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<h1>Using SCGI with Apache 2.x and Python</h1>
<p>This guide describes how to use SCGI with Apache 2.x, to serve an
application written in Python. The SCGI package also comes with modules
to use the protocol with Apache 1.x or <tt>lighttpd</tt>, and
frameworks for applications in other languages are available (including
LISP).</p>
<p>A complete setup for an SCGI application involves these four
components:</p>
<ol>
<li>
<p style="margin-bottom: 0in">A web server to process incoming
<tt>http</tt> requests--in our case, Apache 2.</p>
</li>
<li>
<p style="margin-bottom: 0in">The Apache SCGI module to delegate
requests over the SCGI protocol.</p>
</li>
<li>
<p style="margin-bottom: 0in">An SCGI server process based on the
Python <tt>scgi_server</tt> module.</p>
</li>
<li>
<p>An application to be run in child processes forked off by the
server process.</p>
</li>
</ol>
<p>A single SCGI server process serves only one application, but
multiple instances of it. Each instance lives in its own child process
of the SCGI server. More child processes are created on demand.</p>
<h2>Apache module</h2>
<p>To install the module by hand, <tt>cd</tt> into the <tt>apache2</tt>
subdirectory of the SCGI source tree, and, as root, <tt>make
install</tt>. On Debian-based systems and Gentoo, however, just install
the <tt>libapache2-mod-scgi</tt> package. RPM users can install
<tt>apache2-mod_scgi</tt>.</p>
<p>Next, set up the webserver to load the module and delegate requests
(in this example, for the <tt>http</tt> path "<tt>/dynamic</tt>") to a
separate server process accepting SCGI requests over a TCP socket. That
server will typically run on the same machine as the <tt>http</tt>
server, but it doesn't have to. Here's a snippet of Apache 2 config to
delegate requests for the <tt>http</tt> path <tt>/dynamic</tt> to local
TCP port 4000, which is the default:</p>
<pre>
# (This actually better set up permanently with the command line
# "a2enmod scgi" but shown here for completeness)
LoadModule scgi_module /usr/lib/apache2/modules/mod_scgi.so
# Set up a location to be served by an SCGI server process
SCGIMount /dynamic/ 127.0.0.1:4000
</pre>
<p>The deprecated way of delegating requests to an SCGI server
is as follows:</p>
<pre>
<Location "/dynamic">
# Enable SCGI delegation
SCGIHandler On
# Delegate requests in the "/dynamic" path to daemon on local
# server, port 4000
SCGIServer 127.0.0.1:4000
</Location>
</pre>
<p>Note that using <tt>SCGIMount</tt> instead of <tt>SCGIServer</tt>
allows Apache to compute <tt>PATH_INFO</tt> as most software
expects.</p>
<p>Here's a description of the configuration directives accepted by
this Apache module. The information is largely derived from the source,
so please excuse (or better yet: help improve) poor descriptions:</p>
<table width="99%" border="0" cellpadding="2" cellspacing="0">
<col width="43*" />
<col width="52*" />
<col width="57*" />
<col width="103*" />
<tr>
<th width="17%" bgcolor="#E6E6E6">
<p>
<strong>Directive</strong>
</p>
</th>
<th width="20%" bgcolor="#E6E6E6">
<p>
<strong>Arguments</strong>
</p>
</th>
<th width="22%" bgcolor="#E6E6E6">
<p>
<strong>Example</strong>
</p>
</th>
<th width="40%" bgcolor="#E6E6E6">
<p>
<strong>Description</strong>
</p>
</th>
</tr>
<tr>
<td width="17%" bgcolor="#E6E6E6">
<p>
<tt>SCGIMount</tt>
</p>
</td>
<td width="20%" bgcolor="#E6E6E6">
<p><tt>http</tt> path and IP/port pair</p>
</td>
<td width="22%" bgcolor="#E6E6E6">
<p>
<tt>/dynamic 127.0.0.1:4000</tt>
</p>
</td>
<td width="40%" bgcolor="#E6E6E6">
<p>Path prefix and address of SCGI server</p>
</td>
</tr>
<tr>
<td width="17%" bgcolor="#E6E6E6">
<p>
<tt>SCGIServer</tt>
</p>
</td>
<td width="20%" bgcolor="#E6E6E6">
<p>IP address and port</p>
</td>
<td width="22%" bgcolor="#E6E6E6">
<p>
<tt>127.0.0.1:4000</tt>
</p>
</td>
<td width="40%" bgcolor="#E6E6E6">
<p>Address and port of an SCGI server</p>
</td>
</tr>
<tr>
<td width="17%" bgcolor="#E6E6E6">
<p>
<tt>SCGIHandler</tt>
</p>
</td>
<td width="20%" bgcolor="#E6E6E6">
<p><em>On</em> or <em>Off</em></p>
</td>
<td width="22%" bgcolor="#E6E6E6">
<p>
<tt>On</tt>
</p>
</td>
<td width="40%" bgcolor="#E6E6E6">
<p>Enable delegation of requests through SCGI</p>
</td>
</tr>
<tr>
<td width="17%" bgcolor="#E6E6E6">
<p>
<tt>SCGIServerTimeout</tt>
</p>
</td>
<td width="20%" bgcolor="#E6E6E6">
<p>Number of seconds</p>
</td>
<td width="22%" bgcolor="#E6E6E6">
<p>10</p>
</td>
<td width="40%" bgcolor="#E6E6E6">
<p>Timeout for communication with SCGI server</p>
</td>
</tr>
</table>
<h2>Server process</h2>
<p>To install the Python module for creating SCGI applications, go to
the main SCGI source directory and run <tt><em>python setup.py
build</em></tt> and then, as <tt>root</tt>, <em><tt>python setup.py
install</tt>.</em> Or, on most GNU/Linux systems, just install the
<tt>python-scgi</tt> package.</p>
<p>The hard part is creating that application server process. The SCGI
home page provides a server implementation in Python, called
<tt>scgi_server.py</tt>, but this is really just a kind of support
module. In order to port an application to use this server, write a
main program in Python that imports this module and uses it. There is
one, fairly complex, example that makes an application called <a href="http://www.quixote.ca/">Quixote</a> run over
SCGI.</p>
<p>The main program that you write for yourself creates a handler class
describing how requests should be processed. You then tell the SCGI
server module to loop forever, handling incoming requests and creating
processes running your handler class as needed. <em>It is your own
responsibility to ensure that the server process is running.</em></p>
<p>Once the SCGI server process runs, it can accept requests forwarded
by the <tt>http</tt> server. It will attempt to delegate every request
that comes in to an existing child process, invoking its handler. If no
child process is free to take the request, an additional child process
is spawned (if the configured maximum number of child processes is not
exceeded; if it is, the request blocks until a child process becomes
free). If a handler "dies" (e.g. exits with an exception), a new child
is spawned to replace it.</p>
<h3>Writing a handler</h3>
<p>The server process must <tt>import scgi_server</tt>, then derive
handler classes from <tt>scgi_server.SCGIHandler</tt>. Just
<tt>scgi_server.py</tt> can also be run standalone, in which case it
return pages displaying the details of your browser's request.</p>
<p>Your handler class should be derived from the <tt>SCGIHandler</tt>
class defined in <tt>scgi_server.py</tt>, and override its
<tt>produce()</tt> function to process a request. This is new in SCGI
1.12. Before that, the function to override was
<tt>handle_connection()</tt> which involved a lot more work.</p>
<p>Besides <em><tt>self</tt>,</em> the <tt>produce()</tt> function
takes several arguments:</p>
<ol>
<li>
<p style="margin-bottom: 0in">
<em>a dict mapping names of CGI
parameters to request details</em>
</p>
</li>
<li>
<p style="margin-bottom: 0in">
<em>size (in bytes) of the request
body</em>
</p>
</li>
<li>
<p style="margin-bottom: 0in">
<em>an input stream providing the
request body</em>
</p>
</li>
<li>
<p>
<em>an output stream to write a page to.</em>
</p>
</li>
</ol>
<p>Alternatively, override <tt>SCGIHandler.produce_cgilike()</tt> which
can read the request body from standard input, write its output to
standard output, and receives its request details both as an argument
dict and added to its environment variables.</p>
<p>Your output should start out with an <tt>http</tt> header, so
normally you'd want to start out with something like <tt>"Content-Type:
text/plain\r\n\r\n"</tt> followed by page content.</p>
<p>Besides a header containing CGI variables, the request may also
contain a body. The length of this body is passed as the CGI parameter
<tt>CONTENT_LENGTH</tt>, but for your convenience is also converted to
an integer and passed separately to the <tt>produce()</tt> function. If
your handler needs the request body, it can read this number of bytes
from its input socket. Do not rely on EOF; explicitly read the correct
number of bytes (or less, if that's what you want).</p>
<h3>Writing a server</h3>
<p>Your main SCGI server program should create an <tt>SCGIServer</tt>
object (defined in <tt>scgi_server.py</tt>), passing in your handler
class for the <tt>handler_class</tt> parameter, and then call its
<tt>serve()</tt> method which will loop indefinitely to process
requests:</p>
<pre>
def main():
SCGIServer(handler_class=MyAppHandler).serve()
if __name__ == "__main__":
main()
</pre>
<p>You may want to support command-line options to influence various
options of the server's operation. The <tt>SCGIServer</tt>
initialization function takes several arguments:</p>
<center>
<table width="67%" border="0" cellpadding="2" cellspacing="0">
<col width="53*" />
<col width="157*" />
<col width="46*" />
<tr>
<th width="21%" bgcolor="#E6E6E6">
<p>
<strong>Name</strong>
</p>
</th>
<th width="61%" bgcolor="#E6E6E6">
<p>
<strong>Meaning</strong>
</p>
</th>
<th width="18%" bgcolor="#E6E6E6">
<p>
<strong>Default</strong>
</p>
</th>
</tr>
<tr>
<td width="21%" bgcolor="#E6E6E6">
<p>
<tt>handler_class</tt>
</p>
</td>
<td width="61%" bgcolor="#E6E6E6">
<p>Class (not object!) of handler to invoke for requests</p>
</td>
<td width="18%" bgcolor="#E6E6E6">
<p>
<tt>SCGIHandler</tt>
</p>
</td>
</tr>
<tr>
<td width="21%" bgcolor="#E6E6E6">
<p>
<tt>host</tt>
</p>
</td>
<td width="61%" bgcolor="#E6E6E6">
<p>Local IP address to bind to</p>
</td>
<td width="18%" bgcolor="#E6E6E6">
<p>
<em>empty string</em>
</p>
</td>
</tr>
<tr>
<td width="21%" bgcolor="#E6E6E6">
<p>
<tt>port</tt>
</p>
</td>
<td width="61%" bgcolor="#E6E6E6">
<p>TCP port to listen on</p>
</td>
<td width="18%" bgcolor="#E6E6E6">
<p>4000</p>
</td>
</tr>
<tr>
<td width="21%" bgcolor="#E6E6E6">
<p>
<tt>max_children</tt>
</p>
</td>
<td width="61%" bgcolor="#E6E6E6">
<p>Maximum number of child processes to spawn</p>
</td>
<td width="18%" bgcolor="#E6E6E6">
<p>5</p>
</td>
</tr>
</table>
</center>
<h2>Links</h2>
<ul>
<li>
<p style="margin-bottom: 0in">
<a href="http://python.ca/nas/scgi/">SCGI home
page</a>
</p>
</li>
<li>
<p style="margin-bottom: 0in">
<a href="http://quixote.python.ca/releases/">SCGI download</a>
</p>
</li>
<li>
<p style="margin-bottom: 0in">
<a href="http://simon.bofh.ms/cgi-bin/trac-django-projects.cgi/wiki/DjangoScgi">
Setting up Django with SCGI</a>
</p>
</li>
<li>
<p style="margin-bottom: 0in">
<a href="http://www.rexx.com/%7Edkuhlman/quixote_scgi.html">Setting up
Quixote with SCGI</a>
</p>
</li>
<li>
<p style="margin-bottom: 0in">
<a href="http://www.zedshaw.com/projects/scgi_rails/">Using Ruby on Rails
with SCGI</a>
</p>
</li>
<li>
<p>bazaar-ng (<em>bzr</em>) repository: <a href="http://quixote.python.ca/scgi.dev/">http://quixote.python.ca/scgi.dev/</a></p>
</li>
</ul>
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