PlantUML on Sphinx/Read The Docs¶

Motivation¶

We want to use PlantUML to add diagrams (using the concept as Diagrams as Code, just like we have Infrastructure as Code, etc.). That is, we use some markup language specific to write diagrams (PlantUML), and have their result embedded in our web pages just like these:

$```plantuml @startuml skinparam DefaultFontName Source Code Pro skinparam DefaultFontSize 15 ' hide footbox mainframe **Request Cycle** Actor Client participant Express boundary Handler entity Model control Service control Parser participant Axios Client -> Express ++: sends request to the application Express -> Handler ++: handles route Handler -> Service ++: sends request params\nto service module Service -> Parser ++: parses params to be\nused in the request ||| Parser -> Service --: request params now ready Service -> Axios ++: asks HTTPs client\nto fetch data ||| Axios -> Service --: fetches data and feeds\nit back to service module Service -> Handler --: just hands over the\nresponse data Handler -> Model ++: asks model to prepare\nand format the data for\nthe client ||| Model -> Handler --: data is now ready to\nbe sent to the client Handler -> Express --: asks application to\nsend data to the client Express -> Client: application sends data\nto the client @enduml$

Check out a few more diagrams on our SOLID Principle pages.

Getting Started¶

We’ll need:

Sphinx, the documentation generator which allows writing in both reStructuredText and Markdown.
PlantUML to generate diagrams of any kind.
sphinx-contrib/plantuml to allow embedding diagrams into the output content during build.

PlantUML Sphinx Contrib¶

Add something like this to your ./docs/requirements.txt:

sphinxcontrib-plantuml==0.25

The file should look like this:

./docs/requirements.txt¶

##
# https://myst-parser.readthedocs.io/en/latest/index.html
#
myst-parser==2.0.0

##
# https://github.com/pradyunsg/furo
#
furo==2023.5.20

##
# https://github.com/sphinx-contrib/plantuml
#
sphinxcontrib-plantuml==0.25

Extensions on conf.py¶

Enable myst_parser and plantuml Sphinx extensions:

extensions section of docs/conf.py¶

extensions = [
  ##
  # https://www.sphinx-doc.org/en/master/usage/markdown.html
  #
  'myst_parser',

  ##
  # https://github.com/sphinx-contrib/plantuml
  #
  'sphinxcontrib.plantuml',
]

Enable the colon_fence and attrs_block myst parser extensions in ./docs/conf.py:

myst_parser extensions section of docs/conf.py¶

myst_enable_extensions = [
  'colon_fence',
  'attrs_block',
  # ... other extensions
]

Take a look at the conf.py config file for this very project for more info.

PlantUML package on the container¶

Read The Docs uses (probably Docker) containers to build our docs. The possible settings for .readthedocs.yaml are mentioned in their docs.

Let’s use one of the allowed values for the OS as per ReadTheDocs configuration schema.

We should have a .readthedocs.yaml that looks like this:

./.readthedocs.yaml¶

#
# https://docs.readthedocs.io/en/stable/config-file/v2.html
#

version: 2

build:
  ##
  # Which OSes are available? See:
  #
  # • https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os
  #
  os: ubuntu-22.04

  tools:
    python: "3.11"

  ##
  # We need this in combination with sphinxcontrib-plantuml (python/pip
  # dependency)so PlantUML diagrams can be generated and embedded into
  # the output during build.
  #
  apt_packages:
    - plantuml

##
# Build documentation in the "docs/" directory with Sphinx
#
sphinx:
   configuration: docs/conf.py

##
# • https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
#
python:
   install:
   - requirements: docs/requirements.txt

Note

As of this writing (Jun 2023), they allow these:

{
  "os": {
    "title": "Operating System",
    "description": "Operating system to be used in the build.",
    "enum": [
      "ubuntu-20.04",
      "ubuntu-22.04"
    ]
  }
}

The docs should have an up to date list, though.

An example diagram¶

So now, to generate and embed the diagram output image into the output HTML, use a syntax like this:

@startuml
skinparam DefaultFontName Source Code Pro
skinparam DefaultFontSize 15

... diagram code ...

@enduml

For example, this code:

PlantUML Example Diagram Markup¶

@startuml
skinparam DefaultFontName Source Code Pro
skinparam DefaultFontSize 15
skinparam RankSep 50

package "eCommerce" {
  node "High Level Components" {
    component ProductCatalog
  }

  node "Abstractions" {
    component ProductFactory
  }

  node "Low Level Components" {
    component SQLProductRepository
  }

  ProductCatalog ..> ProductFactory: depends on
  ProductFactory ..> SQLProductRepository: depends on
}
@enduml

…results in this output:

Conclusion¶

At this point, whenever changes are pushed to the repository, a new build will be triggered on ReadTheDocs (depending on your own project settings, of course), and you should have the diagrams built and embedded onto the output web pages.