Changes between Initial Version and Version 1 of AppLoadingRefactor

Timestamp:

Jun 11, 2012, 3:24:29 PM (12 years ago)

Author:

Marc Tamlyn

Comment:

--

AppLoadingRefactor

@@ +1 @@
+This is an attempt to bring together the current state and long term plans of how
+application loading is done in Django, what it could do, and why.
+
+== Application loading refactor ==
+
+At present application loading is handled in different ways through Django.
+Because of some of the limitations of Python, and some peculiarities of how
+models work, modules should not be reimported. Consequently Django maintains
+a global static, called the application cache. Models are imported from the
+application cache transparently when interacting with the database. But
+applications listed in `INSTALLED_APPS` contain more than just models. Some
+contain template tags, `admin.py` files, `search_indexes.py` files from
+haystack, template folders and many other features which applications use to
+inspect each other, or to contribute objects to a site-wide collection.
+
+For the majority of applications, even reusable applications, anything more
+than the current system is unnecessary. Models are automatically registered,
+template tags can be loaded, and templates are available. (Tests are also
+available but this is a separate issue.)
+
+However, not every application has all of these components - there are many
+useful pluggable applications which do not contain models, but must still be
+registered in `INSTALLED_APPS`. A simple example is `django-pagination` -
+which consists of a single template tag. This module must ship with an empty
+`models.py` module to allow it to be registered as an installed app, for that
+template tag to be available.
+
+The problem is even more difficult for ''"meta-applications"'' - that is to say
+applications which exist solely to interact with other applications. The
+canonical example of this is the django admin, but there are many third party
+applications which do a similar thing. Haystack (the search engine framework)
+is a good example. Also there are applications which have a plugin archicture,
+whereby adding a second application to installed apps changes the behaviour of
+the first application. All of these third party systems have implemented their
+own ways to do this, with varying degrees of success.
+
+This is an opportunity for Django to do something great for the third party
+application community. It will be a very powerful tool, and opens up a range of
+magical things which could be done. App loading is magical, but Magic is not
+always a bad thing - most Django developers are very pleased they don't have to
+individually register every model with Django as they do to register a model
+with the admin. You just put it in the `models.py` file and it works! This
+will allow the magic to be more explicit, and ultimately more consistent
+between third party applications.
+
+This is a big change, and almost redefines what an "App" is within Django. To
+my mind, an app was originally intended to be a fairly self contained part of
+the system, normally in fact a self contained part of the ''Web site''.
+
+
+=== Specification ===
+
+Here is a specification of what an application is, and what it can do. Perhaps
+not all of these points need to be implemented to be able to integrate this
+refactor into core, but they should be considered to avoid future backwards
+incompatibilities.
+
+* An application is a self contained piece of code which '''contributes'''
+  something to the site. It may contibute any number of the following:
+    * models
+    * templates
+    * template tags
+    * static files
+    * admin configuration
+  and these are just (some of?) the things which are picked up on by other
+  parts of the Django core. Other things (seach indexes, migrations etc) may
+  also be contributed by the application, and processed by other applications.
+
+'''Note''' Some things are explicitly registered in settings, such as middleware,
+database routers and cache backends. These things may be contained within
+an application's code base, but they will not be registered automatically
+by the presence of that application in the `INSTALLED_APPS`.
+
+* An application may contain urls and views. These will generally be
+  explicitly registered, but some applications may choose to attach dynamic
+  urls offer a way of getting the urls from the application rather than
+  directly (similar to how the admin does to it's site class).
+
+* An application may modify another application. That is to say, an application
+  may provide an interface on it's application class for other applications to
+  modify it. This will facilitate a plugin architecture. A classic example of
+  where this will be useful is a Content Management System.
+
+* An application may depend on the presence of another application. It is
+  preferable for the application to depend on a defined ''contract'' with another
+  application, rather than explicity depend on that application. This should
+  allow applications to be more loosely coupled, leading potentially to
+  pluggable auth/messages applications. It also allows such applications to be
+  tested perhaps without the presence of the dependant app - a mocked object
+  may suffice.
+
+* An application may inspect other applications to gather information about
+  them. The system should provide tools to easily and safely access this
+  information, whether it is accessing a folder or a python module. Where
+  possible, we should modify parts of Django core & contrib to use this system.
+  Examples would include the app directories template loader, admin site and
+  template tag libraries.
+
+* An exact copy of an application can be deployed within the same project by
+  changing its label. This will have some subtle issues regarding imports.
+
+
+=== Implementation plan ===
+
+Application class. Talk about what APIs would bee needed on that class to
+achieve the above. Separate by the above points. Ideally, does not need to be
+contained within the python module it relates to (thus allowing me to subclass
+an app's Application Class in my own project to tweak how it works, this may or
+may not be better than passing configuration options)
+
+Application cache. Discuss what hooks this would need to provide, including
+signals, for the above to work.
+
+Examine any backwards incompatibilities which may result. The app refactor
+itself shouldn't introduce any, but the new functionality which it offers and
+the resulting changes in other applications such as `contrib.auth` may need
+deprecation paths.
+
+
+=== Current status ===
+
+Preston has merged master into the existing branch with work done by Jannis and
+Arthur. (https://github.com/ptone/django/commit/8722bdcc17293baa8b527ae7169b9a465e728d41)
+
+We need to evaluate which of the current features are implemented, and which are
+not.
+
+Update tests so that everything passes again, including outside of the appcache
+tests.
+
+Try to work out which parts of the implementation plan are tangential to
+each other, and therefore could easily be worked on by other people.