Technical Diagrams

Mermaid is the standard for all technical diagrams in this project. It renders natively in GitHub, GitLab, MkDocs (with Material theme), and most modern documentation platforms.

This skill provides:

• Critical styling rules to ensure readability (especially color contrast)
• Quick reference examples for common diagram types
• Reference files for advanced syntax when building complex diagrams

Always wrap Mermaid code in fenced code blocks with the mermaid language identifier.

Why Mermaid

Native rendering -- GitHub, GitLab, Notion, MkDocs, and Docusaurus render Mermaid blocks without plugins or build steps. No external image generation tools needed.

Text-based and diffable -- Diagrams live alongside code in version control. Changes appear in pull request diffs, making reviews straightforward and history trackable.

No external tools -- No Lucidchart exports, no draw.io XML files, no PNG screenshots that go stale. The diagram source is the single source of truth.

Maintainable -- Updating a diagram means editing text, not wrestling with a GUI. Refactoring a component name? Find-and-replace works on diagrams too.

Consistent -- A shared syntax produces visually consistent diagrams across all documentation, regardless of who authored them.

Critical Styling Rules

This is the most important section. Light text on light backgrounds is the most common Mermaid readability issue. Follow these rules strictly.

Rule 1: Always use dark text on nodes

Every node must have color:#000 (or another dark color like #1a1a1a, #333). Never use white, light gray, or any light-colored text.

Rule 2: Use classDef for consistent styling

Define reusable styles at the bottom of the diagram and apply them with ::: syntax:

flowchart LR
    A[Input]:::primary --> B[Process]:::secondary --> C[Output]:::success

    classDef primary fill:#dbeafe,stroke:#2563eb,color:#000
    classDef secondary fill:#f3e8ff,stroke:#7c3aed,color:#000
    classDef success fill:#dcfce7,stroke:#16a34a,color:#000

Rule 3: Safe color palettes

Use these pre-tested combinations that guarantee readability:

Bad vs Good

Bad -- light text is invisible on light background:

classDef bad fill:#dbeafe,stroke:#2563eb,color:#93c5fd

Good -- dark text is always readable:

classDef good fill:#dbeafe,stroke:#2563eb,color:#000

Supported Diagram Types

Quick Reference

Minimal copy-paste examples for simple diagrams. For complex use cases, see the corresponding reference section or file.

Flowchart

flowchart TD
    A[Start]:::primary --> B{Decision}:::neutral
    B -->|Yes| C[Action A]:::success
    B -->|No| D[Action B]:::warning
    C --> E[End]:::primary
    D --> E

    classDef primary fill:#dbeafe,stroke:#2563eb,color:#000
    classDef success fill:#dcfce7,stroke:#16a34a,color:#000
    classDef warning fill:#fef3c7,stroke:#d97706,color:#000
    classDef neutral fill:#f3f4f6,stroke:#6b7280,color:#000

Sequence Diagram

sequenceDiagram
    participant C as Client
    participant S as Server
    participant D as Database

    C->>S: POST /api/resource
    activate S
    S->>D: INSERT INTO resources
    D-->>S: OK
    S-->>C: 201 Created
    deactivate S

Class Diagram

classDiagram
    class Service {
        -repository: Repository
        +create(data: CreateDTO): Entity
        +findById(id: string): Entity
    }
    class Repository {
        <<interface>>
        +save(entity: Entity): void
        +findById(id: string): Entity
    }
    Service --> Repository : uses

State Diagram

stateDiagram-v2
    [*] --> Draft
    Draft --> Review : submit
    Review --> Approved : approve
    Review --> Draft : reject
    Approved --> Published : publish
    Published --> [*]

ER Diagram

erDiagram
    USER ||--o{ ORDER : places
    ORDER ||--|{ LINE_ITEM : contains
    PRODUCT ||--o{ LINE_ITEM : "appears in"

    USER {
        int id PK
        string email UK
        string name
    }
    ORDER {
        int id PK
        int user_id FK
        date created_at
    }

C4 Context Diagram

C4Context
    title System Context Diagram

    Person(user, "User", "End user of the system")
    System(system, "Application", "Main system under design")
    System_Ext(ext, "External API", "Third-party service")

    Rel(user, system, "Uses", "HTTPS")
    Rel(system, ext, "Calls", "REST API")

Styling and Theming

classDef -- Reusable Style Classes

Define once, apply to many nodes:

flowchart LR
    A[Node A]:::primary --> B[Node B]:::secondary

    classDef primary fill:#dbeafe,stroke:#2563eb,color:#000
    classDef secondary fill:#f3e8ff,stroke:#7c3aed,color:#000

::: Shorthand -- Apply Class Inline

A[Label]:::className

style -- One-Off Inline Styling

For single-node overrides (prefer classDef for consistency):

style nodeId fill:#dbeafe,stroke:#2563eb,color:#000

Standard Style Classes

Define these at the bottom of any diagram that uses multiple styles:

classDef primary fill:#dbeafe,stroke:#2563eb,color:#000
classDef secondary fill:#f3e8ff,stroke:#7c3aed,color:#000
classDef success fill:#dcfce7,stroke:#16a34a,color:#000
classDef warning fill:#fef3c7,stroke:#d97706,color:#000
classDef danger fill:#fee2e2,stroke:#dc2626,color:#000
classDef neutral fill:#f3f4f6,stroke:#6b7280,color:#000

Subgraph Styling

Subgraphs can be styled via style directives:

flowchart LR
    subgraph backend["Backend Services"]
        A[API]:::primary --> B[Worker]:::secondary
    end
    style backend fill:#f8fafc,stroke:#94a3b8,color:#000

Edge Styling with linkStyle

Style specific edges by their index (0-based, in order of definition):

linkStyle 0 stroke:#2563eb,stroke-width:2px
linkStyle 1 stroke:#dc2626,stroke-width:2px,stroke-dasharray:5

Best Practices

Keep diagrams focused

Limit to 15-20 nodes maximum. If a diagram grows beyond that, split it into multiple diagrams or use subgraphs to manage complexity.

Choose direction deliberately

• TD (top-down) -- Hierarchies, data flow, process steps
• LR (left-right) -- Timelines, pipelines, request flows
• BT (bottom-up) -- Dependency trees (leaves at top)
• RL (right-left) -- Rarely used, avoid unless it matches a specific mental model

Use meaningful labels

A[User Service] --> B[Auth Service]    %% Good: descriptive
A --> B                                 %% Bad: meaningless

Label edges

A -->|validates| B    %% Good: explains the relationship
A --> B               %% Acceptable only if the relationship is obvious

Group with subgraphs

Use subgraphs to visually separate layers, domains, or subsystems:

flowchart TD
    subgraph frontend["Frontend"]
        A[React App]:::primary
    end
    subgraph backend["Backend"]
        B[API Server]:::secondary --> C[Database]:::neutral
    end
    A --> B

    classDef primary fill:#dbeafe,stroke:#2563eb,color:#000
    classDef secondary fill:#f3e8ff,stroke:#7c3aed,color:#000
    classDef neutral fill:#f3f4f6,stroke:#6b7280,color:#000

Use consistent arrow types

Within a single diagram, stick to one arrow style unless you need to distinguish different relationship types:

• --> solid arrow (primary flow)
• -.-> dotted arrow (optional or async)
• ==> thick arrow (critical path)

Prefer flowchart over graph

flowchart is the modern syntax with more features (subgraph styling, ::: shorthand, more shapes). graph is legacy -- use flowchart for all new diagrams.

Platform compatibility

• GitHub/GitLab: Full support for flowcharts, sequence, class, state, ER, Gantt, pie
• C4 diagrams: Require Mermaid 10.6+ -- verify platform support before using
• MkDocs: Requires pymdownx.superfences with custom Mermaid fence config

Flowcharts Reference

Flowcharts visualize process flows, decision trees, pipelines, and system architectures. They are the most versatile Mermaid diagram type.

Keyword: flowchart (prefer over legacy graph)

Direction Keywords

Node Shapes

Edge Types

#### Solid Edges

#### Dotted Edges

#### Thick Edges

#### Multi-Directional

#### Edge Length

Add extra dashes/dots/equals to make edges longer:

• A ---> B (longer than A --> B)
• A -----> B (even longer)

Subgraphs

Group related nodes into labeled regions:

flowchart TD
    subgraph frontend["Frontend Layer"]
        A[React App] --> B[State Manager]
    end
    subgraph backend["Backend Layer"]
        C[API Server] --> D[Database]
    end
    B --> C

#### Nested Subgraphs

flowchart TD
    subgraph cloud["Cloud Infrastructure"]
        subgraph compute["Compute"]
            A[App Server]
        end
        subgraph storage["Storage"]
            B[Object Store]
        end
    end
    A --> B

#### Subgraph Direction

Override direction inside a subgraph:

subgraph section["Section"]
    direction LR
    A --> B --> C
end

Flowchart Styling

#### classDef and :::

flowchart LR
    A[Input]:::primary --> B[Process]:::secondary --> C[Output]:::success

    classDef primary fill:#dbeafe,stroke:#2563eb,color:#000
    classDef secondary fill:#f3e8ff,stroke:#7c3aed,color:#000
    classDef success fill:#dcfce7,stroke:#16a34a,color:#000

#### Inline style

style A fill:#dbeafe,stroke:#2563eb,color:#000

#### Subgraph styling

style subgraphId fill:#f8fafc,stroke:#94a3b8,color:#000

#### linkStyle

Style edges by their 0-based index:

linkStyle 0 stroke:#2563eb,stroke-width:2px
linkStyle 1 stroke:#dc2626,stroke-dasharray:5

#### Apply class to multiple nodes

class A,B,C primary

Flowchart Examples

#### CI/CD Pipeline

flowchart LR
    A([Push]):::neutral --> B[Lint]:::primary
    B --> C[Test]:::primary
    C --> D{Pass?}:::neutral
    D -->|Yes| E[Build]:::primary
    D -->|No| F[Notify]:::danger
    E --> G[Deploy Staging]:::warning
    G --> H{Approve?}:::neutral
    H -->|Yes| I[Deploy Prod]:::success
    H -->|No| J[Rollback]:::danger

    classDef primary fill:#dbeafe,stroke:#2563eb,color:#000
    classDef success fill:#dcfce7,stroke:#16a34a,color:#000
    classDef warning fill:#fef3c7,stroke:#d97706,color:#000
    classDef danger fill:#fee2e2,stroke:#dc2626,color:#000
    classDef neutral fill:#f3f4f6,stroke:#6b7280,color:#000

#### Layered Architecture

flowchart TD
    subgraph presentation["Presentation Layer"]
        direction LR
        A[Web UI]:::primary
        B[Mobile App]:::primary
        C[CLI]:::primary
    end

    subgraph application["Application Layer"]
        direction LR
        D[Auth Service]:::secondary
        E[User Service]:::secondary
        F[Order Service]:::secondary
    end

    subgraph data["Data Layer"]
        direction LR
        G[(PostgreSQL)]:::neutral
        H[(Redis Cache)]:::neutral
        I[(S3 Storage)]:::neutral
    end

    A & B & C --> D & E & F
    D --> G
    E --> G & H
    F --> G & I

    classDef primary fill:#dbeafe,stroke:#2563eb,color:#000
    classDef secondary fill:#f3e8ff,stroke:#7c3aed,color:#000
    classDef neutral fill:#f3f4f6,stroke:#6b7280,color:#000

    style presentation fill:#eff6ff,stroke:#93c5fd,color:#000
    style application fill:#f5f3ff,stroke:#c4b5fd,color:#000
    style data fill:#f9fafb,stroke:#d1d5db,color:#000

C4 Diagrams Reference

C4 diagrams model software architecture at multiple zoom levels -- from system context down to individual components. C4 uses a unique function-call syntax that differs from other Mermaid diagram types.

Keywords: C4Context, C4Container, C4Component, C4Dynamic, C4Deployment

Compatibility note: C4 requires Mermaid 10.6+. Verify your rendering platform supports it.

Diagram Levels

Element Functions

#### People

Person(alias, "Label", "Description")
Person_Ext(alias, "Label", "Description")

• Person -- Internal user
• Person_Ext -- External user

#### Systems

System(alias, "Label", "Description")
System_Ext(alias, "Label", "Description")
System_Boundary(alias, "Label") { ... }

• System -- System under design
• System_Ext -- External system
• System_Boundary -- Groups elements belonging to one system

#### Containers

Container(alias, "Label", "Technology", "Description")
ContainerDb(alias, "Label", "Technology", "Description")
ContainerQueue(alias, "Label", "Technology", "Description")
Container_Ext(alias, "Label", "Technology", "Description")
Container_Boundary(alias, "Label") { ... }

• Container -- Application/service
• ContainerDb -- Database
• ContainerQueue -- Message queue
• Container_Ext -- External container
• Container_Boundary -- Groups containers

#### Components

Component(alias, "Label", "Technology", "Description")
ComponentDb(alias, "Label", "Technology", "Description")
ComponentQueue(alias, "Label", "Technology", "Description")
Component_Ext(alias, "Label", "Technology", "Description")

#### Deployment Nodes

Deployment_Node(alias, "Label", "Technology") { ... }
Node(alias, "Label", "Technology") { ... }

C4 Relationships

Rel(from, to, "Label")
Rel(from, to, "Label", "Technology/Protocol")

Rel_D(from, to, "Label")    %% Downward
Rel_U(from, to, "Label")    %% Upward
Rel_L(from, to, "Label")    %% Left
Rel_R(from, to, "Label")    %% Right

Rel_Back(from, to, "Label")          %% Reverse direction
BiRel(from, to, "Label")             %% Bidirectional

C4 Styling

#### UpdateElementStyle

UpdateElementStyle(alias, $bgColor="color", $fontColor="color", $borderColor="color")

#### UpdateRelStyle

UpdateRelStyle(from, to, $textColor="color", $lineColor="color", $offsetX="n", $offsetY="n")

#### Layout

UpdateLayoutConfig($c4ShapeInRow="3", $c4BoundaryInRow="1")

Title and Description

Always include a title for context:

title System Context Diagram for My Application

C4 Examples

#### System Context Diagram

C4Context
    title System Context -- SaaS Platform

    Person(customer, "Customer", "Uses the web application to manage their account and resources")
    Person(admin, "Admin", "Manages platform settings, users, and billing")
    Person_Ext(developer, "Developer", "Integrates via API")

    System(platform, "SaaS Platform", "Core platform providing resource management and analytics")

    System_Ext(email, "Email Service", "SendGrid -- transactional and marketing emails")
    System_Ext(payment, "Payment Gateway", "Stripe -- subscription billing and payments")
    System_Ext(monitoring, "Monitoring", "Datadog -- metrics, logs, and alerting")
    System_Ext(cdn, "CDN", "CloudFront -- static asset delivery")

    Rel(customer, platform, "Uses", "HTTPS")
    Rel(admin, platform, "Manages", "HTTPS")
    Rel(developer, platform, "Integrates", "REST API")
    Rel(platform, email, "Sends emails", "SMTP/API")
    Rel(platform, payment, "Processes payments", "API")
    Rel(platform, monitoring, "Sends telemetry", "Agent")
    Rel(platform, cdn, "Serves assets", "HTTPS")

    UpdateLayoutConfig($c4ShapeInRow="3", $c4BoundaryInRow="1")

#### Container Diagram

C4Container
    title Container Diagram -- SaaS Platform

    Person(customer, "Customer", "End user")
    Person(developer, "Developer", "API consumer")

    System_Boundary(platform, "SaaS Platform") {
        Container(spa, "Web App", "React, TypeScript", "Single-page application for customers")
        Container(api, "API Server", "Node.js, Express", "REST API handling all business logic")
        Container(worker, "Background Worker", "Node.js, Bull", "Async job processing")
        ContainerDb(db, "Database", "PostgreSQL", "Stores users, resources, billing")
        ContainerDb(cache, "Cache", "Redis", "Session store, rate limiting, job queue")
        ContainerQueue(queue, "Message Queue", "Redis/Bull", "Job queue for async processing")
        Container(scheduler, "Task Scheduler", "Node.js, cron", "Periodic maintenance tasks")
    }

    System_Ext(email, "Email Service", "SendGrid")
    System_Ext(payment, "Payment Gateway", "Stripe")
    System_Ext(storage, "Object Storage", "S3")

    Rel(customer, spa, "Uses", "HTTPS")
    Rel(developer, api, "Calls", "REST/HTTPS")
    Rel(spa, api, "Calls", "HTTPS/JSON")
    Rel(api, db, "Reads/writes", "SQL")
    Rel(api, cache, "Reads/writes", "Redis protocol")
    Rel(api, queue, "Enqueues jobs", "Redis protocol")
    Rel(worker, queue, "Processes jobs", "Redis protocol")
    Rel(worker, db, "Reads/writes", "SQL")
    Rel(worker, email, "Sends", "API")
    Rel(worker, storage, "Uploads", "S3 API")
    Rel(api, payment, "Charges", "API")
    Rel(scheduler, db, "Queries", "SQL")
    Rel(scheduler, queue, "Enqueues", "Redis protocol")

    UpdateLayoutConfig($c4ShapeInRow="4", $c4BoundaryInRow="1")

#### Component Diagram

C4Component
    title Component Diagram -- API Server

    Container_Boundary(api, "API Server") {
        Component(router, "Router", "Express Router", "Routes HTTP requests to controllers")
        Component(auth, "Auth Middleware", "Passport.js", "JWT validation and session management")
        Component(controllers, "Controllers", "Express", "Request handling and response formatting")
        Component(services, "Service Layer", "TypeScript", "Business logic and orchestration")
        Component(repos, "Repositories", "TypeORM", "Data access and query building")
        Component(validators, "Validators", "Zod", "Request payload validation")
        Component(events, "Event Emitter", "EventEmitter2", "Domain event publishing")
    }

    ContainerDb(db, "Database", "PostgreSQL")
    ContainerDb(cache, "Cache", "Redis")
    ContainerQueue(queue, "Job Queue", "Bull/Redis")

    Rel(router, auth, "Authenticates via")
    Rel(router, controllers, "Delegates to")
    Rel(controllers, validators, "Validates with")
    Rel(controllers, services, "Calls")
    Rel(services, repos, "Queries via")
    Rel(services, events, "Publishes to")
    Rel(repos, db, "Reads/writes")
    Rel(services, cache, "Caches in")
    Rel(events, queue, "Enqueues to")

#### Dynamic Diagram

C4Dynamic
    title User Login Flow

    ContainerDb(db, "Database", "PostgreSQL")
    ContainerDb(cache, "Cache", "Redis")
    Container(api, "API Server", "Node.js")
    Container(spa, "Web App", "React")

    Rel(spa, api, "1. POST /auth/login", "HTTPS")
    Rel(api, db, "2. Validate credentials", "SQL")
    Rel(api, api, "3. Generate JWT")
    Rel(api, cache, "4. Store session", "Redis")
    Rel(api, spa, "5. Return token", "HTTPS")

When to Load Reference Files

Simple diagrams -- The quick reference above is sufficient. Use it for:

• Basic flowcharts with fewer than 10 nodes
• Simple sequence diagrams with 2-3 participants
• Standard ER diagrams with straightforward relationships

Complex or unfamiliar diagrams -- Load the reference file when:

• Using advanced features (composite states, parallel blocks, fork/join)
• Building class diagrams with generics, namespaces, or cardinality
• Needing the full set of node shapes, arrow types, or relationship notations
• Working with a diagram type for the first time

C4 diagrams -- The C4 reference above covers the full syntax. Review it before creating C4 diagrams, as C4 uses a unique function-call syntax.

For complex sequence, class, state, or ER diagrams, see the corresponding reference file in references/.

Integration Notes

What this component does: Provides comprehensive Mermaid diagram syntax, styling rules, and examples for creating readable, consistent technical visualizations across all documentation. Capabilities needed: None (passive reference material -- no tool access required). When reference files are needed for complex diagrams, file reading capability is required to load them from references/. Adaptation guidance: This is a static knowledge resource. The styling rules and diagram syntax are platform-agnostic Mermaid. Reference files in references/ contain advanced syntax for sequence, class, state, and ER diagrams.