Top 11 Open‑Source Diagram‑as‑Code Tools Every Software Architect Should Know

A few months ago I decided to step into the field of software architecture, aiming to elevate my development skills to a new level. I realized that understanding how to build and design scalable, maintainable software and write clean code is essential.

I began exploring basic best practices, principles, and methods in software architecture, consulting experienced architects about the tools they use daily. Choosing the right tools is crucial for any architect, from system component diagramming to infrastructure modeling.

For software architects, diagrams are vital for understanding system components, data flows, and infrastructure.

I categorized architecture tools into logical groups: modeling and diagramming, design and analysis, cloud architecture design, collaboration and documentation, code analysis and visualization, and testing. Below is a curated list of the most useful and easy‑to‑use tools.

1. Graphviz

Graphviz is a code‑based graph generation tool that uses the DOT language. It offers a command‑line interface, multiple layout engines, customizable positioning, and supports output formats such as PNG, SVG, PDF, and JSON.

Uses simple yet powerful DOT syntax.

CLI utility converts DOT scripts to diagrams.

Multiple layout engines (hierarchical, radial, force‑directed).

Customizable layouts for node positioning and hierarchy.

Exports to PNG, SVG, PDF, JSON.

Website: https://gitlab.com/graphviz/graphviz

2. PlantUML

PlantUML is a browser‑based diagram tool that lets architects create diagrams using a simple text‑based syntax.

Key features

Generates UML diagrams (sequence, use case, class, deployment, etc.) and non‑UML diagrams (ER, Gantt, EBNF trees).

Supports hyperlinks and tooltips for interactive diagrams.

Allows Unicode, icons, and rich text for enhanced readability.

Uses various rendering engines, including Graphviz, Smetana, and Puma.

Exports to PNG, SVG, LaTeX, and ASCII art.

3. D2

D2 is a code‑based declarative diagram tool created by Terrastruct. It simplifies creation and maintenance of diagrams through modularization, containers, glob‑based changes, tooltips, hyperlinks, animation, an online playground, natural‑language input, and easy CLI export to SVG, PNG, and PDF.

4. Gaphor

Gaphor is a UML 2 modeling application supporting UML, SysML, RAAML, and the C4 model, making it suitable for various architecture needs.

Supports multiple modeling standards.

Visualizes each model element directly as a diagram.

Extensible via plugins for code generation and documentation.

Tree‑view navigation for large projects.

5. Mermaid

Mermaid is a browser‑based tool that converts text definitions into dynamic visual diagrams using a Markdown‑like syntax.

Easy‑to‑use syntax similar to Markdown.

Live editing with immediate rendering.

Supports flowcharts, sequence diagrams, state diagrams, etc.

Integrates with GitHub, GitLab, Notion, Obsidian, and more.

6. ArchiMate (Archi)

Archi is an open‑source ArchiMate modeling tool designed for enterprise architects of any size, offering features like Magic Connector, viewpoints, visual feedback, relationship visualizer, and sketch/canvas modeling.

7. Modelio

Modelio is an enterprise‑architecture modeling tool supporting UML, BPMN, ArchiMate, and TOGAF, with XMI import/export, HTML publishing, extensibility, and Jython scripting for automation.

8. Diagrams.net (formerly Draw.io)

Diagrams.net is a simple yet powerful diagram tool that integrates with cloud platforms, version control, and offers extensive templates, auto‑layout, collaboration, and support for PlantUML and Mermaid.

9. Excalidraw

Excalidraw is a lightweight browser‑based whiteboard tool that feels like hand‑drawn sketches, supporting offline work, end‑to‑end encryption, multi‑user editing, image uploads, and a library of shapes.

10. tldraw

tldraw offers similar functionality to Excalidraw, with an additional SDK for embedding the whiteboard into applications.

11. Cerbos PDP

Cerbos Policy Decision Point provides an open‑source authorization solution, allowing centralized policy management, fine‑grained access control, real‑time evaluation, and integration with any identity provider.

Choosing the right tool depends on the architecture type, your role, and organization size; lightweight code‑first tools suit small SaaS projects, while enterprise architects may need formal modeling frameworks.

In conclusion, these open‑source tools offer a range of capabilities for software architects. Consider factors such as open‑source license, maintenance, supported standards, learning curve, and collaboration features when selecting the best fit for your use case.