Add docs on writing & using plugins

docs/contributing/plugins.rst

@@ -0,0 +1,119 @@
+.. _contributing/plugins:
+
+============
+TLJH Plugins
+============
+
+TLJH plugins are the official way to make customized 'spins' or 'stacks' 
+with TLJH as the base. For example, the earth sciences community can make
+a plugin that installs commonly used packages, set up authentication
+and pre-download useful datasets. The mybinder.org community can
+make a plugin that gives you a single-node, single-repository mybinder.org.
+Plugins are very powerful, so the possibilities are endless.
+
+Design
+======
+
+`pluggy <https://github.com/pytest-dev/pluggy>`_ is used to implement
+plugin functionality. TLJH exposes specific **hooks** that your plugin
+can provide implementations for. This allows us to have specific hook
+points in the application that can be explicitly extended by plugins,
+balancing the need to change TLJH internals in the future with the
+stability required for a good plugin ecosystem. 
+
+Writing a simple plugins
+========================
+
+We shall try to write a simple plugin that installs a few libraries,
+and use it to explain how the plugin mechanism works. We shall call
+this plugin ``tljh-simple``.
+
+Plugin directory layout
+-----------------------
+
+We recommend creating a new git repo for your plugin. Plugins are
+normal python packages - however, since they are usually simpler,
+we recommend they live in one file.
+
+For ``tljh-simple``, the repository's structure should look like:
+
+.. code-block:: none
+   
+   tljh_simple:
+    - tljh_simple.py
+    - setup.py
+    - README.md
+    - LICENSE
+
+The ``README.md`` (or ``README.rst`` file) contains human readable
+information about what your plugin does for your users. ``LICENSE``
+specifies the license used by your plugin - we recommend the 
+3-Clause BSD License, since that is what is used by TLJH itself.
+
+``setup.py`` - metadata & registration
+--------------------------------------
+
+``setup.py`` marks this as a python package, and contains metadata
+about the package itself. It should look something like:
+
+.. code-block:: python
+
+    from setuptools import setup
+
+    setup(
+        name="tljh-simple",
+        author="YuviPanda",
+        version="0.1",
+        license="3-clause BSD",
+        url='https://github.com/yuvipanda/tljh-simple',
+        entry_points={"tljh": ["simple = tljh_simple"]},
+        py_modules=["tljh_simple"],
+    )
+
+
+This is a mostly standard ``setup.py`` file. ``entry_points={"tljh": ["simple = tljh_simple]}``
+'registers' the module ``tljh_simple`` (in file ``tljh_simple.py``) with TLJH as a plugin.
+
+``tljh_simple.py`` - implementation
+-----------------------------------
+
+In ``tljh_simple.py``, you provide implementations for whichever hooks
+you want to extend. 
+
+A hook implementation is a function that has the following characteristics:
+
+#. Has same name as the hook
+#. Accepts some or all of the parameters defined for the hook
+#. Is decorated with the ``hookimpl`` decorator function, imported from
+   ``tljh.hooks``.
+
+The current list of available hooks and when they are called can be
+seen in ```tljh/hooks.py`` <https://github.com/jupyterhub/the-littlest-jupyterhub/blob/master/tljh/hooks.py>`_
+in the source repository.
+
+
+This example provides an implementation for the ``tljh_extra_user_conda_packages``
+hook, which can return a list of conda packages that'll be installed in users' 
+environment from conda-forge.
+
+.. code-block:: python
+
+    from tljh.hooks import hookimpl
+
+    @hookimpl
+    def tljh_extra_user_conda_packages():
+        return [
+            'xarray',
+            'iris',
+            'dask',
+        ]
+
+
+Publishing plugins
+==================
+
+Plugins are python packages and should be published on PyPI. Users
+can also install them directly from GitHub - although this is
+not good long term practice. 
+
+The python package should be named ``tljh-<pluginname>``.