Model

Overview

Model provides a way to describe software architecture models using diagrams as code. This approach provides many benefit over the use of graphical tools, in particular:

• Built-in history via source control versioning: Each change to the software architecture model results in a commit making it natural to implement Architectural Decision Records.
• Simple and consistent reuse of shared architecture components: The code of shared architecture components can be imported by other software systems. Apart from the obvious time saving a major advantage is ensuring that all references are automatically kept up-to-date as the shared architecture component evolves. The shared architecture component code can be versioned using traditional source code versioning techniques as well.
• Consistent notation and visual styles: The same mechanism used to import shared architecture diagram can also be used to import shared style definitions.
• Ability to control drift between reality and diagrams: Static code analysis allows writing tools that compare the software models with actual code to detect discrepencies (this repo does not provide such a tool at this time).

The Model DSL is implemented in Go and follows the C4 Model to describe the software architecture. Using Go to implement the DSL makes it possible to leverage Go packages to share and version models. It also allows for extending or customizing the DSL by writing simple Go functions.

The C4 (Context, Containers, Components and Code) model provides a clean and simple set of constructs (software elements, deployment nodes and views) that can be learned in minutes. The C4 model is very flexible and only focuses on a few key concepts making it possible to express many different styles of architectures while still adding value.

Example

Here is a complete design describing a simple architecture model composed of a user and a software system:

package design

import . "goa.design/model/dsl"

var _ = Design("Getting Started", "This is a model of my software system.", func() {
    var System = SoftwareSystem("Software System", "My software system.", func() {
        Tag("system")
    })

    Person("User", "A user of my software system.", func() {
        Uses(System, "Uses")
        Tag("person")
    })

    Views(func() {
        SystemContextView(System, "SystemContext", "An example of a System Context diagram.", func() {
            AddAll()
            AutoLayout(RankLeftRight)
        })
        Styles(func() {
            ElementStyle("system", func() {
                Background("#1168bd")
                Color("#ffffff")
            })
            ElementStyle("person", func() {
                Shape(ShapePerson)
                Background("#08427b")
                Color("#ffffff")
            })
        })
    })
})

This code creates a model containing two elements, a relationship, a single view and some styling information. Running the mdl tool (see installation instructions below) renders the following diagram:

Additional examples can be found in the examples directory.

Installation

Model requires Go version 1.23 or greater. Assuming a working installation of Go, the mdl and stz tools can be installed using:

go install goa.design/model/cmd/mdl
go install goa.design/model/cmd/stz

Usage

Model includes two command line tools supporting two different workflows:

• The mdl tool serves a graphical editor that makes it possible to position the elements and relationships in each view defined in the design. The graphical editor saves the rendered views as SVG files in a directory specified when running the tool. mdl can also generate a JSON representation of the model.

• The stz tool uploads the software architecture described in the DSL to the Structurizr service. This service renders the model and includes a visual editor to rearrange the results (the tool takes care of keeping any change made graphically on the next upload).

Diagram source code: model.go

Model also provides a Goa plugin so that the design of APIs and microservices written in Goa can be augmented with a description of the corresponding software architecture.

Using mdl

The mdl serve command starts a local HTTP server that serves an interactive graphical editor with advanced layout and editing capabilities. The command takes the Go import path to the package containing the DSL as argument. The path to the directory used to save the SVG files can be provided via the -dir flag, by default the editor creates a gen folder under the current path. For example:

mdl serve goa.design/model/examples/basic/model -dir gen
Watching: /home/raphael/go/src/goa.design/model/examples/basic/model
mdl v1.16.10, editor started. Open http://localhost:8080 in your browser.

Modifying and saving the DSL while the editor is running causes it to automatically update and reflect the latest changes making it convenient to work on the model while editing the view layouts.

Interactive Editor Features

The graphical editor provides a rich set of interactive features:

• Intuitive Navigation: Pan by dragging, zoom with scroll wheel, or use keyboard shortcuts
• Smart Selection: Click to select, Shift+click for multi-selection, Shift+drag for box selection
• Automatic Layout: Multiple algorithms (Layered, Force, Tree, Radial, etc.) powered by ELK.js
• Precise Alignment: Align and distribute selected elements with toolbar buttons or keyboard shortcuts
• Flexible Connections: Multiple routing styles (orthogonal, straight, curved)
• Full Undo/Redo: Complete history tracking for all changes with Ctrl+Z/Ctrl+Y (Cmd+Z/Cmd+Y on Mac)
• Cross-Platform Keyboard Shortcuts: Full keyboard support with platform-specific shortcuts (Cmd on Mac, Ctrl on PC)
• Dynamic Page Titles: Browser tab title updates to show the current view name being edited
• Toolbar Integration: Modern toolbar with tooltips showing platform-appropriate keyboard shortcuts
• Zoom Controls: Fit-to-view, zoom to 100%, and incremental zoom with proper coordinate handling
• Grid System: Toggle-able grid overlay with snap-to-grid functionality for precise element positioning

The mdl gen command generates the JSON representation of a design, it accepts the Go import path to the package containing the design DSL as argument. For example the following command generates the JSON for the basic example

mdl gen goa.design/model/examples/basic/model -out design.json

The generated file design.json contains a JSON representation of the Design struct.

Installing the diagram-editing skill

mdl includes a Cursor Agent Skill that teaches coding agents how to edit, render, and review Model diagrams while preserving C4 ownership and boundary semantics. Run this command from the root of a repository that contains Model DSL:

mdl skill install

The command detects supported coding-agent executables and existing project configuration directories, then installs the skill for each detected agent:

• Cursor and Codex: .agents/skills/editing-model-diagrams/SKILL.md
• Claude Code: .claude/skills/editing-model-diagrams/SKILL.md

Cursor and Codex both discover the standard .agents/skills location, so one copy serves both without duplicate skill entries. If neither supported location can be detected, mdl uses .agents/skills as the portable fallback. An unchanged skill installed by an earlier release under .cursor/skills is removed after the shared copy is written. Re-running the command is safe when installed copies are unchanged. If any current or legacy copy contains local changes, mdl preserves every location without writing and reports the conflict; use -force only to replace those local changes explicitly.

Using stz

Alternatively, the stz tool generates a file containing a JSON representation of a Structurize service workspace that corresponds to the design described in the DSL. The tool can then upload the workspace to the Structurizr service. The tool can also retrieve the JSON representation of a workspace from the service.

This example uploads a workspace corresponding to the DSL defined in the Go package goa.design/model/examples/basic:

stz gen goa.design/model/examples/basic/model
stz put workspace.json -id ID -key KEY -secret SECRET

In this example ID is the Structurizr service workspace ID, KEY the Structurizr service API key and SECRET the corresponding secret.

The example below retrieves the JSON representation of a workspace from Structurizr:

stz get -id ID -key KEY -secret SECRET -out workspace.json

Using the Goa Plugin

This package can also be used as a Goa plugin by including the DSL package in the Goa design:

package design

import . "goa.design/goa/v3/dsl"
import "goa.design/model/dsl"

// ... DSL describing API, services and architecture model

Running goa gen creates both a design.json and a workspace.json file in the gen folder. The workspace.json file follows the structurizr JSON schema and can be uploaded to the Structurizr service for example using the stz tool included in this repo.

Using Model as a library

The mdl package RunDSL method runs the DSL and produces data structures that contain all the information needed to render the views it defines.

The stz package RunDSL method runs the DSL and produces a data structure that can be serialized into JSON and uploaded to the Structurizr service.

The stz package also contains a client library for the Structurizr service APIs.

Here is a complete example that uploads the design described in a DSL to the Structurizr service:

package main

import (
    "fmt"
    "os"

    . "goa.design/model/dsl"
    "goa.design/model/stz"
)

// DSL that describes software architecture model.
var _ = Design("Getting Started", "This is a model of my software system.", func() {
    var System = SoftwareSystem("Software System", "My software system.", func() {
        Tag("system")
    })

    Person("User", "A user of my software system.", func() {
        Uses(System, "Uses")
        Tag("person")
    })

    Views(func() {
        SystemContextView(System, "SystemContext", "An example of a System Context diagram.", func() {
            AddAll()
            AutoLayout(RankLeftRight)
        })
        Styles(func() {
            ElementStyle("system", func() {
                Background("#1168bd")
                Color("#ffffff")
            })
            ElementStyle("person", func() {
                Shape(ShapePerson)
                Background("#08427b")
                Color("#ffffff")
            })
        })
    })
})

// Executes the DSL and uploads the corresponding workspace to Structurizr.
func main() {
    // Run the model DSL
    w, err := stz.RunDSL()
    if err != nil {
        fmt.Fprintf(os.Stderr, "invalid design: %s", err.Error())
        os.Exit(1)
    }

    // Upload the design to the Structurizr service.
    // The API key and secret must be set in the STRUCTURIZR_KEY and
    // STRUCTURIZR_SECRET environment variables respectively. The
    // workspace ID must be set in STRUCTURIZR_WORKSPACE_ID.
    var (
        key    = os.Getenv("STRUCTURIZR_KEY")
        secret = os.Getenv("STRUCTURIZR_SECRET")
        wid    = os.Getenv("STRUCTURIZR_WORKSPACE_ID")
    )
    if key == "" || secret == "" || wid == "" {
        fmt.Fprintln(os.Stderr, "missing STRUCTURIZR_KEY, STRUCTURIZR_SECRET or STRUCTURIZR_WORKSPACE_ID environment variable.")
        os.Exit(1)
    }
    c := stz.NewClient(key, secret)
    if err := c.Put(wid, w); err != nil {
        fmt.Fprintf(os.Stderr, "failed to store workspace: %s\n", err.Error())
        os.Exit(1)
    }
}

DSL Syntax

Rules

The following rules apply to all elements and views declared in a model:

• Software and people names must be unique.
• Container names must be unique within the context of a software system.
• Component names must be unique within the context of a container.
• Deployment node names must be unique with their parent context.
• Infrastructure node names must be unique with their parent context.
• All relationships from a given source element to a given destination element must have a unique description.
• View keys must be unique.

Note that uniqueness of names is enforced by combining the evaluated definitions for a given element. For example if a model contained:

var Person1 = Person("User", "A user", func() {
    Uses("System 1", "Uses")
})
var Person2 = Person("User", "The same user again", func() {
    Uses("System 2", "Uses")
})

Then the final model would only define a single person named "User" with the description "The same user again" and both relationships. This makes it possible to import shared models and "edit" existing elements, for example to add new relationships.

References to Elements

Some DSL functions such as Uses, Delivers, InteractsWith, Add and Link accept references to elements as argument. The references can be done either through a variable (which holds the element being referred to) or by the path of the element. The path of an element is constructured by appending the parent element names separated by slashes (/) and the name of the element. For example the path of the component 'C' in the container 'CO' and software system 'S' is 'S/CO/C'. The path can be relative when the reference is made within a scoped function, for example when adding an element to a view that is scoped to a parent element.

Resources

The DSL package documentation lists all the DSL keywords and their usage.

The file DSL.md illustrates the complete syntax in one design.

Graphical Editor

The graphical editor is started using the mdl serve command. This command accepts the import path to the Go package containing the model DSL. The editor makes it possible to position the elements and their relationships in each view.

The editor makes it possible to select a view described in the DSL and to position its elements and relationships graphically. Multiple elements can be selected by either dragging over the selection or by using SHIFT + CLICK. Selecting multiple elements automatically selects the relationships between these elements as well.

New vertices can be created on relationship lines using ALT + CLICK (Option + Click on Mac), existing vertices can be deleted with BACKSPACE or DELETE. See the table below for a complete list of editor shortcuts.

Saving

The Save View button causes the editor to create a SVG rendering of the currently edited view in the target directory (specified via the -dir flag). The SVG encodes the positions of all the elements and relationships as well so that restarting the editor using the same target directory automatically restores the element positions.

Diagram source code: model.go

Keyboard Shortcuts

The editor supports a number of keyboard shortcuts listed below. On Mac, use CMD instead of CTRL for all shortcuts:

TBD

While functional, the editor isn't considered feature complete yet. Here is the list of features that will be added in the future:

• Grid system with snap-to-grid functionality
• Better SVG clipping on save
• Autolayout on start if no pre-existing layout information
• Keyboard shortcuts for vertical and horizontal align
• Autosave toggle
• Distribute horizontally and vertically
• Toolbar icons with keyboard shortcut tooltips
• Cross-platform keyboard shortcuts (Mac/PC support)
• Dynamic page titles showing current view

Examples

Refer to the examples directory for working examples.