PlantUML and Kroki

Main considerations for diagrams

Diagrams as text

When diagram sources are plain text, it becomes a lot easier to integrate them in a development workflow, using the same tools and processes as for textual content and source code.

Semantic diagrams

Diagrams in documentation usually specify a structure or behaviour. The source of diagrams therefore describes concepts and relations, rather than forms and shapes. Diagrams as text stimulates using Diagram standards.

Diagrams and source code

Diagrams as text can be included as documentation in source code, or be generated from sources. In stricter situations, source code could also be generated from specifications in diagrams, but this is not considered important in this stack.

Layout control trade-off

Most diagram rendering engines offer very limited control over layout. Styling can also be difficult, and keeping the style consistent with the rest of the documentation requires additional work.

Stack overview

There are two popular approaches to diagrams as text rendering engines.

  1. Generate images in a compile stage when producing a static website, and include (links to) these images in the pages.

    This approach often uses GraphViz as part of the stack, to do layout. There are many diagram syntaxes, for various domains. A popular tool is PlantUML.

  2. Include a Javascript library to render images in the browser after a page is loaded.

    This approach usually replaces the diagram text and embeds SVG in a page. The SVG image can use stylesheet information. A popular tool is MermaidJS (which also can compile static images).

The first approach is older, and there seem to be a wider choice of tools to write diagram text.

Kroki is an image processing tool and service that makes many different rendering engines available via a uniform API. Kroki supports both PlantUML and MermaidJS.

Kroki is available as an online service. It can also be run as local stand-alone service, although that takes some effort to set up.

Antora can use the asciidoctor-kroki module to convert diagram text in a page into an image. This keeps the choice of diagram tool flexible.

Diagram standards

UML

UML is actually a modeling language. In its full form, it lets you write a model for a system, and produce different graphical views of this model.

UML provides a rich set of diagrams for both structural and behavioural documentation of a software system. These diagrams provide a highly standardised visual notation for the concepts in the model.

PlantUML supports drawing visual UML diagrams from textual descriptions, using its own syntax. It does not offer any support for conceptual modeling itself. This is sufficient for the documentation I do.

It is easy to write up a sufficiently detailed diagram to explain an approach. It is also possible to include PlantUML documentation in a source file, and produce a diagram from that. This makes keeping sources and documentation synchronised easier.

The specific notation of UML makes it harder to use in more general overviews and high-level descriptions.

C4

The C4 Model was developed to bridge the gap between the detailed UML standard, and informal diagrams without any standardised semantics. It offers four levels of abstraction (context, container, component, code, hence C4). although it recommendeds only using the first three levels [1].

In its full form, using a tool like Structurizr, it can also capture a model, and generate different diagrams as views of that model.

C4 limits itself to structural views, and actively promotes the use of additional diagram and model techniques to provide more context or detail.

PlantUML also supports drawing C4 diagrams, using its own syntax.

Structurizr has an open source commandline tool that can generate PlantUML descriptions from C4 models.

Rendering Structurizr diagrams directly can only be done via the online service.

Closing remarks

Both UML and C4 provide description languages as well as diagram notations. PlantUML can draw diagrams that follow the diagram notation standards, but uses its own description languages for the specification. This is fine for the documentation I do, but does lock those diagrams to PlantUML specifically.

There are additional diagram types that may be included as "standards" in the future.

  • Flowcharts

  • Gantt charts

  • Work breakdown structures

Background references

1.
Simon Brown. Visualising software architecture with the C4 model. 16-Aug-2019. Agile on the Beach 2019. https://www.youtube.com/watch?v=x2-rSnhpw0g
2.
Simon Brown. The C4 model for visualising software architecture. https://c4model.com/
3.
Kymberly Fergusson. UML diagrams - which diagram to use and why. 26-Oct-2018. draw.io. https://drawio-app.com/uml-diagrams/
4.
The Unified Modeling Language. https://www.uml-diagrams.org/

1. The code level best used only if there are a lot of classes or interfaces to document, and it preferably can be generated from source code
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