<section id="request">
    <title>Apache Child Processes Lifecycle and Request Processing</title>

    <simplesect>
    	<title>Apache Child Process Lifecycle</title>
	   <para>
            Apache Rivet devolves to the <ulink url="">Multi-Processing Module (MPM)</ulink>
            the task of managing the agents responding to network requests. 
            An MPM is responsible for creating such agents during the start-up, 
            and is in charge for terminating existing ones and recreating new 
            agents when the workload is requiring it. 
       </para>
       <para>
            Apache Rivet &version2-generic; supported only the
            <ulink url="&apachedoc-prefork;">prefork</ulink>
            MPM which creates child processes as independent agents responding to network requests. 
            Starting with &version30; also the <ulink url="&apachedoc-worker;">worker</ulink> and
            <ulink url="&apachedoc-event;">event</ulink> MPM are supported. The worker MPM is
            an hybrid model where forked child processes in turn create threads as real
            network agents. Also Apache on Windows&copy; is now supported and tested
            with the <ulink url="&apachedoc-winnt;">winnt</ulink> MPM, 
            where a single process creates and manages a large number of thread agents.
       </para>
       <para>
	    		Configuration parameters about this critical point can be read in the
	    		<ulink url="&apachedoc-perftuning;">Apache
	    		documentation</ulink>. 
	    </para>
	    <para>
	    		There are 4 stages in the lifetime of an Apache webserver that are relevant
	    		to Rivet: 
	    </para>
	    <orderedlist>
	    	<listitem>
	    		<bridgehead>Server Initialization</bridgehead>
	    		<para>
               Apaches starts up as a single process. During this stage Apache performs 
               various preliminary tasks including reading and parsing the configuration. 
					After the configuration has been read Rivet sets up some internal resources
					and if a Tcl script is set as argument of a <command>ServerInitScript</command>
					directive the script is executed. 
					Variables, arrays or dictionaries created during 
					the execution of this script will be preserved and later replicated in the 
					child process intepreters if the prefork MPM is loaded (which restricts
					this feature to the Unix systems). 
					The prefork MPM creates new child processes with a fork() 
					system call, which involves only in memory copy of sections of a
					process address space. Tcl is careful about reproducing an environment
					across a fork call in order to have a functional interpreter.
				</para>
				<para>
					Still, regardless the OS and loaded MPM <command>ServerInitScript</command>
		    		is a good place to do global initialization that doesn't involve
		    		creation of private data. Example of tasks that can be done
		    		in this context are IPC methods that must be initialized at this stage.
		    		With the prefork MPM also importing from namespaces and loading packages
		    		can be done here removing the burden from the child initialization stage. 	
	    		</para>
	    	</listitem>
	    	<listitem>
	    		<bridgehead>Child Process Initialization</bridgehead>

	    		<para>
					A child process calls the MPM bridge interface function to set up
					the Tcl run time environment,either creating multiple threads each running its
					Tcl interpreters or, in the case of the prefork MPM bridge, setting up
					the environment within a the child process itself as a single Tcl thread.
	    		</para>

	    		<para>
               This is the stage where most likely you want to open I/O channels,
               database connections or any other resource that has to be private to an 
               interpreter and has to persist over a whole thread lifespan.
               When the option <command>SeparateVirtualInterps</command> is 
               turned off child processes will have a single interpreter regardless
		    		the number of virtual hosts configured. The
               <command>GlobalInitScript</command> is the configuration script 
               the child process will run once before getting ready to 
               serve requests
	    		</para>

	    		<para>
	    			When <command>SeparateVirtualInterps</command> is turned on 
	    			each configured virtual host will have its own slave interpreter which
	    			can will run the <command>ChildInitScript</command> directive as
	    			initialization script. The	    			
	    			<command>ChildInitScript</command> has to be
	    			placed within a &lt;VirtualHost...&gt;...&lt;/VirtualHost ...&gt;
	    			stanza to associate a script to a specific virtual host initialization.
	    			This scenario of interpreter separation is extremely useful to
	    			prevent resource conflicts when different virtual hosts are 
	    			serving different web applications. 
	    		</para>
	    		<para>
	    			<note>
	    				<command>GlobalInitScript</command> has no effect to working interpreters
	    				when <command>SeparateVirtualInterps</command> is set.
	    			</note>
	    			<note>
	    				The lazy MPM bridge implements a model where
	    				every worker thread has exactly one interpreter and belongs to 
	    				a single virtual host, therefore <command>SeparateVirtualInterps</command>
	    				is ignored and you can't share the same interpreter among virtual host
	    			</note>
	    		</para>
	    	</listitem>
	    	<listitem>
	    		<bridgehead>Request Processing and Content Generation</bridgehead>
		  		<para>
			    		<note>
			    			This section explain the default request handling procedure which
			    			was written to let Rivet &version30; work as a drop in replacement 
			    			of any &version2-generic; module. For a in-depth understanding of
			    			the new request processing mechanics please read the
			    			<link linkend="processing">request processing</link> section of the manual
			    		</note>
			   </para>

	    		<para>
                 After a child has been initialized it's ready to serve requests. 
                 A child process' lifetime is almost entirely spent in this phase, waiting
                 for connections and responding to requests. At every request the URL 
                 goes through filter processing and, in case, rewritten
                 (mod_rewrite, Alias directives, etc). 
                 Parameter values encoded in the request are made available to the 
                 environment and finally the script encoded in the URL is run. 
                 The developer can tell Rivet if optionally the execution has to
                 be  preceded by a <command>BeforeScript</command> and followed by an
                 <command>AfterScript</command>. The real script mod_rivet will
                 execute is the result of the concatenation of the 
                 <command>BeforeScript</command>,
                 the script encoded in the URL and the <command>AfterScript</command>.
                 Thus the whole ensemble of code that makes up a web application might
                 be running within the same "before" and "after" scripts to which 
                 the programmer can devolve tasks common to every 
                 page of an application.
            </para>
         </listitem>
         <listitem>
            <bridgehead>Child Process Exit</bridgehead>
            <para>
                 If no error condition forces the child process to a premature exit, his
                 life is determined by the Apache configuration parameters. To reduce
                 the effects of memory leaks in buggy applications the Apache webserver 
                 forces a child process to exit after a
                 certain number of requests served. A child process gets replaced 
                 with a brand new one if the workload of webserver requires so. 
                 Before the process quits an exit handler can be run
                 to do some housekeeping, just in case something the could have been 
                 left behind has to be cleaned up. Like the initialization scripts
                 <command>ChildExitScript</command> too is a "one shot" script.
             </para>
             <para>
                 The Tcl <command>exit</command> command forces an interpreter to
                 quit, thus removing the ability of the process embedding it 
                 to run more Tcl scripts. The child process then is forced
                 to exit and be replaced by a new one when the workload demands it.
                 This operation implies the <command>ChildExitScript</command> be
                 run before the interpreter is actually deleted.
             </para>
       	</listitem>
	   </orderedlist>
	</simplesect>
	<simplesect>
    	<title>Apache Rivet Error and Exception Scripts Directives</title>
    	
        <para>
         Rivet is highly configurable and each of the webserver lifecycle stages 
         can be exploited to control a web application. 
         Not only the orderly sequence of stages
         in a child lifecycle can be controlled with Tcl scripts, but also
         Tcl error or abnormal conditions taking place during
         the execution can be caught and handled with specific scripts.
       </para>
	    <para>
	    	Tcl errors (conditions generated when a command exits with code TCL_ERROR) 
	    	usually result in the printing of a backtrace of the code fragment
	    	relevant to the error.
	    	Rivet can set up scripts to trap these errors and run instead
	    	an <command>ErrorScript</command> to handle it and conceal details
	    	that usually have no interest for the end user and it
	    	may show lines of code that ought to remain private. The ErrorScript
	    	handler might create a polite error page where things
	    	can be explained in human readable form, thus enabling the end user
	    	to provide meaningful feedback information. 
	    </para>
	    <para>
	    	In other cases an unmanageable conditions might take place in the data and
	    	this could demand an immediate interruption of the content generation. These abort 
	    	conditions can be fired by the <xref linkend="abort_page"/> command, which
	    	in turn fires the execution of an <command>AbortScript</command> to handle
	    	the abnormal condition. Starting with Rivet 2.1.0 <xref linkend="abort_page"/>
	    	accepts a free form parameter that can be retrieved later with the command 
	    	<xref linkend="abort_code"/>
	    </para>
	</simplesect>
	<simplesect>
		<title>Tcl Namespaces in Rivet and the <command>::request</command> Namespace</title>

  		<para>
    		<note>
    			This section explain the default request handling procedure which
    			was written to let Rivet &version30; work as a drop in replacement 
    			of any &version2-generic; module. For a in-depth understanding of
    			the new request processing mechanics please read the
    			<link linkend="processing">request processing</link> section of the manual
    		</note>
	   </para>

		<para>
			With the sole exception of .rvt templates, mod_rivet runs pure Tcl scripts 
			at the global namespace. That means that every variable or procedure 
			created in Tcl scripts resides by default in the 
			"::" namespace (just like in traditional Tcl scripting) and they
			are persistent across different requests until explicitly unset or
			until the interpreter is deleted.
			You can create your own application namespaces to store data but
			it is important to remember that subsequent requests will in general be served 
			by different child processes. Your application can rely on the fact that 
			certain application data will be in the interpreter, but you shouldn't 
			assume the state of a transaction spanning several pages 
			can be stored in this way and be safely kept available to a 
			specific client. Sessions exist for this purpose and Rivet ships its own 
			session package with support for most of popular DBMS. Nonetheless 
			storing data in the global namespace can be useful, even though scoping 
			data in a namespace is recommended. I/O channels and
			database connections are examples of information usually specific 
			to a process for which you don't want to pay the overhead of creating them
			at every request, probably causing a dramatic loss in the application
			performance.
		</para>
		<para>
			A special role in the interpreter is played by the <command>::request</command> 
			namespace.	The <command>::request</command> namespace is deleted and recreated
			at every request and Rivet templates (.rvt files) are executed within it.
		</para>
		<para> 
			Unless you're fully qualifying variable names outside the <command>::request</command> 
			namespace, every variable and procedure created in .rvt files is by default placed in
			it and deleted before any other requests gets processed. It is therefore safe to
			create variables or object instances in template files and foresake about them: Rivet
			will take care of cleaning the namespace up and everything created inside the namespace
			will be destroyed.
		</para>
		<table align="center" title="Apache Rivet Scripts" class="namespaces">
			<thead>
				<td>Stage</td><td>Script</td><td>Namespace</td>
			</thead>
			<tbody>
				<tr class="init"><td>Apache Initialization</td><td>ServerInitScript</td><td>::</td></tr>
				<tr class="childinit"><td rowspan="2">Child Initialization</td><td>GlobalInitScript</td><td>::</td></tr>
				<tr class="childinit"><td>ChildInitScript</td><td>::</td></tr>
				<tr class="processing"><td rowspan="6">Request Processing</td><td>BeforeScript</td><td>::</td></tr>
				<tr class="processing"><td>.rvt</td><td>::request</td></tr>
				<tr class="processing"><td>.tcl</td><td>::</td></tr>
				<tr class="processing"><td>AfterScript</td><td>::</td></tr>
				<tr class="processing"><td>AbortScript</td><td>::</td></tr>
				<tr class="processing"><td>AfterEveryScript</td><td>::</td></tr>
				<tr class="childexit"><td>Child Termination</td><td>ChildExitScript</td><td>::</td></tr>
				<tr class="processing"><td>Error Handling</td><td>ErrorScript</td><td>::</td></tr>
			</tbody>
		</table>
	</simplesect>
</section>
