<!-- $ Id: $ -->

  <section id="internals">
    <title>Rivet Internals</title>
    <para>
      This section easily falls out of date, as new code is added, old
      code is removed, and changes are made.  The best place to look
      is the source code itself.  If you are interested in the changes
      themselves, the Subversion revision control system
      (<command>svn</command>) can provide you with information about
      what has been happening with the code.
    </para>
    <section>
      <title>Initialization</title>
      <para>
	When Apache is started, (or when child Apache processes are
	started if a threaded Tcl is used),
	<function>Rivet_InitTclStuff</function> is called, which
	creates a new interpreter, or one interpreter per virtual
	host, depending on the configuration. It also initializes
	various things, like the <structname>RivetChan</structname>
	channel system, creates the Rivet-specific Tcl commands, and
	executes Rivet's <filename>init.tcl</filename>.  The caching
	system is also set up, and if there is a
	<command>GlobalInitScript</command>, it is run.
      </para>
    </section>
    <section>
      <title>RivetChan</title>
      <para>
	The <structname>RivetChan</structname> system was created in
	order to have an actual Tcl channel that we could redirect
	standard output to.  This lets us use, for instance, the
	regular <command>puts</command> command in .rvt pages.  It
	works by creating a channel that buffers output, and, at
	predetermined times, passes it on to Apache's IO system.
	Tcl's regular standard output is replaced with an instance of
	this channel type, so that, by default, output will go to the
	web page.
      </para>
    </section>
    <section>
      <title>The <command>global</command> Command</title>
      <para>
	Rivet aims to run standard Tcl code with as few surprises as
	possible.  At times this involves some compromises - in this
	case regarding the <command>global</command> command.  The
	problem is that the command will create truly global
	variables.  If the user is just cut'n'pasting some Tcl code
	into Rivet, they most likely just want to be able to share the
	variable in question with other procs, and don't really care
	if the variable is actually persistant between pages.  The
	solution we have created is to create a proc
	<command>::request::global</command> that takes the place of
	the <command>global</command> command in Rivet templates.  If
	you really need a true global variable, use either
	<command>::global</command> or add the :: namespace qualifier
	to variables you wish to make global.
      </para>
    </section>
    <section>
      <title>Page Parsing, Execution and Caching</title>
      <para>
	When a Rivet page is requested, it is transformed into an
	ordinary Tcl script by parsing the file for the &lt;? ?&gt;
	processing instruction tags.  Everything outside these tags
	becomes a large <command>puts</command> statement, and
	everything inside them remains Tcl code.
      </para>
      <para>
	Each .rvt file is evaluated in its own
	<constant>::request</constant> namespace, so that it is not
	necessary to create and tear down interpreters after each
	page.  By running in its own namespace, though, each page will
	not run afoul of local variables created by other scripts,
	because they will be deleted automatically when the namespace
	goes away after Apache finishes handling the request.
      <note>
	    One current problem with this system is that while
	    variables are garbage collected, file handles are not, so
	    that it is very important that Rivet script authors make
	    sure to close all the files they open.
      </note>
      </para>
      <para>
	After a script has been loaded and parsed into it's "pure Tcl"
	form, it is also cached, so that it may be used in the future
	without having to reload it (and re-parse it) from the disk.
	The number of scripts stored in memory is configurable.  This
	feature can significantly improve performance.
      </para>
    </section>

    <section>
      <title>Debugging Rivet and Apache</title>
      <para>
	If you are interested in hacking on Rivet, you're welcome to
	contribute!  Invariably, when working with code, things go
	wrong, and it's necessary to do some debugging.  In a server
	environment like Apache, it can be a bit more difficult to
	find the right way to do this.  Here are some techniques to
	try.
      </para>
      <para>
	The first thing you should know is that Apache can be launched
	as a <emphasis>single process</emphasis> with the
	<option>-X</option> argument:</para>
      <programlisting>httpd -X</programlisting>.
      <para>
	On Linux, one of the first things to try is the system call
	tracer, <command>strace</command>.  You don't even have to
	recompile Rivet or Apache for this to work.
      </para>

      <programlisting>strace -o /tmp/outputfile -S 1000 httpd -X</programlisting>

      <para>This command will run httpd in the system call tracer,
	which leaves its output (there is potentially a lot of it) in
	<filename>/tmp/outputfile</filename>.  The <option>-S</option>
	option tells <command></command>strace to only record the
	first 1000 bytes of a syscall.  Some calls such as
	<function>write</function> can potentially be much longer than
	this, so you may want to increase this number.  The results
	are a list of all the system calls made by the program.  You
	want to look at the end, where the failure presumably occured,
	to see if you can find anything that looks like an error.  If
	you're not sure what to make of the results, you can always
	ask on the Rivet development mailing list.
      </para>

      <para>
	If <command>strace</command> (or its equivalent on your
	operating system) doesn't answer your question, it may be time
	to debug Apache and Rivet.  To do this, you will need to run
	the <command>./configure.tcl</command> script with the
	<option>-enable-symbols</option> option, and recompile.
      </para>
      <para>
	Since it's easier to debug a single process, we'll still run
	Apache in single process mode with -X:
      </para>

      <programlisting>
@ashland [~] $ gdb /usr/sbin/apache.dbg
GNU gdb 5.3-debian
Copyright 2002 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "powerpc-linux"...
(gdb) run -X
Starting program: /usr/sbin/apache.dbg -X
[New Thread 16384 (LWP 13598)]
.
.
.
      </programlisting>

      <para>
	When your apache session is up and running, you can request a
	web page with the browser, and see where things go wrong (if
	you are dealing with a crash, for instance).
      </para>

    </section>

  </section>

