WebKit User's Guide
+++++++++++++++++++

Webware for Python

:Version: 0.9
:Released: 11/13/05

.. contents:: Contents

Synopsis
========

WebKit provides Python classes for generating dynamic content from a
web-based, server-side application. It is a significantly more
powerful alternative to CGI scripts for application-oriented
development.

Feedback
========

You can e-mail webware-discuss@lists.sourceforge.net to give feedback,
discuss features, and get help using WebKit.  If you have a bug to
report, use the `bug tracker`__.

__  http://sourceforge.net/tracker/?atid=104866&group_id=4866&func=browse


Introduction
============

Overview
--------

The core concepts of the WebKit are the Application, Servlet, Request,
Response and Transaction, for which there are one or more Python
classes.

The application resides on the server-side and manages incoming
requests in order to deliver them to servlets which then produce
responses that get sent back to the client. A transaction is a simple
container object that holds references to all of these objects and is
accessible to all of them.

Content is normally served in HTML or XML format over an HTTP
connection. However, applications can provide other forms of content
and the framework is designed to allow new classes for supporting
protocols other than HTTP.

In order to connect the web server and the application server a small
program called an *adapter* is used. It bundles a web browser
request and sends it to the application server, which then processes
it and sends the response back to the adapter which then outputs the
results for use by the web server. Adapters come in various flavors
including CGI, FastCGI and Apache mod. See the `Install Guide`__ for
more information.

__ InstallGuide.html

At a more detailed level, the process looks like this:

* At some point, someone has configured and run both a web server
  (such as Apache) and the WebKit app server (WebKit/AppServer).
* A user requests a web page by typing a URL or submitting a form.
* The user's browser sends the request to the remote web server.
* The web server invokes the adapter.
* The adapter simply collects information about the request and sends
  it to the WebKit app server which is ready and waiting.
* The app server asks the Application object to dispatch the raw
  request.
* The application instantiates an HTTPRequest object and asks the
  appropriate Servlet (as determined by examining the URL) to process
  it.
* The servlet generates content into a given HTTPResponse object,
  whose content is then sent back by the app server to the adapter.
* The adapter sends the content through the web server and ultimately to
  the user's web browser.

For a more detailed guide, see `Anatomy of a Webware Transaction`__.

__ http://colorstudy.com/software/webware/Anatomy.html

Compared to CGI applications
----------------------------

The alternative to a server-side application is a set of CGI
scripts. However, a CGI script must always be launched from scratch
and many common tasks will be performed repeatedly for each
request. For example, loading libraries, opening database connections,
reading configuration files, etc.

With the long-running server-side application, the majority of these
tasks can be done once at launch time and important results can be
easily cached. This makes the application significantly more
efficient.

Of course, CGIs can still be appropriate for "one shot" deals or
simple applications. Webware includes a `CGI Wrapper`__ if you'd like
to encapsulate your CGI scripts with robust error handling, e-mail
notifications, etc.

__ ../../CGIWrapper/Docs/index.html

Papers
------

You should also consider reading the papers__ which have been
written about Webware for Python, particularly the "Introduction to
Webware for Python" that was presented by Chuck Esterbrook at the
9th International Python Conference in 2001.

__ http://www.webwareforpython.org/Papers

Errors / Uncaught Exceptions
============================

One of the conveniences provided by WebKit is the handling of uncaught
exceptions. The response to an uncaught exception is:

* Log the time, error, script name and traceback to AppServer's console.
* Display a web page containing an apologetic message to the user.
* Save a technical web page with debugging information so that
  developers can look at it after-the-fact. These HTML-based error
  messages are stored one-per-file, if the ``SaveErrorMessages``
  setting is true (the default). They are stored in the directory
  named by the ``ErrorMessagesDir`` (defaults to ``"ErrorMsgs"``).
* Add an entry to the error log, found by default in
  ``Logs/Errors.csv``
* E-mail the error message if the ``EmailErrors`` setting is true,
  using the settings ``ErrorEmailServer`` and
  ``ErrorEmailHeaders``. See Configuration__ for more information.

__ Configuration.html#errors

Here is a `sample error page`__.

__ SampleError.html

Archived error messages can be browsed through the administration_ page.

Error handling behavior can be configured as described in Configuration__.

__ Configuration.html#errors

Configuration
=============

There are several configuration parameters through which you can alter
how WebKit behaves. They are described below, including their default
values. Note that you can override the defaults by placing config
files in the ``Configs/`` directory. A config file simply contains a
Python dictionary containing the items you wish to override. For
example::

    {
        'SessionStore': 'Memory',
        'ShowDebugInfoOnErrors': 1
    }

See the `Configuration Guide`__ for more information on settings.

__ Configuration.html

Administration
==============

WebKit has a built-in administration page that you can access via the
``Admin`` context. You can see a list of all contexts in the sidebar
of any ``Example`` or ``Admin`` page.

The admin pages allows you to view WebKit's configuration, logs, and
servlet cache, and perform actions such as clearing the cache,
reloading selected modules and shutting down the app server.

More sensitive pages that give control over the app server require a
user name and password, the username is ``admin``, and you'll set the
password when you run ``install.py``. You can change the password in
``WebKit/Configs/Application.config``.

The adminstration scripts provide further examples of writing pages
with WebKit, so you may wish to examine their source in
``WebKit/Admin/``.

Debugging
=========

print
-----

The most common technique is the infamous ``print`` statement. The
results of ``print`` statements go to the console where the WebKit
application server was started (not to the HTML page as would happen
with CGI). Prefixing the debugging output with a special tag (such as
>>) is useful because it stands out on the console and you can search
for the tag in source code to remove the print statements after they
are no longer useful. For example::

    print '>> fields =', self.request().fields()

.. Note that if you are using <a
   href=InstallGuide.html#Adapters_OneShot>OneShot.cgi</a>, then you will
   need to set <span class=name>ShowConsole</span> to 1 in <span
   class=filename>WebKit/Configs/OneShotAdapter.config</span>.

Raising Exceptions
------------------

Uncaught expections are trapped at the application level where a
useful error page is saved with information such as the traceback,
environment, fields, etc. You can configure the application to
automatically e-mail you this information. Here is an `example error
page`__.

__ SampleError.html

When an application isn't behaving correctly, raising an exception can
be useful because of the additional information that comes with
it. Exceptions can be coupled with messages, thereby turning them into
more powerful versions of the ``print`` statement. For example::

    raise Exception, 'self = %s' % self

Restarting the Server
---------------------

When a servlet's source code changes, it is reloaded. However,
ancestor classes of servlets and library modules are not.
You may wish to enable the auto-reloading feature in the AppServer.config__
file to mitigate this problem.

__ Configuration.html#appserver-config

In any case, when having problems, consider restarting the app server.

Another option is to use the AppControl page of the Admin__ context
to clear the servlet instance and class cache.

__ administration_

Assertions
----------

Assertions are used to ensure that the internal conditions of the
application are as expected. An assertion is equivalent to an ``if``
statement coupled with an exception. For example::

    assert shoppingCart.total()>=0.0, \
        'shopping cart total is %0.2f' % shoppingCart.total()

HTML Validation
---------------

You can validate the HTML in your pages using the `Web Designer
Group's HTML Validator`__.  It is available as a `RPM package`__, in
Debian as ``wdg-html-validator`` and you can also install the source__.

__ http://www.htmlhelp.com/tools/validator/
__ http://www.htmlhelp.com/tools/validator/packages/
__ http://www.htmlhelp.com/tools/validator/source.html

To enable the validation, you have to override ``.writeBodyParts()``
in your ``SitePage``, as::

    def writeBodyParts(self):
        Page.writeBodyParts()
        self.validateHTML()

If your pages contain invalid HTML, a message will be appended to the
page.

Naming Conventions
==================

Cookies and form values that are named with surrounding underscores
(such as ``_sid_`` and ``_action_``) are generally reserved by WebKit
and various plugins and extensions for their own internal purposes. If
you refrain from using surrounding underscores in your own names, then
[a] you won't accidentally clobber an already existing internal name
and [b] when new names are introduced by future versions of WebKit,
they won't break your application.

Actions
=======

Suppose you have a web page with a form and one or more
buttons. Normally, when the form is submitted, a method such as
Servlet's ``respondToPost()`` or Page's ``writeBody()``, will be
invoked. However, you may find it more useful to bind the button to a
specific method of your servlet such as ``new()``, ``remove()``
etc. to implement the command, and reserve ``writeBody()`` for
displaying the page and the form that invokes these methods.  Note
that your "command methods" can then invoke ``writeBody()`` after
performing their task.

The *action* feature of ``Page`` let's you do this. The process goes
like this:

1. Add buttons to your HTML form of type ``submit`` and name
   ``_action_``. For example::

       <input name="_action_" type="submit" value="New">
       <input name="_action_" type="submit" value="Delete">

2. Alternately, name the submit button ``_action_methodName``.  For
   example::

       <input name="_action_New" type="submit" value="Create New Item">

3. Add an ``actions()`` method to your class to state which actions
   are valid.  (If WebKit didn't force you to do this, someone could
   potentially submit data that would cause any method of your servlet
   to be run).  For example::

       def actions(self):
           return SuperClass.actions(self) + ['New', 'Delete']

4. Now you implement your action methods.

The ``ListBox`` example shows the use of actions (in
``WebKit/Examples/ListBox.py``).

Note that if you procede as in 1., you can add a ``methodNameForAction()``
method to your class transforming the value from the submit button (its
label) to a valid method name. This will be needed, for instance, if there
is a blank in the label on the button. However, usually it's simpler to
procede as in 2. in such cases.



Plug-ins
========

A plug-in is a software component that is loaded by WebKit in order to
provide additional WebKit functionality without necessarily having to
modify WebKit's source.

The most infamous plug-in is PSP (Python Server Pages) which ships
with Webware.

Plug-ins often provide additional servlet factories, servlet
subclasses, examples and documentation. Ultimately, it is the plug-in
author's choice as to what to provide and in what manner.

Technically, plug-ins are Python packages that follow a few simple
conventions in order to work with WebKit. See `Creating Plugins`__ for
information about writing your own.

__ Developing.html#creatingplugins

How do I develop an app?
========================

The answer to that question might not seem clear after being deluged
with all the details. Here's a summary:

* Make sure you can run the WebKit AppServer. See the `Install Guide`__
  for more information.

__ InstallGuide.html

* Read the source to the examples (in ``WebKit/Examples``), then
  modify one of them to get your toes wet.

* Create your own new example from scratch. Ninety-nine percent of the
  time you will be subclassing the ``Page`` class.

* Familiarize yourself with the class docs in order to take advantage
  of classes like Page, HTTPRequest, HTTPResponse and
  Session. Unfortunately, I couldn't get generated class docs working
  for this release, so you'll have to resort to breezing through the
  source code which is coupled with documentation strings. Read the
  examples first.

* With this additional knowledge, create more sophisticated pages.

* If you need to secure your pages using a login screen, you'll want
  to look at the SecurePage, LoginPage, and SecureCountVisits examples
  in ``WebKit/Examples``.  You'll need to modify them to suit your
  particular needs.

* The `Webware For Python Wiki`__ provides a lot of useful HowTos,
  Recipes and other useful informaiton for Application developers.

__ http://wiki.webwareforpython.org

* Contribute enhancements and bug fixes back to the project. ``:-)``

* The Webware user community is quite supportive and friendly:

  - Discussion mailing list:
    http://lists.sourceforge.net/mailman/listinfo/webware-discuss

  - Make sure you find out about new versions when they're released,
    via the announce list:
    http://lists.sourceforge.net/mailman/listinfo/webware-announce

Known Bugs
==========

Known bugs and future work in general are documented in Future__.

__ Future.html

Credit

Authors: Chuck Esterbrook, Jay Love, Geoff Talvola, Ian Bicking
and others (improvements and additions).

Many people, mostly on the webware-discuss mailing list, have provided
feedback, testing and patches.

The design was inspired by both Apple's WebObjects and Java's
Servlets.
