Web Server Configuration for Quixote
====================================

For a simple Quixote installation, there are two things you have to get
right:

* installation of the Quixote modules to Python's library (the
  trick here is that the ``quixote`` package must be visible to the user
  that CGI scripts run as, not necessarily to you as an interactive
  command-line user)

* configuration of your web server to run Quixote driver scripts
  (such as demo.cgi)

This document is concerned with the second of these.


Which web servers?
------------------

We are only familiar with Apache, and we develop Quixote for use under
Apache.  However, Quixote doesn't rely on any Apache-specific tricks;
if you can execute CGI scripts, then you can run Quixote applications
(although they'll run a lot faster with mod_scgi or FastCGI).  If you
can redirect arbitrary URLs to a CGI script and preserve parts of the
URL as an add-on to the script name (with ``PATH_INFO``), then you can
run Quixote applications in the ideal manner, ie. with superfluous
implementation details hidden from the user.


Which operating systems?
------------------------

We are mainly familiar with Unix, and develop and deploy Quixote under
Linux.  However, we've had several reports of people using Quixote under
Windows, more-or-less successfully.  There are still a few Unix-isms in
the code, but they are being rooted out in favour of portability.

Remember that your system is only as secure as its weakest link.
Quixote can't help you write secure web applications on an inherently
insecure operating system.


Basic CGI configuration
-----------------------

If you don't know how to configure your web server to run CGI scripts,
you probably shouldn't be installing Quixote, running a web server, or
writing complex web applications.  However, the sad truth is that lots
of people with little relevant experience or knowledge are running web
servers and writing web applications -- hence, this explanation.

Throughout this document, I'm going to assume that:
  
* CGI scripts live in the ``/www/cgi-bin`` directory of your web server,
  and have the extension ``.cgi``

* HTTP requests for ``/cgi-bin/foo.cgi`` will result in the execution
  of ``/www/cgi-bin/foo.cgi`` (for various values of ``foo``)

* if the web server is instructed to serve an executable file
  ``bar.cgi``, the file is treated as a CGI script

With Apache, these configuration directives will do the trick::

    AddHandler cgi-script .cgi
    ScriptAlias /cgi-bin/ /www/cgi-bin/

Consult the Apache documentation for other ways of configuring CGI
script execution.

For other web servers, consult your server's documentation.


Installing driver scripts
-------------------------

Given the above configuration, installing a Quixote driver script is the
same as installing any other CGI script: copy it to ``/www/cgi-bin`` (or
whatever).  To install the Quixote demo's driver script::

    cp -p demo/demo.cgi /www/cgi-bin

(The ``-p`` option ensures that ``cp`` preserves the file mode, so that
it remains executable.)


URL rewriting
-------------

With the above configuration, users need to use URLs like ::

    http://www.example.com/cgi-bin/demo.cgi

to access the Quixote demo (or other Quixote applications installed in
the same way).  This works, but it's ugly and unnecessarily exposes
implementation details.

In our view, it's preferable to give each Quixote application its own
chunk of URL-space -- a "virtual directory" if you like.  For example,
you might want ::

    http://www.example.com/qdemo

to handle the Quixote demo.

With Apache, this is quite easy, as long as mod_rewrite is compiled,
loaded, and enabled.  (Building and loading Apache modules is beyond the
scope of this document; consult the Apache documentation.)

To enable the rewrite engine, use the ::

    RewriteEngine on

directive.  If you have virtual hosts, make sure to repeat this for each
``<VirtualHost>`` section of your config file.

The rewrite rule to use in this case is ::

    RewriteRule ^/qdemo(/.*) /www/cgi-bin/demo.cgi$1 [last]

This is *not* a redirect; this is all handled with one HTTP
request/response cycle, and the user never sees ``/cgi-bin/demo.cgi`` in
a URL.

Note that requests for ``/qdemo/`` and ``/qdemo`` are *not* the same; in
particular, with the above rewrite rule, the former will succeed and the
latter will not.  (Look at the regex again if you don't believe me:
``/qdemo`` doesn't match the regex, so ``demo.cgi`` is never invoked.)

The solution for ``/qdemo`` is the same as if it corresponded to a
directory in your document tree: redirect it to ``/qdemo/``.  Apache
(and, presumably, other web servers) does this automatically for "real"
directories; however, ``/qdemo/`` is just a directory-like chunk of
URL-space, so either you or Quixote have to take care of the redirect.

It's almost certainly faster for you to take care of it in the web
server's configuration.  With Apache, simply insert this directive
*before* the above rewrite rule::

    RewriteRule ^/qdemo$ /qdemo/ [redirect=permanent]

If, for some reason, you are unwilling or unable to instruct your web
server to perform this redirection, Quixote will do it for you.
However, you have to make sure that the ``/qdemo`` URL is handled by
Quixote.  Change the rewrite rule to::

    RewriteRule ^/qdemo(/.*)?$ /www/cgi-bin/demo.cgi$1 [last]

Now a request for ``/qdemo`` will be handled by Quixote, and it will
generate a redirect to ``/qdemo/``.  If you're using a CGI driver
script, this will be painfully slow, but it will work.

For redirecting and rewriting URLs with other web servers, consult your
server's documentation.


Long-running processes
----------------------

For serious web applications, CGI is unacceptably slow.  For a CGI-based
Quixote application, you have to start a Python interpreter, load the
Quixote modules, and load your application's modules before you can
start working.  For sophisticated, database-backed applications, you'll
probably have to open a new database connection as well for every hit.

Small wonder so many high-performance alternatives to CGI exist.  (The
main advantages of CGI are that it is widely supported and easy to
develop with.  Even for large Quixote applications, running in CGI mode
is nice in development, because you don't have to kill a long-running
driver script every time the code changes.)  Currently, Quixote supports
three such alternatives: mod_scgi, FastCGI, and mod_python.


mod_scgi configuration
----------------------

SCGI is a CGI replacement written by Neil Schemenauer, one of
Quixote's developers, and is similar to FastCGI but is designed to be
easier to implement.  mod_scgi simply forwards requests to an
already-running SCGI server on a different TCP port, and doesn't try
to start or stop processes, leaving that up to the SCGI server.

The SCGI code is available from http://www.mems-exchange.org/software/scgi/ .  
It contains a Python module, scgi.quixote_handler, that will publish a
Quixote-based application via SCGI.  Here's an example script to publish
an application::

    #!/usr/bin/python
    from scgi.quixote_handler import QuixoteHandler, main
    from quixote.publisher import SessionPublisher

    class MyAppHandler(QuixoteHandler):
        publisher_class = SessionPublisher
        root_namespace = "myapp.ui"
        prefix = ""

    if __name__ == '__main__':
        main(MyAppHandler)

When run, this script will take various command-line arguments.  ``-p
<port>`` specifies the TCP port that the SCGI server will listen to.
The following Apache directive will direct requests to an SCGI server
running on port 3001::

    <Location />
      SCGIServer 127.0.0.1 3001
      SCGIHandler On
    </Location>


FastCGI configuration
---------------------

If your web server supports FastCGI, you can significantly speed up your
Quixote applications with a simple change to your configuration.  You
don't have to change your code at all (unless it makes assumptions about
how many requests are handled by each process).  (See
http://www.fastcgi.com/ for more information on FastCGI.)

To use FastCGI with Apache, you'll need to download mod_fastcgi from
http://www.fastcgi.com/ and add it to your Apache installation.

Configuring a FastCGI driver script is best done after reading the fine
documentation for mod_fastcgi at
http://www.fastcgi.com/mod_fastcgi/docs/mod_fastcgi.html

However, if you just want to try it with the Quixote demo to see if it
works, add this directive to your Apache configuration::

    AddHandler fastcgi-script .fcgi

and rename demo.cgi to demo.fcgi.  If you're using a URL rewrite to map
requests for (eg.) ``/qdemo`` to ``/www/cgi-bin/demo.cgi``, be sure to
change the rewrite -- it should now point to ``/www/cgi-bin/demo.fcgi``.

After the first access to ``demo.fcgi`` (or ``/qdemo/`` with the
modified rewrite rule), the demo should be noticeably faster.  You
should also see a ``demo.fcgi`` process running if you do ``ps -le``
(``ps -aux`` on BSD-ish systems, or maybe ``ps aux``).  (On my 800 MHz
Athlon machine, there are slight but perceptible delays navigating the
Quixote demo in CGI mode.  In FastCGI mode, the delay between pages is
no longer perceptible -- navigation is instantaneous.)  The larger your
application is, the more code it loads, and the more work it does at
startup, the bigger a win FastCGI will be for you.


mod_python configuration
------------------------

mod_python is an Apache module for embedding a Python interpreter into
the Apache server.  To use mod_python as the interface layer between
Apache and Quixote, add something like this to your httpd.conf::

    LoadModule python_module /usr/lib/apache/1.3/mod_python.so
    <LocationMatch "^/qdemo(/|$)">
        SetHandler python-program
        PythonHandler quixote.mod_python_handler
        PythonOption quixote-root-namespace quixote.demo
        PythonInterpreter quixote.demo
        PythonDebug On
    </LocationMatch>

This will attach URLs starting with ``/qdemo`` to the Quixote demo.
When you use mod_python, there's no need for rewrite rules (because of
the pattern in the ``LocationMatch`` directive), and no need for a
driver script.

mod_python support was contributed to Quixote by Erno Kuusela
<erno@iki.fi>.


$Id: web-server.txt,v 1.15 2002/10/07 14:08:38 akuchlin Exp $
