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<section id="processing">
<title>Apache Rivet HTTP Request Processing</title>
<section>
<title>Tcl Scripts Processing</title>
<para>
Until &version2-generic; versions Rivet handled requests
following the vision that inspired the original design
which was summarized in the definition <quote>Web programming
like in PHP, but with Tcl</quote>. HTTP requests triggered the
execution of Tcl scripts or a Rivet (.rvt file) templates
whose path is encoded in the URI (an alias translation or
URL rewriting might occur to establish the real path).
The execution of such scripts can be preceded and/or
followed by the execution of scripts common to a whole web site
or to a hierarchy of directories the BeforeScript and AfterScript
directives. These scripts can be configured on a per virtual host,
per directory or per user basis. Execution of such combined
scripts can break because of coding errors (thus triggering the
ErrorScript execution) or it can deliberately interrupt
ordinary execution by calling ::rivet::abort_page (which triggers
the execution of a script defined by the directive AbortScript).
This scheme is in case terminated by a further configurable script
(AfterEveryScript). This model of request handling was coded within
the module mod_rivet.so itself.
</para>
<para>
With Rivet &version30; we changed this approach and landed to
a new much simpler and flexible model where each request is
by default handled by the following Tcl procedure
</para>
<programlisting>&request_handler.tcl;</programlisting>
<para>
Note the call to new &version3-generic; command ::rivet::url_script
that returns the body of the Tcl script or Rivet template
pointed by the URL.
</para>
<para>
This procedure emulates the &version2-generic; scheme
and as such works as a fully compatible request handling
but opens to the programmers the option of replacing it
with their own application request handling procedure
</para>
<note>
Note that if you redefine the core request handler
you'll need to handle yourself any error conditions
and any code interruption brought about by calling
<command>::rivet::abort_page</command>.
The current procedure might work as a template to be
reworked into your own request handler.
</note>
</section>
<section>
<title>Example: basic OO Rivet Application</title>
<para>
An applications may have no interest in running
a script pointed by the URL as in the traditional approach
followed by rivet inspired to the PHP philosophy of <quote>scripting
the HTML</quote>. A web based application
could be driven entirely by the URL encoded arguments and by the
data POSTed with HTML forms, still retaining the ability of exploiting
the template engine of Rivet through the <command>::rivet::parse</command>.
In other words an application could hinge on a single entry point to
handle requests, regardless the complexity of its internal design.
</para>
<para>This section shows a template for such an application
(let's call it MyRivetApp) based on an Itcl (or TclOO for what
it matters) object instance. In myrivetapp.tcl
the application class is defined and an instance of it is
created in the global namespace.
</para>
<programlisting>&myrivetapp.tcl;</programlisting>
<para>
which provides a very basic interface for both initialization
and request processing. Such script will be sourced into the
Tcl interpreter at the mod_rivet initialization stage. In the
Apache configuration (most
likely within a <VirtualHost myrivetapp.com:80>...</VirtualHost>
definition block)
</para>
<programlisting><IfModule rivet_module>
RivetServerConf ChildInitScript "source myrivetapp.tcl"
</IfModule></programlisting>
<para>
By running this script when an a thread is started
we set it up to respond requests, but we still need to
tell mod_rivet what code will eventually handle requests
and how the method MyRivetApp::request_processing will
be called with appropriate arguments
</para>
<programlisting>&myapp_request_handler.tcl;</programlisting>
</section>
<para>
Finally we have to tell mod_rivet to run this script when a
request is delivered to myApplication and we do so
using the &version30; directive <command>RequestHandler</command>
</para>
<programlisting><IfModule rivet_module>
RivetServerConf ChildInitScript "source myrivetapp.tcl"
RivetServerConf RequestHandler "myapp_request_handler.tcl"
</IfModule></programlisting>
<para>
Notice that the argument of the directive <command>RequestHandler</command>
is a file name not a Tcl script as for <command>ChildInitScript</command>
</para>
<para>
With such approach only the <command>ChildInitScript</command>, <command>ChildExitScript</command>
and <command>GlobalInitScript</command> configuration directives are effective, while
the effect of other handler is devolved to our request handler script.
</para>
</section>
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