Captcha Plugin Architecture
===========================

This document contains design notes for the plone.app.discussion Captcha plugin
architecture.


Introduction
------------

When a Captcha plugin (e.g. plone.formwidget.captcha or 
plone.formwidget.recaptcha) is installed, plone.app.discussion 
extends the comment form with a Captcha field/widget and a Captcha validator.

The form extender and validator are only registered if there is a plugin 
installed that claims to provide the "plone.app.discussion-captcha" feature in 
its configure.zcml file::

    <configure ...
        xmlns:meta="http://namespaces.zope.org/meta">
        <!-- Declare that plone.formwidget.captcha provides a Captcha field that 
             can be used by plone.app.discussion to add a Captcha field to comment 
             forms. -->
        <meta:provides feature="plone.app.discussion-captcha" />
    
        ...
    </configure>

For an example see:

https://svn.plone.org/svn/plone/plone.formwidget.captcha/trunk/plone/formwidget/captcha/configure.zcml

Currently plone.formwidget.captcha and plone.formwidget.recaptcha provide such a
feature. If you want to write your own Captcha plugin, it has to provide this 
feature as well.

    
CaptchaExtender
---------------

The CaptchaExtender class extends the comment form with a Captcha field and
widget. The CaptchaExtender currently uses either the CaptchaFieldWidget from
plone.formwidget.captcha or the ReCaptchaFieldWidget from 
plone.formwidget.recaptcha. If you want to write your own Captcha solution, you
have to override the update() method of the CaptchaExtender or write your own
CaptchaExtender class.


CaptchaValidator
----------------

The CaptchaValidator class provides custom versions of the 
plone.formwidget.captcha and the plone.formwidget.recaptcha validators. It does 
this, because we want to be able to have more than one Captcha solution 
installed in one Plone instance. We also want to be able to easily switch 
between different Captcha implementations inside a single Plone instance. 

Therefore we have to check which Captcha solution is enabled in the discussion 
control panel and use only the selected Captcha validator. It is not enough to 
check if a Captcha plugin is just installed, because there could be more than 
one.

We do two checks. First we check for a suitable Captcha solution (check for the 
plone.app.discussion-captcha feature, see notes above). Second, we check which 
Captcha solution is enabled in the discussion control panel and apply the 
corresponding field validator.