Mailing List Archive

stas 01/09/15 09:01:48

Added: . README
src/devel/core_explained core_explained.pod
src/devel/writing_tests writing_tests.pod
src/user/install install.pod
src/user/intro start_fast.pod
src/world/templates choosing.pod
Log:
adding first docs

Revision Changes Path
1.1 modperl-docs/README

Index: README
===================================================================
This is mod_perl 2.0 docs documentation project repository

The repository is called modperl-docs, but it'll be automatically
placed into modperl-2.0/docs when you check out mod_perl-2.0. So you
can get the repository in one of these ways:

% cvs -d :pserver:anoncvs@cvs.apache.org:/home/cvspublic login
% cvs -d :pserver:anoncvs@cvs.apache.org:/home/cvspublic co modperl-2.0

and then look in modperl-2.0/docs or:

% cvs -d :pserver:anoncvs@cvs.apache.org:/home/cvspublic co modperl-docs

You probably want to subscribe to these two mailing lists:

docs-cvs-subscribe@perl.apache.org
docs-dev-subscribe@perl.apache.org

The first one is for cvs commits and the second is for discussions.

Then you can post to the modperl-docs discussion list. To post take
the docs-dev-subscribe@perl.apache.org address and remove the
-subscribe part from it. Please don't advertise the list's direct
address so we won't get spammed. Thanks.

If you are interested in helping us creating great documentation by
reviewing what gets committed and submitting new docs and patches,
please subscribe to these two mailing list.

Welcome to the yet another cool project brought to you by Apache
Software Foundation.



1.1 modperl-docs/src/devel/core_explained/core_explained.pod

Index: core_explained.pod
===================================================================
=head1 mod_perl 2.0 Source Code Explained

=head1 Code Layout

Apache-Test/ - test kit for mod_perl and Apache::* modules
Changes - Changes file
LICENSE - ASF LICENSE doc
Makefile.PL - Generates all the needed Makefiles
Makefile - autogenerated
WrapXS/ - autogenerated
lib/
src/
t/ - mod_perl tests
todo/ - things to be done
util/ - useful utilities for developers
xs/


=head1

most of the xs/ *.h files have code in them. they are .h because they are
always #include-d, never compiled into their own object file. and only
the file that #include-s a .h file from xs/ should be able to see what's
in there. anything else belongs in a src/modules/perl/foo.c public api.

=cut



1.1 modperl-docs/src/devel/writing_tests/writing_tests.pod

Index: writing_tests.pod
===================================================================
=head1 Writing tests with Apache::Test framework

=head1 Running Tests

Running test is usual just like for any perl module, first we have to
create the I<Makefile> and build everything. So we run:

% perl Makefile.PL [options]
% make

Now we can do the testing. You can run the tests in two ways. The
first one is usual:

% make test

but it adds the overhead of checking all the directories that
everything is built (the usual make modification control). So it's
faster to run the tests directly:

% t/TEST

In case something goes wrong you should run the tests in the verbose
mode:

% t/TEST -v

When debugging problems it's helps to keep the I<error_log> file open
in another console, and see the debug output in the real time via
tail(1):

% tail -f t/logs/error_log

Of course this file gets created only when the server starts, so you
cannot run tail(1) on it before the server starts.

If you have to run the same tests repeatedly, in most cases you don't
want to wait for the server to start every time, so you can start it
once:

% t/TEST -start

and then run all the tests via:

% t/TEST

or only specific tests:

% t/TEST protocol/echo

note that you don't have to add the I<t/> prefix for the test
filenames if you specify them explicitly.

There is one bit you should be aware of. If you run the tests without
restarting the server any changes to the response handlers that you
apply won't take effect, untill the server is restarted. Therefore you
may want to use C<Apache::Reload> module (META: not working with 2.0
yet), or use the following trick:

PerlModule Apache::Foo
<Location /cgi-test>
PerlOptions +GlobalRequest
SetHandler modperl
PerlResponseHandler "sub { delete $INC{'Apache/Foo.pm'}; require Apache::Foo; Apache::Foo::handler(shift);}"
</Location>

This will force the response handler C<Apache::Foo> to be reloaded on
every request.

Since the request test files don't reside in memory you can change
them and the changes will take effect without restarting the server.

% t/TEST -start

always stops the server first if any is running. In case you have a
server runnning on the same port, (for example if you develop the a
few tests at the same time in different trees), you should either kill
that server, or run the server on a different port.

% t/TEST -start Port 8799

or by setting an evironment variable C<APACHE_PORT> to the desired
value before starting the server.

=head1 Writing tests with C<Apache::Test> framework

The C<Apache::Test> tests framework is designed for easy writing of
tests that need to be run under Apache webserver. Originally designed
for the mod_perl Apache module, it was extended to be used for any
Apache module.

You can write tests in Perl and C, and the framework will provide an
extensive functionality which makes the tests writing a simple and
therefore enjoy-able process.

If you have ever written or looked at the tests most Perl modules come
with, C<Apache::Test> uses the same concept. I<t/TEST> is running all the
files ending with I<.t> it can found in the I<t/> directory, and looks
at what they print. A typical test prints the following:

1..3 # going to run 3 tests
ok 1 # the first test has passed
ok 2 # the second test has passed
not ok 3 # the third test has failed

C<t/TEST> uses a standard Perl's C<Test::Harness> module which
intercepts the STDOUT parses it and at the end of the tests print the
results of the tests running: how many tests were run, how many
failed, how many suceeded and more.

Some tests may be skipped by printing:

1..0 # all tests in this file are going to be skipped.

You usually want to do that when some feature is optional and the
prerequisites are not installed on the system. Once you test that you
cannot proceed with the tests and it's not a must pass test, you just
skip it.

It's important to know, that there is a special debug mode, enabled
with I<-v> option, in which everything printed by the test goes to
STDOUT. So for example if your test does this:

print "testing : feature foo\n";
print "expected: $expected\n";
print "received: $received\n";
ok $expected eq $received;

in the normal mode, you won't see any of these prints. But if you run
the test with I<t/TEST -v>, you will see something like this:

testing : feature foo
expected: 2
received: 2
ok 2

So when you develop your tests and need to debug them, keep the debug
statements there, don't even comment them out. In fact when you
develop the test you should always put the debug statements
there. This is because if some user reports a failure in some test,
you can ask him to run the failing test in the verbose mode and send
you back the report. Using this report it'll probably be much easier
for you to discover the problem.

In the section L<"Apache::TestUtil"> we discuss a few helper functions
which make the tests writing easier.

For more details about the C<Test::Harness> module please refer to its
manpage.

=head2 One Part Perl tests: Response only

If you write only a response part of the test, C<Apache::Test> will
automatically generate the corresponding test part that will generated
the response. In this case your test should print I<'ok 1'>, I<'not ok
2'> responses as usual tests do. The autogenerated request part will
receive the response and print them out automatically completing the
C<Test::Harness> expectations.

The corresponding request part of the test is named just like the
response part, using the following translation:

$response_test =~ s|t/[^/]+/Test([^/]+)/(.*).pm$|t/\L$1\E/$2.t|;

so for example I<t/response/TestApache/write.pm> becomes:
I<t/apache/write.t>.

If we look at the autogenerated test I<t/apache/write.t>, we can see
that it start with the warning that it has been autogenerated, so you
won't attempt to change it, following by the trace of the calls that
generated this test, in case you want to trace back to who generated
the test, and finally it loads the C<Apache::TestConfig> module and
prints a raw response from the the response part:

use Apache::TestConfig ();
print Apache::TestConfig->thaw->http_raw_get("/TestApache::write");

As you can see the request URI is autogenerated from the response test
name:

$response_test =~ s|.*/([^/]+)/(.*).pm$|/$1::$2|;

So I<t/response/TestApache/write.pm> becomes: I</TestApache::write>.

Now a simple response test may look like this:

package TestApache::write;

use strict;
use warnings FATAL => 'all';

use constant BUFSIZ => 512; #small for testing
use Apache::Const -compile => 'OK';

sub handler {
my $r = shift;
$r->content_type('text/plain');

$r->write("1..2\n");
$r->write("ok 1")
$r->write("not ok 2")

Apache::OK;
}
1;

The configuration part for this test will be autogenerated by the
C<Apache::Test> framework and added to the autogenerated file
I<t/conf/httpd.conf>. In our case the following configuration section
will be added.

<Location /TestApache::write>
SetHandler modperl
PerlResponseHandler TestApache::write
</Location>

You should remember to run:

% t/TEST -clean

so when you run your new tests the new configuration will be added.

=head2 Two parts Perl tests: Request and Response

But in most cases you want to write a two parts test where the client
(request) parts generates various requests and tests the responses.

It's possible that the client part tests a static file or some other
feature that doesn't require a dynamic response. In this case, only
the request part of the test should be written.

If you need to write the complete test, with two parts, you proceed
just like in the previous section, but now you write the client part
of the test by yourself. It's quite easy, all you have to do is to
generate requests and check the response. So a typical test will look
like this:

t/apache/cool.t
-----------
use strict;
use warnings FATAL => 'all';

use Apache::Test;
use Apache::TestUtil;
use Apache::TestRequest;

plan tests => 1; # plan one test.

Apache::TestRequest::module('default');

my $config = Apache::Test::config();
my $hostport = Apache::TestRequest::hostport($config) || '';
print "connecting to $hostport\n";

my $received = $config->http_raw_get("/TestApache::cool", undef);
my $expected = "COOL";

ok t_cmp(
$expected,
$received,
"testing TestApache::cool",
);

See the L<Apache::TestUtil> manpage for more info on the t_cmp()
function.

And the corresponding response part:

t/response/TestApache/cool.pm:
--------------------------
package TestApache::cool;

use strict;
use warnings FATAL => 'all';

use Apache::Const -compile => 'OK';

sub handler {
my $r = shift;
$r->content_type('text/plain');

$r->write("COOL");

Apache::OK;
}
1;

Again, remember to run I<t/TEST -clean> before running the new test so
the configuration will be created for it.

As you can see the test generates a request to I</TestApache::cool>,
and expects it to return I<"COOL">. If we run the test:

% ./t/TEST t/apache/cool

We see:

apache/cool....ok
All tests successful.
Files=1, Tests=1, 1 wallclock secs ( 0.52 cusr + 0.02 csys = 0.54 CPU)

But if we run it in the debug (verbose) mode, we can actually see what
we are testing, what was expected and what was received:

apache/cool....1..1
connecting to localhost:8529
testing : testing TestApache::cool
expected: COOL
received: COOL
ok 1
ok
All tests successful.
Files=1, Tests=1, 1 wallclock secs ( 0.49 cusr + 0.03 csys = 0.52 CPU)

So in case in our simple test we have received something different
from I<COOL> or nothing at all, we can immediately see what's the
problem.

The name of the request part of the test is very important. If
C<Apache::Test> cannot find the corresponding test for the response part
it'll automatically generate one and in this case it's probably not
what you want. Therefore when you choose the filename for the test,
make sure to pick the same C<Apache::Test> will pick. So if the response
part is named: I<t/response/TestApache/cool.pm> the request part
should be named I<t/apache/cool.t>. See the regular expression that
does that in the previous section.

=head2 Tests Written in C

META: to be written

=head1 Writing Test Methodology

META: to be written

=head1 Using Apache::TestUtil

META: to be written

=head1 Using C<Apache::Test> framework

META: to be written

=head2 C<Apache::Test> Inside mod_perl 2.0

There is nothing to be done to add new tests for the mod_perl 2.0,
other than writing the tests as explained before. The rest of the
setup is already in place.

=head2 C<Apache::Test> Standalone

If you have developed an Apache module that you want to develop the
tests for with C<Apache::Test>, you have to prepare a special setup
for it.

Let's say that your package is called C<Apache::Amazing> and we are
going to prepare a setup for it. If the module is going to be
distributed on CPAN or you simply want to take a benefit of all the
package distribution and developing features Perl provides, you
probably already have the right layout in place, but in case you
aren't let's go fast through it:

% mkdir Apache-Amazing
% cd Apache-Amazing

As you have noticed we have created the C<Apache-Amazing> directory
and from now on will be working from it. Next let's prepare a
directory for our module:

% mkdir lib
% mkdir lib/Apache

Now put the module into I<lib/Apache/Amazing.pm>:

package Apache::Amazing;

$Apache::Amazing::VERSION = '0.01';

use Apache::Const -compile => qw(:common);
use Apache::compat ();

sub handler {
$r = shift;
$r->send_http_header('text/plain');
$r->print("Amazing!");
return Apache::OK;
}
1;

The only thing it does is setting the I<text/plan> header and
responding with I<"Amazing!">.

Next prepare the I<Makefile.PL> file:

require 5.6.1;

use ExtUtils::MakeMaker;

use lib qw(../blib/lib lib );

use Apache::TestMM qw(test clean); #enable 'make test'

# prerequisites
my %require =
(
"Apache::Test" => "0.1",
);

# accept the configs from comman line
Apache::TestMM::filter_args();
Apache::TestMM::generate_script('t/TEST');

WriteMakefile
(
NAME => 'Apache::Registry',
VERSION_FROM => 'lib/Apache/Registry.pm',
PREREQ_PM => \%require,
clean => {
FILES => "@{ clean_files() }",
},
);

sub clean_files {
return [@scripts];
}

C<Apache::TestMM> will do a lot of thing for us, such as building a
complete Makefile with prope I<'test'> and I<'clean'> targets,
automatically converting I<.PL> and I<conf/*.in> files and more.

As you see we specify a prerequisites hash with I<Apache::Test> in it,
so if the package gets distributed on CPAN, C<CPAN.pm> shell will know
to fetch and install this required package.

Next we create the test suite. First we create I<t/TEST.PL> which will
be automatically converted into I<t/TEST> during I<perl Makefile.PL>
stage:

#!perl

use strict;
use warnings FATAL => 'all';

use lib qw(lib);

use Apache::TestRunPerl ();

Apache::TestRunPerl->new->run(@ARGV);

Assuming that C<Apache::Test> is already installed on your system and
Perl can find it. If not you should Perl where to find it. For example
you could add:

use lib "../Apache-Test/lib";

if C<Apache::Test> is located in a parallel directory.

As you can see we didn't write the real path to the Perl executable,
but C<#!perl>. When I<t/TEST> is created the correct path will be
placed there automatically.

Next we need to prepare extra Apache configuration bits, so we create
the I<t/conf/extra.conf.in> file which will be automatically converted
into I<t/conf/extra.conf> before the server starts. If the file has
any placeholders like C<@documentroot@>, these will be replaced with
the real values for a specific server that is run. In our case we put
the following configuration bits into this file:

# this file will be Include-d by @ServerRoot@/httpd.conf

# where Apache::Amazing can be found
PerlSwitches -Mlib=@ServerRoot@/../lib
# preload the module
PerlModule Apache::Amazing
<Location /test/amazing>
PerlOptions +GlobalRequest
SetHandler modperl
PerlResponseHandler Apache::Amazing
</Location>

As you can see we just add a simple E<lt>LocationE<gt> container and
tell Apache that the namespace I</test/amazing> should be handled by
C<Apache::Amazing> module.

In case you do a development you may want to force a reload of this
module on each request:

PerlResponseHandler "sub { delete $INC{'Apache/Amazing.pm'}; require Apache::Amazing; Apache::Amazing::handler(shift);}"

Alternatively you can put this code into:

lib/Apache/AmazingReload.pm:
----------------------------
package Apache::AmazingReload;

use lib qw(../lib);

sub handler{
delete $INC{'Apache/Amazing.pm'};
undef *Apache::Amazing::handler; # avoid reload warnings.
eval { require Apache::Amazing; };
if ($@) {
warn "reason: $@";
}
else {
Apache::Amazing::handler(shift);
}
}
1;

And now use this configuration instead:

PerlSwitches -Mlib=@ServerRoot@/../lib
# preload the module
PerlModule Apache::AmazingReload
<Location /test/amazing>
PerlOptions +GlobalRequest
SetHandler modperl
PerlResponseHandler Apache::AmazingReload
</Location>

C<Apache::AmazingReload> will worry to forward all the requests to the
real handler, but first it'll reload it, so any changes will
immediately take an effect.

[META: update this when Apache::Reload will work with 2.0!]

Now we can create a simple test:

t/basic.t
-----------
use strict;
use warnings FATAL => 'all';

use Apache::Amazing;
use Apache::Test;
use Apache::TestUtil;

plan tests => 2;

ok 1; # simple load test

my $config = Apache::Test::config();
my $url = '/test/amazing';
my $data = $config->http_raw_get($url);

ok t_cmp(
"Amazing!",
$data,
"basic test",
);

Now create the README file.

% touch README

Don't forget to put in the relevant information about your module, or
arrange for C<ExtUtils::MakeMaker::WriteMakefile()> to do this for you
with:

WriteMakefile(
...
dist => {
PREOP => 'pod2text lib/Apache/Amazing.pm > README',
},
...
);

in this case C<README> will be created from the pod in
I<lib/Apache/Amazing.pm>.

and finally we create the C<MANIFEST> file, so we can prepare a
complete distribution. Therefore we list all the files that should
enter the distribution including the C<MANIFEST> file itself:

MANIFEST:
---------
lib/Apache/Amazing.pm
t/TEST.PL
t/basic.t
t/conf/extra.conf.in
Makefile.PL
README
MANIFEST

That's it. Now we can build the package. But we need to know where
C<apxs> utility from the installed on our system Apache is located. We
pass its path as an option:

% perl Makefile.PL apxs /path/to/httpd-2.0/bin/apxs
% make
% make test

basic...........ok
All tests successful.
Files=1, Tests=2, 1 wallclock secs ( 0.52 cusr + 0.02 csys = 0.54 CPU)

Now we are ready to distribute the package on CPAN:

% make dist

will create the package which can be immediately uploaded to CPAN. In
this example the generated source package with all the required files
will be called: I<Apache-Amazing-0.01.tar.gz>.

The only thing that we haven't done and hope that you will do is to
write the POD sections for the C<Apache::Amazing> module, explainingg
how amazingly it works and how amazingly it can be deployed by other
users.

=head1 When Tests Should Be Written

=head2 New feature is Added

Every time a new feature is added new tests should be added to cover
the new feature.

=head2 A Bug is Reported

Every time a bug gets reported, before you even attempt to fix the
bug, write a test that exposes the bug. This will make much easier for
you to test whether your fix actually fixes the bug.

Now fix the bug and make sure that test passes ok.

It's possible that a few tests can be written to expose the same
bug. Write them all -- the more tests you have the less chances are
that there is a bug in your code.

If the person reporting the bug is a programmer you may try to ask her
to write the test for you. But usually if the report includes a simple
code that reproduces the bug, it should probably be easy to convert
this code into a test.

=head1 Authors

Stas Bekman E<lt>stas@stason.orgE<gt>

=cut







1.1 modperl-docs/src/user/install/install.pod

Index: install.pod
===================================================================
=head1 Installing mod_perl

=head1 Prerequisites

=head1 Installing from Source

=head1 Installing from Binary Packages

=cut



1.1 modperl-docs/src/user/intro/start_fast.pod

Index: start_fast.pod
===================================================================
=head1 Getting Your Feet Wet with mod_perl

=head1 A Quick Guide to Getting Started with mod_perl



1.1 modperl-docs/src/world/templates/choosing.pod

Index: choosing.pod
===================================================================
=head1 Choosing a Templating System

by Perrin Harkins

=head1 Introduction

Go on, admit it: you've written a templating system. It's okay,
nearly everyone has at some point. You start out with something
beautifully simple like C<$HTML =~ s/\$(\w+)/${$1}/g> and end up
adding conditionals and loops and includes until you've created your
very own unmaintainable monster.

Luckily for you, you are not the first to think it might be nice to get
the HTML out of your code. Many have come before, and more than a few
have put their contributions up on CPAN. At this time, there are so
many templating modules on CPAN that it's almost certain you can find
one that meets your needs. This document aims to be your guide to
those modules, leading you down the path to the templating system of
your dreams.

And, if you just went straight to CPAN in the first place and never
bothered to write your own, congratulations: you're one step ahead of
the rest of us.

=head2 On A Personal Note

Nothing can start an argument faster on the mod_perl mailing list than
a claim that one approach to templating is better than another.
People get very attached to the tools they've chosen. Therefore, let
me say up front that I am biased. I've been at this for a while and I
have opinions about what works best. I've tried to present a balanced
appraisal of the features of various systems in this document, but it
probably won't take you long to figure out what I like. Besides,
attempts to be completely unbiased lead to useless documents that
don't contain any real information. So take it all with a pound of
salt and if you think I've been unfair to a particular tool through a
factual error or omission, let me know.

=head1 Why Use Templates?

Why bother using templates at all? Print statements and CGI.pm were
good enough for Grandpa, so why should you bother learning a new way
to do things?

=head2 Consistency of Appearance

It doesn't take a genius to see that making one navigation bar
template and using it in all of your pages is easier to manage than
hard-coding it every where. If you build your whole site like this,
it's much easier to make site-wide changes in the look and feel.

=head2 Reusability

Along the same lines, building a set of commonly used components makes
it easier to create new pages.

=head2 Better Isolation from Changes

Which one changes more often, the logic of your application or the
HTML used to display it? It actually doesn't matter which you
answered, as long as it's one of them. Templates can be a great
abstraction layer between the application logic and the display logic,
allowing one to be updated without touching the other.

=head2 Division of Labor

Separating your Perl code from your HTML means that when your
marketing department decides everything should be green instead of
blue, you don't have to lift a finger. Just send them to the HTML
coder down the hall. It's a beautiful thing, getting out of the HTML
business.

Even if the same people in your organization write the Perl code and
the HTML, you at last have the opportunity for more people to be
working on the project in parallel.

=head1 What Are the Differences?

Before we look at the available options, let's go through an
explanation of some of the things that make them different.

=head2 Execution Models

Although some try to be flexible about it, most templating systems
expect you to use some variation of the two basic execution models,
which I will refer to as "pipeline" and "callback." In the callback
style, you let the template take over and it has the application's
control flow coded into it. It uses callbacks to modules or snippets
of in-line Perl code to retrieve data for display or perform actions
like user authentication. Some popular examples of systems using this
model include Mason, Embperl, and Apache::ASP.

The pipeline style does all the work up front in a standard CGI or
mod_perl handler, then decides which template to run and passes some
data to it. The template has no control flow logic in it, just
presentation logic, e.g. show this graphic if this item is on sale.
Popular systems supporting this approach include HTML::Template and
Template Toolkit.

The callback model works very well for publishing-oriented sites where
the pages are essentially mix and match sets of articles and lists.
Ideally, a site can be broken down into visual "components" or pieces
of pages which are general enough for an HTML coder to re-combine them
into entirely new kinds of pages without any help from a programmer.

The callback model can get a bit hairy when you have to code logic
that can result in totally different content being returned. For
example, if you have a system that processes some form input and takes
the user to different pages depending on the data submitted. In these
situations, it's easy to end up coding a spaghetti of includes and
redirects, or putting what are really multiple pages in the same file.

On the other hand, a callback approach can result in fewer files (if
the Perl code is in the HTML file), and feels easier and more
intuitive to many developers. It's a simple step from static files to
static files with a few in-line snippets of code in them. This is part
of why PHP is so popular with new developers.

The pipeline model is more like a traditional model-view-controller
design. Working this way can provide additional performance tuning
opportunities over an approach where you don't know what data will be
needed at the beginning of the request. You can aggregate database
queries, make smarter choices about caching, etc. It can also promote
a cleaner separation of application logic and presentation. However,
this approach takes longer to get started with since it's a bigger
conceptual hurdle and always involves at least two files: one for the
Perl code and one for the template.

Keep in mind, many systems offer significant flexibility for
customizing their execution models. For example, Mason users could
write separate components for application logic and display, letting
the logic components choose which display component to run after
fetching their data. This allows it to be used in a pipeline style.
A Template Toolkit application could be written to use a simple
generic handler (like the Apache::Template module included in the
distribution) with all the application logic placed in the template
using object calls or in-line Perl. This would be using it in a
callback style.

HTML::Template and some of the AxKit XML processors are fairly rigid
about insisting on a pipeline approach. Neither provide methods for
calling back into Perl code during the HTML formatting stage; you have
to do the work before running the template. The authors of these
tools consider this a feature since it prevents developers from
cheating on the separation of application code and presentation.

=head2 Languages

Here's the big issue with templating systems. This is the one that
always cranks up the flame on web development mailing lists.

Some systems use in-line Perl statements. They may provide some extra
semantics, like Embperl's operators for specifying whether the code's
output should be displayed or Mason's C<E<lt>%initE<gt>> sections for specifying
when the code gets run, but at the end of the day your templates are
written in Perl.

Other systems provide a specialized mini-language instead of (or in
addition to) in-line Perl. These will typically have just enough
syntax to handle variable substitution, conditionals, and looping.
HTML::Template and Template Toolkit are popular systems using this
approach. AxKit straddles the fence, providing both a (not-so-)
mini-language - XSLT - and an in-line Perl approach - XPathScript.

Here's how a typical discussion of the merits of these approaches might go:

B<IN-LINE:> Mini-languages are stupid. I already know Perl and it's easy
enough. Why would you want to use something different?

B<MINI-LANG:> Because my HTML coder doesn't know Perl, and this is easier
for him.

B<IN-LINE:> Maybe he should learn some Perl. He'd get paid more.

B<MINI-LANG:> Whatever. You just want to use in-line Perl so you can
handle change requests by putting little hacks in the template instead
of changing your modules. That's sloppy coding.

B<IN-LINE:> That's efficient coding. I can knock out data editing
screens in half the time it takes you, and then I can go back through,
putting all the in-line code into modules and just have the templates
call them.

B<MINI-LANG:> You could, but you won't.

B<IN-LINE:> Is it chilly up there in that ivory tower?

B<MINI-LANG:> Go write some VBScript, weenie.

etc.

Most people pick a side in this war and stay there. If you are one of
the few who hasn't fully decided yet, you should take a moment to
think about who will be building and maintaining your templates, what
skills those people have, and what will allow them to work most
efficiently.

Here's an example of a simple chunk of template using first an in-line
style (Apache::ASP in this case) and then a mini-language style
(Template Toolkit). This code fetches an object and displays some
properties of it. The data structures used are identical in both
examples. First Apache::ASP:

<% my $product = Product->load('sku' => 'bar1234'); %>

<% if ($product->isbn) { %>
It's a book!
<% } else { %>
It's NOT a book!
<% } %>

<% foreach my $item (@{$product->related}) { %>
You might also enjoy <% $item->name %>.
<% } %>

And now Template Toolkit:

[% USE product(sku=bar1234) %]

[% IF product.isbn %]
It's a book!
[% ELSE %]
It's NOT a book!
[% END %]

[% FOREACH item = product.related %]
You might also enjoy [% item.name %].
[% END %]

There is a third approach, based on parsing an HTML document into a
DOM tree and then manipulating the contents of the nodes. The only
module using this approach is HTML_Tree. The idea is similar to using
a mini-language, but it doesn't require any non-standard HTML tags and
it doesn't embed any logic about loops or conditionals in the template
itself. This is nice because it means your templates are valid HTML
documents that can be viewed in a browser and worked with in most
standard HTML tools. It also means people working with the templates
can put placeholder data in them for testing and it will simply be
replaced when the template is used. This preview ability only breaks
down when you need an if/else type construct in the template. In that
situation, both the "if" and "else" chunks of HTML would show up when
previewing.

=head2 Parsers and Caching

The parsers for these templating systems are implemented in one of
three ways: they parse the template every time ("repeated parse"),
they parse it and cache the resulting parse tree ("cached parse
tree"), or they parse it, convert it to Perl code, and compile it
("compiled").

Systems that compile templates to Perl take advantage of Perl's
powerful runtime code evaluation capabilities. They examine the
template, generate a chunk of Perl code from it, and C<eval> the
generated code. After that, subsequent requests for the template can
be handled by running the compiled bytecode in memory. The complexity
of the parsing and code generation steps varies based on the number of
bells and whistles the system provides beyond straight in-line Perl
statements.

Compiling to Perl and then to Perl bytecode is slow on the first hit
but provides excellent performance once the template has been
compiled, since the template becomes a Perl subroutine call. This is
the same approach used by systems like JSP (Java ServerPages). It is
most effective in environments with a long-running Perl interpreter,
like mod_perl.

HTML::Template, HTML_Tree, and the 2.0 beta release of Embperl all use
a cached parse tree approach. They parse templates into their
respective internal data structures and then keep the parsed structure
for each processed template in memory. This is similar to the
compiled Perl approach in terms of performance and memory
requirements, but does not actually involve Perl code generation and
thus doesn't require an C<eval> step. Which way is faster, caching
the parse tree or compiling? It's hard to objectively measure, but
anecdotal evidence seems to support compilation. Template Toolkit
used a cached parse tree approach for version 1, but switched to a
compilation approach for version 2 after tests showed it to offer a
significant speed increase. However, as will be discussed later,
either approach is more than fast enough.

In contrast to this, a repeated parse approach may sound very slow.
However, it can be pretty fast if the tokens being parsed for are
simple enough. Systems using this approach generally use very simple
tokens, which allows them to use fast and simple parsers.

Why would you ever use a system with this approach if compilation has
better performance? Well, in an environment without a persistent Perl
interpreter like vanilla CGI this can actually be faster than a
compiled approach since the startup cost is lower. The caching of
Perl bytecode done by compilation systems is useless when the Perl
interpreter doesn't stick around for more than one request.

There are other reasons too. Compiled Perl code takes up a lot of
memory. If you have many unique templates, they can add up fast.
Imagine how much RAM it would take up if every page that used
server-side includes (SSI) had to stay in memory after it had been
accessed. (Don't worry, the C<Apache::SSI> module doesn't use
compilation so it doesn't have this problem.)

=head2 Application Frameworks vs. Just Templates

Some of the templating tools try to offer a comprehensive solution to
the problems of web development. Others offer just a templating
solution and assume you will fit this together with other modules to
build a complete system.

Some common features offered in the frameworks include:

=head3 URL Mapping

All of the frameworks offer a way to map a URL to a template file. In
addition to simple mappings similar to the handling of static
documents, some offer ways to intercept all requests within a certain
directory for pre-processing, or create an object inheritance scheme
out of the directory structure of a site.

=head3 Session Tracking

Most interactive sites need to use some kind of session tracking to
associate application state data with a user. Some tools make this
very easy by handling all the cookies or URL-munging for you and
letting you simply read and write from an object or hash that contains
the current user's session data. A common approach is to use the
Apache::Session module for storage.

=head3 Output Caching

Caching is the key to good performance in many web systems, and some
of these tools provide user-controlled caching of output. This is one
of the major features of both Mason and AxKit. AxKit can cache at the
page level, while Mason also offers fine-grained caching of components
within the page.

=head3 Form Handling

How will you live without CGI.pm to parse incoming form data? Many of
these tools will do it for you, making it available in a convenient
data structure. Some also validate form input, and even provide
"sticky" form widgets that keep their selected values when
re-displayed or set up default values based on data you provide.

=head3 Debugging

Everyone knows how painful it can be to debug a CGI script.
Templating systems can make it worse, by screwing up Perl's line
numbers with generated code. To help fix the problem they've created,
some offer built-in debugging support, including extra logging, or
integration with the Perl debugger.

If you want to use a system that just does templates but you need some
of these other features and don't feel like implementing them
yourself, there are some tools on CPAN which provide a framework you
can build on. The libservlet distribution, which provides an
interface similar to the Java servlet API, is independent of any
particular templating system. Apache::PageKit and CGI::Application
are other options in this vein, but both of these are currently tied
to HTML::Template. OpenInteract is another framework, this time tied
to Template Toolkit. All of these could be adapted for the "just
templates" module of your choice with fairly minimal effort.

=head1 The Contenders

Okay, now that you know something about what separates these tools
from each other, let's take a look at the top choices for Perl
templating systems. This is not an exhaustive list: I've only
included systems that are currently maintained, well-documented, and
have managed to build up a significant user community. In short, I've
left out a dozen or so less popular systems. At the end of this
section, I'll mention a few systems that aren't as commonly used but
may be worth a look.

=head2 SSI

SSI is the granddaddy of templating systems, and the first one that
many people used since it comes as a standard part of most web
servers. With mod_perl installed, mod_include gains some additional
power. Specifically, it is able to take a new #perl directive which
allows for in-line subroutine calls. It can also efficiently include
the output of Apache::Registry scripts by using the Apache::Include
module.

The Apache::SSI module implements the functionality of mod_include
entirely in Perl, including the additional #perl directive. The main
reasons to use it are to post-process the output of another handler
(with Apache::Filter) or to add your own directives. Adding
directives is easy through subclassing. You might be tempted to
implement a complete template processor in this way, by adding loops
and other constructs, but it's probably not worth the trouble with so
many other tools out there.

SSI follows the callback model and is mostly a mini-language, although
you can sneak in bits of Perl code as anonymous subs in #perl
directives. Because SSI uses a repeated parse implementation, it is
safe to use it on large numbers of files without worrying about memory
bloat.

SSI is a great choice for sites with fairly simple templating needs,
especially ones that just want to share some standard headers and
footers between pages. However, you should consider whether or not
your site will eventually need to grow into something with more
flexibility and power before settling on this simple approach.

=head2 HTML::Mason

Mason has been around for a few years now, and has built up a loyal
following. It was originally created as a Perl clone of some of the
most interesting features from Vignette StoryServer, but has since
become it's own unique animal. It comes from a publishing background,
and includes features oriented towards splitting up pages into
re-useable chunks, or "components."

Mason uses in-line Perl with a compilation approach, but has a feature
to help keep the perl code out of the HTML coder's way. Components
(templates) can include a section of Perl at the end of the file which
is wrapped inside a special tag indicating that it should be run
first, before the rest of the template. This allows programmers to
put all the logic for a component down at the bottom away from the
HTML, and then use short in-line Perl snippets in the HTML to insert
values, loop through lists, etc.

Mason is a site development framework, not just a templating tool. It
includes a very handy caching feature that can be used for capturing
the output of components or simply storing data that is expensive to
compute. It is currently the only tool that offers this sort of
caching as a built-in. It also implements an argument parsing scheme
which allows a component to specify the names, types, and default
values that it expects to be passed, either from another component or
from the values passed in the URI query string.

While the documentation mostly demonstrates a callback execution
model, it is possible to use Mason in a pipeline style. This can be
accomplished in various ways, including designating components as
"autohandlers" which run before anything else for requests within a
certain directory structure. An autohandler could do some processing
and set up data for a display template which only includes minimal
in-line Perl. There is also support for an object-oriented site
approach, applying concepts like inheritance to the site directory
structure. For example, the component at /store/book/ might inherit a
standard layout from the component at /store/, but override the
background color and navigation bar. Then /store/music/ can do the
same, with a different color. This can be a very powerful paradigm
for developing large sites.

Mason's approach to debugging is to create "debug files" which run
Mason outside of a web server environment, providing a fake web
request and activating the debugger. This can be helpful if you're
having trouble getting Apache::DB to behave under mod_perl, or using
an execution environment that doesn't provide built-in debugger
support.

Another unique feature is the ability to leave the static text parts
of a large template on disk, and pull them in with a file seek when
needed rather than keeping them in RAM. This exchanges some speed for
a significant savings in memory when dealing with templates that are
mostly static text.

There are many other features in this package, including filtering of
HTML output and a page previewing utility. Session support is not
built-in, but a simple example showing how to integrate with
Apache::Session is included. Mason's feature set can be a bit
overwhelming for newbies, but the high-quality documentation and
helpful user community go a long way.

=head2 HTML::Embperl

Embperl makes its language choice known up front: embedded perl. It
is one of the most popular in-line Perl templating tools and has been
around longer than most of the others. It has a solid reputation for
speed and ease of use.

It is commonly used in a callback style, with Embperl intercepting
URIs and processing the requested file. However, it can optionally be
invoked through a subroutine call from another program, allowing it to
be used in a pipeline style. Templates are compiled to Perl bytecode
and cached.

Embperl has been around long enough to build up an impressive list of
features. It has the ability to run code inside a Safe compartment,
support for automatically cleaning up globals to make mod_perl coding
easier, and extensive debugging tools including the ability to e-mail
errors to an administrator.

The main thing that sets Embperl apart from other in-line Perl systems
is its tight HTML integration. It can recognize C<TABLE> tags and
automatically iterate over them for the length of an array. It
automatically provides sticky form widgets. An array or hash
reference placed at the end of a query string in an C<HREF> or C<SRC>
attribute will be automatically expanded into query string
"name=value" format. C<META HTTP-EQUIV> tags are turned into true HTTP
headers.

Another reason people like Embperl is that it makes some of the common
tasks of web application coding so simple. For example, all form data
is always available just by reading the magic variable %fdat.
Sessions are supported just as easily, by reading and writing to the
magic %udat hash. There is also a hash for storing persistent
application state. HTML-escaping is automatic (though it can be
toggled on and off).

Embperl includes something called EmbperlObject, which allows you to
apply OO concepts to your site hierarchy in a similar way to the
inheritance features mentioned for Mason, above. This is a very
convenient way to code sites with styles that vary by area, and is
worth checking out.

One drawback of older versions of Embperl was the necessity to use
built-in replacements for most of Perl's control structures like "if"
and "foreach" when they are being wrapped around non-Perl sections.
For example:

[$ if ($foo) $]
Looks like a foo!
[$ else $]
Nope, it's a bar.
[$ endif $]

These may seem out of place in a system based around in-line Perl. As
of version 1.2b2, it is possible to use Perl's standard syntax instead:

[$ if ($foo) { $]
Looks like a foo!
[$ } else { $]
Nope, it's a bar.
[$ } $]

At the time of this writing, a new 2.x branch of Embperl is in beta
testing. This includes some interesting features like a more flexible
parsing scheme which can be modified to users' tastes. it also
supports direct use of the Perl debugger on Embperl templates, and
provides performance improvements.

=head2 Apache::AxKit

AxKit is the first mod_perl page generation system to be built from the
ground up around XML. Technically, AxKit itself is not a templating
tool but rather a framework for stringing together different modules
that generate and transform XML data. In fact, it can optionally use
Template Toolkit as an XML transformation language. However, it
deserves coverage here since it is also the home of some templating
tools that are not represented elsewhere.

In its simplest form, AxKit maps XML files to XSL stylesheets which it
can process using commonly available XSLT modules like XML::XSLT or
XML::Sablotron. The rules for mapping a stylesheet to a request are
very flexible, and they can incorporate query strings, cookies, and
other attributes of the request. The idea is that you can use this
feature to handle a wide variety of clients with differing display
capabilities by choosing the right stylesheet.

Recognizing that not everyone is a fan of XSL's somewhat obtuse
syntax, Matt Sergeant has provided an alternate stylesheet language
called XPathScript. XPathScript allows you to write a stylesheet
using text with embedded Perl code. This is similar to the other
embedded Perl templating tools, but the focus is on using the built in
XPath functions for querying an XML document and manipulating the
retrieved data. XPathScript can also be used in a declarative
fashion, specifying the formatting of particular elements in the XML
input. For example this snippet will change all C<E<lt>fooE<gt>> tags
in an XML document to BAR in the output::

<%
$t->{'foo'}{pre} = 'BAR';
$t->{'foo'}{post} = '';
$t->{'foo'}{showtag} = 0;
%>
<%= apply_templates() %>

By using XPathScript's include function (which looks just like SSI),
you can build up libraries of useful transformations that use this
technique.

This is all well and good if you have a bunch of XML files sitting on
a disk somewhere, but what about dynamic content? AxKit handles this
by allowing you to substitute a different data source for the default
file-based one. This can include running some dynamic code on each
request to generate the XML data that will be transformed. The
distribution includes a module for doing this called XSP. XSP is a
language for building an XML DOM using in-line Perl and tag libraries.
The tag libraries are specified as stylesheets which can turn XML tags
into Perl code. This is demonstrated through the included SQL tag
library, which allows you to write an XSP page using XML tags which
will connect to a database, execute queries, and generate an XML
document with the results.

AxKit has some nice performance boosts built into it. It can cache
the full output of a page and serve it as a static file on future
requests. It can also compress output to speed up downloads for
browsers that understand gzip encoding. These can be done with other
systems, but they require you to setup additional software . With
AxKit, you just enable them in the configuration file.

If all of these languages, tag libraries, and stylesheets sound
intimidating to you, AxKit may be overkill for your project. However,
AxKit has the advantage of being built on approved W3C standards, and
many of the skills used in developing for it carry over to other
languages and tools.

=head2 Apache::ASP

Apache::ASP started out as a port of Microsoft's Active Server Pages
technology, and its basic design still follows that model. It uses
in-line Perl with a compilation approach, and provides a set of simple
objects for accessing the request information and formulating a
response. Scripts written for Microsoft's ASP using Perl (via
ActiveState's PerlScript) can usually be run on this system without
changes. (Pages written in VBScript are not supported.)

Like the original ASP, it has hooks for calling specified code when
certain events are triggered, such as the start of a new user session.
It also provides the same easy-to-use state and session management.
Storing and retrieving state data for a whole application or a
specific user is as simple as a single method call. It can even
support user sessions without cookies by munging URLs -- a unique
feature among these systems.

A significant addition that did not come from Microsoft ASP is the XML
and XSLT support. There are two options provided: XMLSubs and XSLT
transforms. XMLSubs is a way of adding custom tags to your pages. It
maps XML tags to your subroutines, so that you can add something like
C<E<lt>site:header page="Page Title" /E<gt>> to your pages and have it
translate into a subroutine call like C<&site::header({title =E<gt>
"Page Title"})>. It can handle processing XML tags with body text as
well.

The XSLT support allows the output of ASP scripts to be filtered
through XSLT for presentation. This allows your ASP scripts to
generate XML data and then format that data with a separate XSL
stylesheet. This support is provided through integration with the
XML::XSLT module.

Apache::ASP provides sticky widgets for forms through the use of the
HTML::FillInForm module. It also has built-in support for removing
extra whitespace from generated output, gzip compressing output (for
browsers that support it), tracking performance using Time::HiRes,
automatically mailing error messages to an administrator, and many
other conveniences and tuning options. This is a mature package which
has evolved to handle real-world problems.

One thing to note about the session and state management in this
system is that it currently only supports clusters through the use of
network filesystems like NFS or SMB. (Joshua Chamas, the module's
author, has reported much better results from Samba file-sharing than
from NFS.) This may be an issue for large-scale server clusters,
which usually rely on a relational database for network storage of
sessions. Support database storage of sessions is planned for a
future release.

=head2 Text::Template

This module has become the de facto standard general purpose
templating module on CPAN. It has an easy interface and thorough
documentation. The examples in the docs show a pipeline execution
style, but it's easy to write a mod_perl handler that directly invokes
templates, allowing a callback style. The module uses in-line Perl.
It has the ability to run the in-line code in a Safe compartment, in
case you are concerned about mistakes in the code crashing your
server.

The module relies on creative uses of in-line code to provide things
that people usually expect from templating tools, like includes. This
can be good or bad. For example, to include a file you could just
call Text::Template::fill_in_file(filename). However, you'll have to
specify the complete file path and nothing will stop you from using
/etc/passwd as the file to be included. Most of the fancier
templating tools have concepts like include paths, which allow you to
specify a list of directories to search for included files. You could
write a subroutine that works this way, and make it available in your
template's namespace, but it's not built in.

Each template is loaded as a separate object. Templates are compiled
to Perl and only parsed the first time they are used. However, to
take full advantage of this caching in a persistent environment like
mod_perl, your program will have to keep track of which templates have
been used, since Text::Template does not have a way of globally
tracking this and returning cached templates when possible.

Text::Template is not tied to HTML, and is just a templating module,
not a web application framework. It is perfectly at home generating
e-mails, PDFs, etc.

=head2 Template Toolkit

One of the more recent additions to the templating scene, Template
Toolkit is a very flexible mini-language system. It has a complete
set of directives for working with data, including loops and
conditionals, and it can be extended in a number of ways. In-line
Perl code can be enabled with a configuration option, but is generally
discouraged. It uses compilation, caching the compiled bytecode in
memory and optionally caching the generated Perl code for templates on
disk. Although it is commonly used in a pipeline style, the included
Apache::Template module allows templates to be invoked directly from
URLs.

Template Toolkit has a large feature set, so we'll only be able cover
some of the highlights here. The TT distribution sets a gold standard
for documentation thoroughness and quality, so it's easy to learn more
if you choose to.

One major difference between TT and other systems is that it provides
simple access to complex data structures through the concept of a dot
operator. This allows people who don't know Perl to access nested
lists and hashes or call object methods. For example, we could pass
in this Perl data structure:

$vars = {
customer => {
name => 'Bubbles',
address => {
city => 'Townsville',
}
}
};

Then we can refer to the nested data in the template:

Hi there, [% customer.name %]!
How are things in [% customer.address.city %]?

This is simpler and more uniform than the equivalent syntax in Perl.
If we pass in an object as part of the data structure, we can use the
same notation to call methods within that object. If you've modeled
your system's data as a set of objects, this can be very convenient.

Templates can define macros and include other templates, and
parameters can be passed to either. Included templates can optionally
localize their variables so that changes made while the included
template is executing do not affect the values of variables in the
larger scope.

There is a filter directive, which can be used for post-processing
output. Uses for this range from simple HTML entity conversion to
automatic truncation (useful for pulldown menus when you want to limit
the size of entries) and printing to STDERR.

TT supports a plugin API, which can be used to add extra capabilities
to your templates. The provided plugins can be broadly organized into
data access and formatting. Standard data access plugins include
modules for accessing XML data or a DBI data source and using that
data within your template. There's a plugin for access to CGI.pm as
well.

Formatting plugins allow you to display things like dates and prices
in a localized style. There's also a table plugin for use in
displaying lists in a multi-column format. These formatting plugins
do a good job of covering the final 5% of data display problems that
often cause people who are using an in-house system to embed a little
bit of HTML in their Perl modules.

In a similar vein, TT includes some nice convenience features for
template writers like eliminating white space around tags and the
ability to change the tag delimiters -- things that may sound a little
esoteric, but can sometimes make templates significantly easier to
work with.

The TT distribution also includes a script called ttree which allows
for processing an entire directory tree of templates. This is useful
for sites that pre-publish their templated pages and serve them
statically. The script checks modification times and only updates
pages that require it, providing a make-like functionality. The
distribution also includes a sample set of template-driven HTML
widgets which can be used to give a consistent look and feel to a
collection of documents.

=head2 HTML::Template

HTML::Template is a popular module among those looking to use a
mini-language rather than in-line Perl. It uses a simple set of tags
which allow looping (even on nested data structures) and conditionals
in addition to basic value insertion. The tags are intentionally
styled to look like HTML tags, which may be useful for some
situations.

As the documentation says, it "does just one thing and it does quickly
and carefully" -- there is no attempt to add application features like
form-handling or session tracking. The module follows a pipeline
execution style. Parsed templates are stored in a Perl data structure
which can be cached in any combination of memory, shared memory (using
IPC::SharedCache), and disk. The documentation is complete and
well-written, with plenty of examples.

You may be wondering how this module is different from Template
Toolkit, the other popular mini-language system. Beyond the obvious
differences in syntax, HTML::Template is faster and simpler, while
Template Toolkit has more advanced features, like plugins and dot
notation. Here's a simple example comparing the syntax:

HTML::Template:

<TMPL_LOOP list>
<a href="<TMPL_VAR url>"><b><TMPL_VAR name></b></A>
</TMPL_LOOP>

Template Toolkit:

[% FOREACH list %]
<a href="[% url %]"><b>[% name %]</a></a>
[% END %]

And now, a few honorable mentions:

=head2 HTML_Tree

As mentioned earlier, HTML Tree uses a fairly unique method of
templating: it loads in an HTML page, parses it to a DOM, and then
programmatically modifies the contents of nodes. This allows it to
use genuine valid HTML documents as templates, something which none of
these other modules can do. The learning curve is a little steeper
than average, but this may be just the thing if you are concerned
about keeping things simple for your HTML coders. Note that the name
is "HTML_Tree", not "HTML::Tree".

=head2 Apache::XPP

XPP is an in-line Perl system that compiles to bytecode. Although it
is a perfectly good implementation, it has little to differentiate it
except for an easy mechanism to define new HTML-like tags which can be
used to replace in-line code in templates.

=head2 ePerl

Possibly the first module to embed Perl code in a text or HTML file,
ePerl is still a viable option in the form of Apache::ePerl. It
caches compiled bytecode in memory to achieve solid performance, and
some people find it refreshingly simple to use.

=head2 CGI::FastTemplate

This module takes a minimalistic approach to templating, which makes
it unusually well suited to use in CGI programs. It parses templates
with a single regular expression and does not support anything in
templates beyond simple variable interpolation. Loops are handled by
including the output of other templates. Unfortunately, this leads to
a Perl coding style that is more confusing than most, and a
proliferation of template files. However, some people swear by this
dirt-simple approach.

=head1 Performance

People always seem to worry about the performance of templating
systems. If you've ever built a large-scale application, you should
have enough perspective on the relative costs of different actions to
know that your templating system is not the first place to look for
performance gains. All of the systems mentioned here have excellent
performance characteristics in persistent execution environments like
mod_perl. Compared to such glacially slow operations as fetching data
from a database or file, the time added by the templating system is
almost negligible.

If you think your templating system is slowing you down, get the
facts: pull out Devel::DProf and see. If one of the tools mentioned
here is at the top of the list for wall clock time used, you should
pat yourself on the back -- you've done a great job tuning your system
and removing bottlenecks! Personally, I have only seen this happen
when I had managed to successfully cache nearly every part of the work
to handle a request except running a template.

However, if you really are in a situation where you need to squeeze a
few extra microseconds out of your page generation time, there are
performance differences between systems. They're pretty much what you
would expect: systems that do the least run the fastest. Using
in-line print() statements is faster than using templates. Using
simple substitution is faster than using in-line Perl code. Using
in-line Perl code is faster than using a mini-language.

The only templating benchmark available at this time is one developed
by Joshua Chamas, author of Apache::ASP. It includes a "hello world"
test, which simply checks how fast each system can spit back those
famous words, and a "hello 2000" test, which exercises the basic
functions used in most dynamic pages. It is available from the
following URL:

http://www.chamas.com/bench/hello.tar.gz

Results from this benchmark currently show SSI, Apache::ASP, and
HTML::Embperl having the best performance of the lot. Not all of the
systems mentioned here are currently included in the test. If your
favorite was missed, you might want to download the benchmark code and
add it. As you can well imagine, benchmarking people's pet projects
is largely a thankless task and Joshua deserves some recognition and
support for this contribution to the community.

=head2 CGI Performance Concerns

If you're running under CGI, you have bigger fish to fry than worrying
about the performance of your templating system. Nevertheless, some
people are stuck with CGI but still want to use a templating system
with reasonable performance. CGI is a tricky situation, since you
have to worry about how much time it will take for Perl to compile the
code for a large templating system on each request. CGI also breaks
the in-memory caching of templates used by most of these systems,
although the slower disk-based caching provided by Mason,
HTML::Template, and Template Toolkit will still work. (HTML::Template
does provide a shared memory cache for templates, which may improve
performance, although shared memory on my Linux system is usually
slower than using the filesystem. Benchmarks and additional
information are welcome.)

Your best performance bet with CGI is to use one of the simpler tools,
like CGI::FastTemplate or Text::Template. They are small and compile
quickly, and CGI::FastTemplate gets an extra boost since it relies on
simple regex parsing and doesn't need to eval any in-line Perl code.
Almost everything else mentioned here will add tenths of seconds to
each page in compilation time alone.

=head1 Matrix

To help you choose a system, I'll summarize the basic characteristics
of the major systems along the decision points I've explained in the
beginning of the article. Keep in mind that in many cases a system
can be used in more than one way, and I've simply shown the dominant
method as seen in the documentation and real world use. You should
not eliminate options based on this chart without reading the more
detailed explanations above.

=begin pod2html

<TABLE BORDER=1>
<TR>
<TH>
</TD>
<TH>
Application Framework
</TD>
<TH>
Pipeline or Callback
</TD>
<TH>
Parsing Method
</TD>
<TH>
Language
</TD>
</TR>
<TR>
<TD>
HTML::Mason
</TD>
<TD>
Framework
</TD>
<TD>
Callback
</TD>
<TD>
Compiled
</TD>
<TD>
Perl
</TD>
</TR>
<TR>
<TD>
Template Toolkit
</TD>
<TD>
Just Templates
</TD>
<TD>
Pipeline
</TD>
<TD>
Compiled
</TD>
<TD>
Mini-Language
</TD>
</TR>
<TR>
<TD>
Apache::ASP
</TD>
<TD>
Framework
</TD>
<TD>
Callback
</TD>
<TD>
Compiled
</TD>
<TD>
Perl and XSL
</TD>
</TR>
<TR>
<TD>
HTML::Embperl
</TD>
<TD>
Framework
</TD>
<TD>
Callback
</TD>
<TD>
Compiled
</TD>
<TD>
Perl
</TD>
</TR>
<TR>
<TD>
SSI
</TD>
<TD>
Just Templates
</TD>
<TD>
Callback
</TD>
<TD>
Repeated Parse
</TD>
<TD>
Mini-Language
</TD>
</TR>
<TR>
<TD>
AxKit
</TD>
<TD>
Framework
</TD>
<TD>
Pipeline
</TD>
<TD>
Compiled or Cached Parse Tree
</TD>
<TD>
Perl and XSL and Mini-Language(s)
</TD>
</TR>
<TR>
<TD>
HTML::Template
</TH>
<TD>
Just Templates
</TD>
<TD>
Pipeline
</TD>
<TD>
Cached Parse Tree
</TD>
<TD>
Mini-Language
</TD>
</TR>
<TR>
<TD>
Text::Template
</TD>
<TD>
Just Templates
</TD>
<TD>
Pipeline
</TD>
<TD>
Compiled
</TD>
<TD>
Perl
</TD>
</TR>
</TABLE>

=end pod2html

=head1 Updates

These modules are moving targets, and a document like this is bound to
contain some mistakes. Send your corrections to perrin@elem.com.
Future versions of this document will be announced on the mod_perl
mailing list, and possibly other popular Perl locations as well.