jekyll-kroki

A Jekyll plugin to convert diagram descriptions into images using Kroki.

Add the jekyll-kroki Gem to the :jekyll_plugins group of your site's Gemfile:

group :jekyll_plugins do
  gem "jekyll-kroki"
end

Kroki supports over 25 popular diagram scripting languages, including Blockdiag, D2, GraphViz, Mermaid, and PlantUML. The examples page and complete list of supported diagram languages provide a taste of what's possible.

In Markdown, simply write your diagram descriptions inside a fenced code block with the language specified:

```plantuml
participant Jekyll
participant Kroki #MediumSpringGreen

Jekyll -> Kroki: Encoded diagram description
Kroki --> Jekyll: Rendered diagram in SVG format
```

When Jekyll builds your site, the jekyll-kroki plugin encodes the diagram descriptions, renders them as SVG images using the Kroki server, then embeds them in the generated HTML:

Since the images are rendered and embedded at build-time, the Jekyll site remains completely static and doesn't depend on access to the Kroki server later on. This also eliminates all of the client-side processing that is typically used to render diagrams into images.

Instead of using Liquid tags, jekyll-kroki leverages the same Markdown fenced code block syntax used by both GitLab and GitHub to display diagrams. Besides being more consistent, this means that diagram descriptions in Markdown files can be displayed consistently as images across the GitLab/GitHub UI and on GitLab/GitHub Pages sites generated using Jekyll. GitLab currently supports Mermaid and PlantUML, while GitHub only supports Mermaid.

Self-managed GitLab instances can additionally enable the Kroki integration, which adds support for all the same diagram scripting languages used by jekyll-kroki. Furthermore, by pointing both GitLab and jekyll-kroki to the same Kroki instance, you can guarantee that diagrams are generated using identical versions of the diagram libraries.

The server-side nature of Kroki means that you don't have to deal with installing or updating any diagram library dependencies on your machine. Jekyll sites that are generated in CI/CD pipelines can bypass these steps and will thus build faster.

Kroki is available either as a free service or self-hosted using Docker. Organisations that frequently build large Jekyll sites with many diagrams or want maximum control over their data can choose to operate their own Kroki instance to ensure consistency and use compute resources efficiently. For individuals, you can also opt to run Kroki locally.

You can specify the following parameters in the Jekyll _config.yml file:

For example:

kroki:
  url: "https://my-kroki.server"
  http_retries: 3
  http_timeout: 15
  max_concurrent_docs: 8

Embedding diagrams as SVGs directly within HTML files can be dangerous. You should only use a Kroki instance that you trust (or run your own!). For additional security, you can configure a Content Security Policy (CSP) using custom Webrick headers in the Jekyll _config.yml file:

webrick:
  headers:
    Content-Security-Policy: "Add a policy here"

Bug reports and pull requests are welcome! For major changes, please open an issue first to discuss what you would like to change.