Create build_pipelines.rst (issue #25)

docs/shared/contribution_guidelines/build_pipelines.rst

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+**Note**: This is a draft for text to be added to the ReadTheDocs documentation pages
+
+openMINDS pipelines
+===================
+
+openMINDS pipelines are specialized branches within a subset of the repositories in the openMetadataInitiative organization. The purpose of these branches is to build an up-to-date version of schemas or derived schema representations. Pipelines are triggered by a set of GitHub Actions workflows whose job it is to ensure the continual synchronization of interdependent repositories.
+
+We’ll first outline the different repository types in the openMetadataInitiative organization before we describe the existing workflows and the pipeline branches in more detail.
+
+Repository types
+----------------
+
+There are three types of repositories in the Open Metadata Initiative Organization:
+
+1. "Module" repositories
+2. "Central" repository
+3. "Target" repositories
+
+Module Repositories
+~~~~~~~~~~~~~~~~~~~
+
+These repositories represent individual openMINDS metadata models, for example `openMINDS_core`_ or `openMINDS_SANDS`_. It also includes the `openMINDS_instances`_ repository which is not a model, but a collection of controlled metadata instances of schemas from the various openMINDS models. Development of openMINDS schemas (and instances) will happen on these module repositories, and any suggestion for a schema modification should happen through a Pull Request to the respective repository.
+
+.. _openMINDS_core: https://github.com/openMetadataInitiative/openMINDS_core
+.. _openMINDS_SANDS: https://github.com/openMetadataInitiative/openMINDS_SANDS
+.. _openMINDS_instances: https://github.com/openMetadataInitiative/openMINDS_instances
+
+Central repository
+~~~~~~~~~~~~~~~~~~
+
+The **central** repository (`openMINDS`_) contains schemas for all the openMINDS models in the omi.json format. These schemas are extended and “resolved” [link to description of the tpl.json and omi.json formats] variants of the schema templates found in the respective **module** repositories. The **central** repository should be considered the ground truth for schemas.
+
+.. _openMINDS: https://github.com/openMetadataInitiative/openMINDS
+
+Target repositories
+~~~~~~~~~~~~~~~~~~~
+
+**Target** repositories are repositories that depend on the ground truth schemas in the **central** repository. Examples include `openMINDS_Python`_, `openMINDS_MATLAB`_, `openMINDS_Java`_, and `openMINDS_documentation`_. These repositories should be updated whenever changes are made to the ground truth schemas, i.e., the **central** `openMINDS`_ repository.
+
+.. _openMINDS_Python: https://github.com/openMetadataInitiative/openMINDS_Python
+.. _openMINDS_MATLAB: https://github.com/openMetadataInitiative/openMINDS_MATLAB
+.. _openMINDS_Java: https://github.com/openMetadataInitiative/openMINDS_Java
+.. _openMINDS_documentation: https://github.com/openMetadataInitiative/openMINDS_documentation
+
+Repository dependencies
+-----------------------
+
+The **central** repository depends on each of the **module** repositories, and the **target** repositories all depend on the **central** repository. Once any of the **module** repositories have a new commit (push), they trigger the build process of **central** which aggregates the information across all of those **modules** and merges them in a single (versioned) view. Once this is done, the **target** build pipelines are triggered to then consume the centrally provided information to build their "language-specific" products.
+
+Overview of Workflows
+---------------------
+
+There are currently two types of GitHub Actions workflows, ``openMINDS_upstream``, and ``build``.
+
+openMINDS_upstream (module repositories)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Each of the module repositories has a workflow called ``openMINDS_upstream``. This workflow is triggered whenever something is pushed to the respective repository (todo: mention version branches), and it has a single job which is to trigger the build pipeline of the **central** repository. An example of such a workflow is found `here`_.
+
+.. _here: https://github.com/openMetadataInitiative/openMINDS_core/blob/e873947bd8cae1de08aebd3ecc59a1c81d218cb3/.github/workflows/openMINDS_upstream.yml
+
+openMINDS_upstream (central repository)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When a push is made on the ``main`` branch of the **central** `openMINDS`_ repository, the **central** ``openMINDS_upstream`` workflow is triggered. This workflow has a single job which is to trigger the ``build`` workflow of each of the **target** repositories.
+
+Suggestion: rename ``openMINDS_upstream`` of **central** repo to ``openMINDS_trigger_targets``
+
+Build (central and target repositories)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The **central** repository (`openMINDS`_) and the **target** repositories have a branch called ``pipeline`` that contains the ``build`` workflow and the build routines for that repository. When the ``build`` workflow is triggered, the build routines will execute and the built product is pushed to the ``main`` branch of the respective repository.
+
+Workflow example flow
+---------------------
+
+Let's say a modification of a schema in the core model is pushed onto `openMINDS_core`_. This will trigger the following series of workflows:
+
+1. The ``openMINDS_upstream`` workflow of the `openMINDS_core`_ repository is triggered, which in turn triggers the ``build`` workflow of the **central** `openMINDS`_ repository’s ``pipeline`` branch.
+2. The ``build`` workflow of the **central** repository executes and pushes an updated version onto the ``main`` branch.
+3. The ``openMINDS_upstream`` workflow of the **central** repository is triggered, which in turn triggers the ``build`` workflows of each of the **target** repositories.
+4. The ``build`` workflows of the **target** repositories execute and the changes are pushed onto the ``main`` repository branches.
+
+The result of this series of workflows is that both the **central** and the **target** repositories are updated with the latest change from the core model.
+
+For target repository developers
+--------------------------------
+
+1. All **target** repositories should have a ``pipeline`` branch whose responsibility it is to build schema representations from the ground-truth schemas of the **central** repository. At the moment the best way to get started is to explore the pipeline branches of one or more existing **target** repositories.
+2. The `openMINDS_upstream.yml`_ of the **central** `openMINDS`_ repository must be updated to trigger the ``build`` workflow on the ``pipeline`` branch of the **target** repository.
+
+.. _openMINDS_upstream.yml: https://github.com/openMetadataInitiative/openMINDS/blob/main/.github/workflows/openMINDS_upstream.yml
+
+For model / module repository developers
+----------------------------------------
+
+There must be an ``openMINDS_upstream`` workflow (todo: add link to example from one of the module repositories).