documentation with protocols and operations
• Links to all contained c4-code-*.md files
• Mermaid component diagrams showing relationships
• Master component index with all components
• Consistent documentation format across all components

🏰 Rei Skills — Curated by Rootcastle Engineering & Innovation | Batuhan Ayrıbaş
Engineering Beyond Boundaries | admin@rootcastle.com