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<section id="request">
<title>Apache Child Processes Lifecycle and Request Processing</title>
<simplesect>
<title>Apache Child Process Lifecycle</title>
<para>
Apache Rivet delegates 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 is currently supporting only the
<ulink url="http://httpd.apache.org/docs/2.2/mod/prefork.html">prefork</ulink>
MPM which creates full fledged child processes as independent agents
responding to network requests.
Efforts are under way to extend the support to
the <ulink url="http://httpd.apache.org/docs/2.2/mod/worker.html">worker</ulink> MPM,
a hybrid model where forked child processes in turn create threads as real
network agents. If we can achieve this the goal would open the possibility of
supporting also the Windows© specific
<ulink url="http://httpd.apache.org/docs/2.2/mod/mpm_winnt.html">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="http://httpd.apache.org/docs/2.2/misc/perf-tuning.html">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>Single Process 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,
since 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). Thus
<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 importing namespace commands and loading packages
providing code of general interest for every application to
be served. Also IPC methods can be initialized in this stage.
</para>
</listitem>
<listitem>
<bridgehead>Child Process Initialization</bridgehead>
<para>
Right after the webserver has forked its child processes
there is a chance to perform specific initialization of their Tcl interpreters.
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. 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 <VirtualHost...>...</VirtualHost ...>
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>
</para>
</listitem>
<listitem>
<bridgehead>Request Processing and Content Generation</bridgehead>
<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>
With the sole exception of .rvt templates, 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>
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