add docs about names and namespaces

Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>

docs/source/tutorial/design.rst

@@ -1,7 +0,0 @@
-===========================
- Designing Your Plugin API
-===========================
-
-.. seealso:: 
-
-   :doc:`/essays/pycon2013`

docs/source/tutorial/implementation.rst

@@ -1,4 +1,68 @@
-=======================
+=====================================
- Implementing a Plugin
+ Guidelines for Implementing Plugins
-=======================
+=====================================
 
+Stevedore uses setuptools entry points to define and load plugins. An
+entry point is standard way to refer to a named object defined inside
+a Python module or package. The name can be a reference to any class,
+function, or instance, as long as it is created when the containing
+module is imported (i.e., it needs to be a module-level global).
+
+Names and Namespaces
+====================
+
+Entry points are registered using a *name* in a *namespace*. 
+
+Entry point names are usually considered user-visible. For example,
+they frequently appear in configuration files where a driver is being
+enabled.  Because they are public, names are typically as short as
+possible while remaining descriptive. For example, database driver
+plugin names might be "mysql", "postgresql", "sqlite", etc.
+
+Namespaces, on the other hand, are an implementation detail, and while
+they are known to developers they are not usually exposed to users.
+The namespace naming syntax looks a lot like Python's package syntax
+(``a.b.c``) but *namespaces do not correspond to Python packages*. It
+can be convenient to use a package name as a namespace, but it's not
+required at all.  The main feature of entry points is that they can be
+discovered *across* packages. That means that a plugin can be
+developed and installed completely separately from the application
+that uses it, as long as they agree on the namespace and API.
+
+Each namespace is owned by the code that consumes the plugins and is
+used to search for entry points. The entry point names are typically
+owned by the plugin, but they can also be defined by the consuming
+code for named hooks (see :class:`~stevedore.hook.HookManager`).  The
+names of entry points must be unique within a given distribution, but
+are not necessarily unique in a namespace (again, for hook patterns).
+
+Keeping it Simple
+=================
+
+After a lot of trial and error, the easiest way I have found to define
+an API is to follow these steps:
+
+1. Define a unique namespace for each API by combining the name of the
+   application (or library) and a name of the API. Keep it
+   shallow. For example, "cliff.formatters" or
+   "ceilometer.pollsters.compute".
+2. Use the `abc module`_ to create a base abstract class to define the
+   behaviors required of plugins of the API. 
+3. Create plugins by subclassing the base class and implementing the
+   required methods.
+
+Developers don't have to subclass from the base class, but it provides
+a convenient way to document the API, and using an abstract base class
+keeps you honest.
+
+.. seealso::
+
+   * `abc module`_
+   * `Using setuptools entry points`_
+   * `Package Discovery and Resource Access using pkg_resources`_
+   * `Using Entry Points to Write Plugins | Pylons`_
+
+.. _Using setuptools entry points: http://reinout.vanrees.org/weblog/2010/01/06/zest-releaser-entry-points.html
+.. _Package Discovery and Resource Access using pkg_resources: http://pythonhosted.org/distribute/pkg_resources.html
+.. _Using Entry Points to Write Plugins | Pylons: http://docs.pylonsproject.org/projects/pylons-webframework/en/latest/advanced_pylons/entry_points_and_plugins.html
+.. _abc module: http://docs.python.org/2/library/abc.html

docs/source/tutorial/index.rst

@@ -9,9 +9,12 @@ application.
 .. toctree::
    :maxdepth: 2
 
-   design
    implementation
    registration
    loading
    calling
    testing
+
+.. seealso:: 
+
+   :doc:`/essays/pycon2013`