This page is an introduction to the development of the preCICE documentation, based on a jekyll theme called documentation-theme-jekyll. You will learn how to run jekyll locally, about the sidebar structure, how to name and where to save documentation pages and what a minimal frontmatter looks like.
Updated 03 Apr 24

About the content

This majority of this documentation focuses on the technical side of writing content. See our content guidelines to learn what the content should look like.

About the theme

This site is based on a jekyll theme by technical writer Tom Joht called documentation-theme-jekyll. At the time of writing this theme was the second most popular documentation-style jekyll theme on and has been selected for its rich feature set and clean, functional design out of the box.

In addition Tom did a great job documenting the theme (using the theme) and you can read about specific features and their implementation and use in his documentation.

Getting started

To develop the website locally it is recommended to install jekyll and run

bundle exec jekyll serve

The theme’s documentation page has a step-by-step guide to install jekyll and the plugin (“gem”) manager bundler. When running jekyll for the first time you might have to install and/or update the gems first:

bundle install
bundle update

Now try again bundle exec jekyll serve and the site should be running at http://localhost:4000/. Jekyll will refresh and rebuild when you change files.

How documentation-theme-jekyll works in a nutshell

The two main ingredients behind this jekyll theme are

  1. The sidebar, i.e. the navigation tree. Jekyll builds the sidebar based on the sidebar.yml in the _data/sidebars directory. The YAML contains the relative structure of the navigation tree as well as the links to the html pages.
  2. A set of pages, i.e. Markdown or html files. Jekyll parses the Markdown or html files in the pages directory, renders them to html (in case of Markdown), and places them in the root folder.

Here is a snippet from the _data\sidebars\docs_sidebar.yml that spans (the maximum) two levels:

- title: sidebar
  product: Docs
  version: 2.1.0

  - title: Configuration
    output: web, pdf

    - title: Basics
      url: /configuration-introduction.html
      output: web, pdf

      - title: Coupling Scheme
        output: web, pdf

        - title: Overview
          url: /configuration-coupling.html
          output: web, pdf

        - title: Multi Coupling
          url: /configuration-coupling-multi.html
          output: web, pdf

    - title: Acceleration
      url: /configuration-acceleration.html
      output: web, pdf

Where to save files

Save Markdown files in the pages directory in an appropriate subdirectory. Jekyll is agnostic to this folder structure - subdirectories are for human ease of organisation only.

|_ docs
  |_ configuration
    |_ configuration-introduction.html
    |_ ...
  |_ installation
    |_ ...

Naming conventions

This can be easily achieved by baking in the category/topic into the filename and adds some welcome robustness, e.g.

|_ configuration
  |_ configuration-introduction.html
  |_ configuration-coupling.html
  |_ configuration-coupling-multi.html

File names should contain hyphens - instead of underscores _ following best practices for SEO.

In addition use the singular form where possible, e.g. configration-action.html instead of configuration-actions.html.

Minimal viable frontmatter

The minimal frontmatter contains only the options title and permalink (required) but should be complemented by keywords and summary (optional).

title: Configuration Basics
permalink: configuration-introduction.html
keywords: configuration, basics, overview
summary: "preCICE needs to be configured at runtime via an `xml` file, typically named `precice-config.xml`. Here, you specify which solvers participate in the coupled simulation, which coupling data values they exchange, which numerical methods are used for the data mapping and the fixed-point acceleration and many other things. "

The permalink has to be the full file name ending in .html with no leading slash \. During the build process jekyll processes the frontmatter and places the file at permalink value, i.e. in the root directory (by default is _site).

The Migration Guide contains more information on how to migrate preCICE documentation pages from the preCICE Github Wiki.

Rendering content from external repositories

While the main content of this website is sourced from the same repository that hosts the mechanics of it, some content is sourced from separate repositories. The main reason is to keep the documentation next to the respective code, so that developers can view it without looking at the website and update it in the same contribution, while users can find everything in the same place. Read more about this concept in the preCICE v2 reference paper. This practice is not yet uniformly adopted, but we are working on migrating more content.

External repositories are included as Git submodules, specified in the .gitmodules file. One example is the tutorials, which is covered by additional documentation for adding new tutorials.

To fetch content from an external repository/project (replace the my-* with the actual names):

  1. Switch to a new branch of the website and specify the new module: git submodule add imported/my-project.
  2. Set the branch to track, if not the default: git submodule set-branch --branch my-branch imported/my-project. This is particularly useful in case you are adding new documentation via a pull request. However, remember to reset the branch after merging.
  3. The above commands should have modified the .gitmodules file and staged changes. Commit the result (remember to push later, after testing).
  4. Update all submodules with git submodule update --remote --merge. If successful, you should see your new project in the imported/ directory. Remember that the branch in your external project must already be published.
  5. Update the commit that the module points to: git add imported/my-project && git commit -m "Update my-project submodule" && git push. You should only see a modified: imported/my-project (new commits) in your git status, not the files of that directory.
  6. In your GitHub pull request to the website, at the “files changed” view, you should see a submodule with a Git reference to your new project in the imported/ directory.

To render the fetched content on the website:

  1. In the file _config.yml, specify the newly imported directory in the list of subprojects:.
  2. In the same file, add an entry under the defaults: list, associating the subproject with some layout, sidebar, a path for the “Edit me” button, and more features.
  3. Remember to make the new pages discoverable, e.g., by adding them to some sidebar, or linking from another page.

After you merge the pull request in the external repository, remember to change the brach in the submodule (step 2) and in the _config.yml (step 1). If you squash-and-merge the pull request, the commit you were pointing to will not exist anymore. The easiest workaround it to delete the imported/my-project folder and update the submodules again (remember to add, commit, and push). You can always check the git diff for the commit it will point the submodule to.

To update the content, push to your repository and then manually trigger the “update submodules” workflow. Alternatively, add a GitHub Actions workflows to your repository, to update the website automatically. You will need to share the WORKFLOW_DISPATCH_TOKEN with the external repository.