avalanche 0.3.0
Web Framework with a focus on testability and reusability
License: MIT

=========
Overview
=========

Avalanche is a Python web-framework that focus on testability and reusability

It uses `Jinja2 <http://jinja.pocoo.org/>`_ as a default template system and
does not includes any persistence layer.


Avalanche goals (or why another web framework?)
================================================


Testability
-------------

Avalanche was designed in a way that it makes it possible (easier for you) to
write *good* unit-tests for your code. That is not only making it easy to write
tests. A unit-test should:

  * give a clear error message when it fails
  * fail only when the feature under test is broken, not on every code change
  * be fast



Reusability
-------------

Mostly every framework claims that some kind of reusability is one of
their design goals.
Avalanche "reusability" goals means **source-code** reusability.

Many frameworks provide some mechanisms for reusable/plugable *sub-applications*
however it is not always easy to re-use
these applications source code in case you need to configure/modify it.
Plugable applications is also a very important feature but as of now
Avalanche has no support for that.

It should not only be possible to write reusable code, the code should be
reusable on the *first* time you write it.
You should not be advised to write the code in one way,
and than later have to modify it to make it reusable.
I.e. it is opposed to saying "Use *view* (handler) functions". And than...
"if you want your *views* to be re-usable convert them to class-based *views*!".



Of course Avalanche does not do miracles. Testability and reusability will
ultimately depends on the application code. But the framework
have a big role on setting up the *right* path.

The goals are presented below, hopefully after reading the design and tutorial
it will be clear to you how these goals are achieved.



Project Details
===============

* `Website/docs <http://packages.python.org/avalanche>`_

* This is an open-source project (`MIT license <http://opensource.org/licenses/mit-license.php>`_) written in python.

* Download from `PyPi <http://pypi.python.org/pypi/avalanche>`_

* Project management (bug tracker, feature requests and source code ) on `bitbucket <https://bitbucket.org/schettino72/avalanche>`_.

* Questions and feedback on `google group <https://groups.google.com/d/forum/python-avalanche>`_.




Avalanche Design
====================


beyond MVC (model-view-controller)
-----------------------------------

`MVC <http://en.wikipedia.org/wiki/Model-view-controller>`_ is a software
architectural pattern created with the goal to isolate "domain logic" from
the user interface. This separation of concern enables the creation of better
application code. This pattern was very successful for many desktop frameworks
and so served as a reference to the creation of web-frameworks. The problem is
that this architecture can not be mapped directly to the way web-applications
work.

Even the so-called MVC frameworks are not really MVC. So let's just keep the
MVC's goal. That is to write clean, re-usable and testable code.


web applications
------------------

Essentially all a web-application do is to receive a
`HTTP <http://en.wikipedia.org/wiki/HTTP>`_ request, process it and generate a HTTP response.

::

                      +------------------+
  HTTP Request ------>| web application  + ----->  HTTP Response
                      +------------------+

Sending and receiving HTTP is handled by a web-server,
so we don't need to worry about that.
Let's take a closer look into how the web application generates a HTTP response:



::

                    +------+      +-------+
  HTTP request ---->|router|----->|handler|----> HTTP response
                    +------+      +-------+

The *router* will analyze the request and dispatch it to a request
*handler* that will create the response.


request handlers styles
------------------------

There are mainly 3 styles of request handlers.
  * a single function
  * a class method
  * a class

Avalanche uses the third style, a class. Using a class as request
handler suits better our goals because it provides a greater flexibility, easier
to modify/extend and re-use parts of the handler.


request handler processing
---------------------------

The request handler processing can be divided in 3 stages:

::

               +-----------------+                        +------------+                  +----------+
  request ---->| param converter |---- param objects ---->| processing |--- context ----->| renderer |----> response
               +-----------------+                        +------------+                  +----------+


1) param converter - get parameters from HTTP request

  HTTP is a text protocol, the application will typically get some
  parameters from the request and convert string values into some native data
  types. These parameters are taken from the URI path, URI query, post-data,
  cookies, etc.

  This is where you will write some *glue code* to map HTTP into your application
  code.


2) processing / context builder

  Processing is the application logic. It will often access a persistence
  layer (sometimes called *Model*, but this is entirely up to the application code
  and the web-framework has no role on that). It might also trigger other
  operations like doing some batch processing or accessing third-party
  applications.
  Note that typically this application logic is independent of HTTP.

  This processing may also consist of simple data retrieval.
  In this case the processing may generate a *context*.
  Context is a term used to represent the data that will be used by a renderer
  to create a HTTP response.

  A web page is often composed of several elements so sometimes it makes sense to
  divide the work into more than one "context builder".



3) renderer - generate output

  The renderer will convert the result of the application processing into text
  to create a HTTP response.
  The renderer will typically use a template system to generate HTML code
  or convert the data to JSON.


On avalanche you should write code for the 3 stages of the handler separately and
let the framework put the different parts together.


Move on to the tutorial to see how it looks like.