=head1 NAME

Overview of mod_perl 2.0

=head1 Description

This chapter should give you a general idea about what mod_perl 2.0 is
and how it differs from mod_perl 1.0.  This chapter presents the new
features of Apache 2.0, Perl 5.6.0 -- 5.8.0 and their influence on
mod_perl 2.0. The new MPM models from Apache 2.0 are also discussed.



=head1 Version Naming Conventions

In order to keep things simple, here and in the rest of the
documentation we refer to mod_perl 1.x series as mod_perl 1.0 and to
2.0.x series as mod_perl 2.0. Similarly we call Apache 1.3.x series as
Apache 1.3 and 2.0.x as Apache 2.0. There is also Apache 2.1, which is
a development track towards Apache 2.2.

=head1 Why mod_perl, The Next Generation

mod_perl was introduced in early 1996, both Perl and Apache have
changed a great deal since that time. mod_perl has adjusted to both
along the way over the past 4 and a half years or so using the same
code base.  Over this course of time, the mod_perl sources have become
more and more difficult to maintain, in large part to provide
compatibility between the many different flavors of Apache and Perl.
And, compatibility across these versions and flavors is a more
difficult goal for mod_perl to reach that a typical Apache or Perl
module, since mod_perl reaches a bit deeper into the corners of Apache
and Perl internals than most.  Discussions of the idea to rewrite
mod_perl as version 2.0 started in 1998, but never made it much
further than an idea.  When Apache 2.0 development was underway it
became clear that a rewrite of mod_perl would be required to adjust to
the new Apache architecture and API.

Of the many changes happening in Apache 2.0, the one which has the
most significant impact on mod_perl is the introduction of threads to
the overall design.  Threads have been a part of Apache on the win32
side since the Apache port was introduced.  The mod_perl port to win32
happened in version 1.00b1, released in June of 1997.  This port
enabled mod_perl to compile and run in a threaded windows environment,
with one major caveat: only one concurrent mod_perl request could be
handled at any given time.  This was due to the fact that Perl did not
introduce thread-safe interpreters until version 5.6.0, released in
March of 2000.  Contrary to popular belief, the "threads support"
implemented in Perl 5.005 (released July 1998), did not make Perl
thread-safe internally.  Well before that version, Perl had the notion
of "Multiplicity", which allowed multiple interpreter instances in the
same process.  However, these instances were not thread safe, that is,
concurrent callbacks into multiple interpreters were not supported.

It just so happens that the release of Perl 5.6.0 was nearly at the
same time as the first alpha version of Apache 2.0.  The development
of mod_perl 2.0 was underway before those releases, but as both Perl
5.6.0 and Apache 2.0 were reaching stability, mod_perl 2.0 was
becoming more of a reality.  In addition to the adjustments for
threads and Apache 2.0 API changes, this rewrite of mod_perl is an
opportunity to clean up the source tree.  This includes both removing
the old backward compatibility bandaids and building a smarter,
stronger and faster implementation based on lessons learned over the
4.5 years since mod_perl was introduced.

The new version includes a mechanism for the automatic building of the
Perl interface to Apache API, which allowed us to easily adjust
mod_perl 2.0 to the ever changing Apache 2.0 API, during its development
period. Another important feature is the
C<L<Apache::Test|docs::general::testing::testing>> framework, which
was originally developed for mod_perl 2.0, but then was adopted by
Apache 2.0 developers to test the core server features and third party
modules. Moreover the tests written using the
C<L<Apache::Test|docs::general::testing::testing>> framework could be
run with Apache 1.0 and 2.0, assuming that both supported the same
features.

There are multiple other interesting changes that have already
happened to mod_perl in version 2.0 and more will be developed in the
future. Some of these are discussed in this chapter, others can be
found in the rest of the mod_perl 2.0 documentation.

=head1 What's new in Apache 2.0

Apache 2.0 has introduced numerous new features and enhancements. Here
are the most important new features:

=over

=item * I<Apache Portable Runtime> (APR)

Apache 1.3 has been ported to a very large number of platforms
including various flavors of unix, win32, os/2, the list goes on.
However, in 1.3 there was no clear-cut, pre-designed portability layer
for third-party modules to take advantage of.  APR provides this API
layer in a very clean way.  APR assists a great deal with mod_perl
portability.  Combined with the portablity of Perl, mod_perl 2.0 needs
only to implement a portable build system, the rest comes "for free".
A Perl interface is provided for certain areas of APR, such as the
shared memory abstraction, but the majority of APR is used by mod_perl
"under the covers".

The APR uses the concept of memory pools, which significantly
simplifies the memory management code and reduces the possibility of
having memory leaks, which always haunt C programmers.

=item * I/O Filtering

Filtering of Perl modules output has been possible for years since
tied filehandle support was added to Perl.  There are several modules,
such as C<Apache2::Filter> and C<Apache::OutputChain> which have been
written to provide mechanisms for filtering the C<STDOUT> stream.
There are several of these modules because no one's approach has quite
been able to offer the ease of use one would expect, which is due
simply to limitations of the Perl tied filehandle design.  Another
problem is that these filters can only filter the output of other Perl
modules. C modules in Apache 1.3 send data directly to the client and
there is no clean way to capture this stream.  Apache 2.0 has solved
this problem by introducing a filtering API.  With the baseline I/O
stream tied to this filter mechansim, any module can filter the output
of any other module, with any number of filters in between. Using this
new feature things like SSL, data (de-)compression and other data
manipulations are done very easily.


=item * I<Multi Processing Model modules> (MPMs).

In Apache 1.3 concurrent requests were handled by multiple processes,
and the logic to manage these processes lived in one place,
I<http_main.c>, 7700 some odd lines of code.  If Apache 1.3 is
compiled on a Win32 system large parts of this source file are
redefined to handle requests using threads.  Now suppose you want to
change the way Apache 1.3 processes requests, say, into a DCE RPC
listener.  This is possible only by slicing and dicing I<http_main.c>
into more pieces or by redefining the I<standalone_main> function,
with a C<-DSTANDALONE_MAIN=your_function> compile time flag.  Neither
of which is a clean, modular mechanism.

Apache-2.0 solves this problem by introducing I<Multi Processing Model
modules>, better known as I<MPMs>.  The task of managing incoming
requests is left to the MPMs, shrinking I<http_main.c> to less than
500 lines of code. Now it's possible to write different processing
modules specific to various platforms.  For example the Apache 2.0 on
Windows is much more efficient now, since it uses I<mpm_winnt> which
deploys the native Windows features.

Here is a partial list of major MPMs available as of this writing.

=over

=item prefork

The I<prefork> MPM emulates Apache 1.3's preforking model, where each
request is handled by a different forked child process.

=item worker

The I<worker> MPM implements a hybrid multi-process multi-threaded
approach based on the I<pthreads> standard. It uses one acceptor
thread, multiple worker threads.

=item mpmt_os2, netware, winnt and beos

These MPMs also implement the hybrid multi-process/multi-threaded
model, with each based on native OS thread implementations.

=item perchild

The I<perchild> MPM is similar to the I<worker> MPM, but is extended
with a mechanism which allows mapping of requests to virtual hosts to
a process running under the user id and group configured for that
host.  This provides a robust replacement for the I<suexec> mechanism.

META: as of this writing this mpm is not working

=back

On platforms that support more than one MPM, it's possible to switch
the used MPMs as the need change. For example on Unix it's possible to
start with a preforked module. Then when the demand is growing and the
code matures, it's possible to migrate to a more efficient threaded
MPM, assuming that the code base is capable of running in the
L<threaded
environment|docs::2.0::user::coding::coding/Threads_Coding_Issues_Under_mod_perl>.

=item * Protocol Modules

Apache 1.3 is hardwired to speak only one protocol, HTTP.  Apache 2.0
has moved to more of a "server framework" architecture making it
possible to plugin handlers for protocols other than HTTP.  The
protocol module design also abstracts the transport layer so protocols
such as SSL can be hooked into the server without requiring
modifications to the Apache source code.  This allows Apache to be
extended much further than in the past, making it possible to add
support for protocols such as FTP, SMTP, RPC flavors and the like.
The main advantage being that protocol plugins can take advantage of
Apache's portability, process/thread management, configuration
mechanism and plugin API.

=item * Parsed Configuration Tree

When configuration files are read by Apache 1.3, it hands off the
parsed text to module configuration directive handlers and discards
that text afterwards.  With Apache 2.0, the configuration files are
first parsed into a tree structure, which is then walked to pass data
down to the modules.  This tree is then left in memory with an API for 
accessing it at request time.  The tree can be quite useful for other
modules.  For example, in 1.3, mod_info has its own configuration
parser and parses the configuration files each time you access it.
With 2.0 there is already a parse tree in memory, which mod_info can
then walk to output its information.

If a mod_perl 1.0 module wants access to configuration information,
there are two approaches.  A module can "subclass" directive handlers,
saving a copy of the data for itself, then returning B<DECLINE_CMD> so
the other modules are also handed the info.  Or, the
C<$Apache2::PerlSections::Save> variable can be set to save
E<lt>PerlE<gt> configuration in the C<%Apache2::ReadConfig::>
namespace.  Both methods are rather kludgy, version 2.0 provides a
L<Perl interface to the Apache configuration
tree|docs::2.0::user::config::config/Perl_Interface_to_the_Apache2_Configuration_Tree>.


=back

All these new features boost the Apache performance, scalability and
flexibility. The APR helps the overall performance by doing lots of
platform specific optimizations in the APR internals, and giving the
developer the API which was already greatly optimized.

Apache 2.0 now includes special modules that can boost
performance. For example the mod_mmap_static module loads webpages
into the virtual memory and serves them directly avoiding the overhead
of I<open()> and I<read()> system calls to pull them in from the
filesystem.

The I/O layering is helping performance too, since now modules don't
need to waste memory and CPU cycles to manually store the data in
shared memory or I<pnotes> in order to pass the data to another
module, e.g., in order to provide response's gzip compression.

And of course a not least important impact of these features is the
simplification and added flexibility for the core and third party
Apache module developers.

=head1 What's new in Perl 5.6.0 - 5.8.0

As we have mentioned earlier Perl 5.6.0 is the minimum requirement for
mod_perl 2.0. Though as we will see later certain new features work
only with Perl 5.8.0 and higher.

These are the important changes in the recent Perl versions that had
an impact on mod_perl. For a complete list of changes see the
corresponding to the used version I<perldelta> manpages
(I<http://perldoc.perl.org/perl56delta.html>,
I<http://perldoc.perl.org/perl561delta.html> and
I<http://perldoc.perl.org/perldelta.html>).

The 5.6 Perl generation has introduced the following features:

=over

=item *

The beginnings of support for running multiple interpreters
concurrently in different threads.  In conjunction with the
perl_clone() API call, which can be used to selectively duplicate the
state of any given interpreter, it is possible to compile a piece of
code once in an interpreter, clone that interpreter one or more times,
and run all the resulting interpreters in distinct threads. See the
I<perlembed> (I<http://perldoc.perl.org/perlembed.html>) and
I<perl561delta>
(I<http://perldoc.perl.org/perl561delta.html>) manpages.

=item *

The core support for declaring subroutine attributes, which is used by
mod_perl 2.0's I<method handlers>. See the I<attributes> manpage.

=item *

The I<warnings> pragma, which allows to force the code to be super
clean, via the setting:

  use warnings FATAL => 'all';

which will abort any code that generates warnings. This pragma also
allows a fine control over what warnings should be reported. See the
I<perllexwarn> (I<http://perldoc.perl.org/perllexwarn.html>)
manpage.

=item *

Certain C<CORE::> functions now can be overridden via
C<CORE::GLOBAL::> namespace. For example mod_perl now can override
C<CORE::exit()> via C<CORE::GLOBAL::exit>. See the I<perlsub>
(I<http://perldoc.perl.org/perlsub.html>) manpage.

=item *

The C<XSLoader> extension as a simpler alternative to C<DynaLoader>.
See the I<XSLoader> manpage.

=item *

The large file support. If you have filesystems that support "large
files" (files larger than 2 gigabytes), you may now also be able to
create and access them from Perl. See the I<perl561delta>
(I<http://perldoc.perl.org/perl561delta.html>) manpage.

=item *

Multiple performance enhancements were made. See the I<perl561delta>
(I<http://perldoc.perl.org/perl561delta.html>) manpage.

=item *

Numerous memory leaks were fixed. See the I<perl561delta>
(I<http://perldoc.perl.org/perl561delta.html>) manpage.

=item *

Improved security features: more potentially unsafe operations taint
their results for improved security. See the I<perlsec>
(I<http://perldoc.perl.org/perlsec.html>) and I<perl561delta>
(I<http://perldoc.perl.org/perl561delta.html>) manpages.

=item *

Available on new platforms: GNU/Hurd, Rhapsody/Darwin, EPOC.

=back

Overall multiple bugs and problems very fixed in the Perl 5.6.1, so if
you plan on running the 5.6 generation, you should run at least
5.6.1. It is possible that when this tutorial is printed 5.6.2 will be
out.

The Perl 5.8.0 has introduced the following features:

=over

=item *

The introduced in 5.6.0 experimental PerlIO layer has been stabilized
and become the default IO layer in 5.8.0. Now the IO stream can be
filtered through multiple layers. See the I<perlapio>
(I<http://perldoc.perl.org/perlapio.html>) and I<perliol>
(I<http://perldoc.perl.org/perliol.html>) manpages.

For example this allows mod_perl to inter-operate with the APR IO
layer and even use the APR IO layer in Perl code. See the
C<L<APR::PerlIO|docs::2.0::api::APR::PerlIO>> manpage.

Another example of using the new feature is the extension of the
C<open()> functionality to create anonymous temporary files via:

   open my $fh, "+>", undef or die $!;

That is a literal C<undef()>, not an undefined value. See the
C<open()> entry in the I<perlfunc> manpage
(I<http://perldoc.perl.org/functions/open.html>).

=item *

More overridable via C<CORE::GLOBAL::> keywords. See the I<perlsub>
(I<http://perldoc.perl.org/perlsub.html>) manpage.

=item * 

The signal handling in Perl has been notoriously unsafe because
signals have been able to arrive at inopportune moments leaving Perl
in inconsistent state.  Now Perl delays signal handling until it is
safe.

=item *

C<File::Temp> was added to allow a creation of temporary files and
directories in an easy, portable, and secure way.  See the
I<File::Temp> manpage.

=item *

A new command-line option, C<-t> is available.  It is the little
brother of C<-T>: instead of dying on taint violations, lexical
warnings are given.  This is only meant as a temporary debugging aid
while securing the code of old legacy applications.  B<This is not a
substitute for C<-T>.> See the I<perlrun>
(I<http://perldoc.perl.org/perlrun.html>) manpage.

A new special variable C<${^TAINT}> was introduced. It indicates
whether taint mode is enabled. See the I<perlvar>
(I<http://perldoc.perl.org/perlvar.html>) manpage.

=item *

Threads implementation is much improved since 5.6.

=item *

A much better support for Unicode.

=item *

Numerous bugs and memory leaks fixed. For example now you can localize
the tied C<Apache::DBI> filehandles without leaking memory.

=item *

Available on new platforms: AtheOS, Mac OS Classic, Mac OS X, MinGW,
NCR MP-RAS, NonStop-UX, NetWare and UTS. The following platforms are
again supported: BeOS, DYNIX/ptx, POSIX-BC, VM/ESA, z/OS (OS/390).


=back


=head1 What's new in mod_perl 2.0

The new features introduced by Apache 2.0 and Perl 5.6 and 5.8
generations provide the base of the new mod_perl 2.0 features. In
addition mod_perl 2.0 re-implements itself from scratch providing such
new features as new build and testing framework. Let's look at the
major changes since mod_perl 1.0.

=head2 Threads Support

In order to adapt to the Apache 2.0 threads architecture (for threaded
MPMs), mod_perl 2.0 needs to use thread-safe Perl interpreters, also
known as "ithreads" (Interpreter Threads). This mechanism can be
enabled at compile time and ensures that each Perl interpreter uses
its private C<PerlInterpreter> structure for storing its symbol
tables, stacks and other Perl runtime mechanisms. When this separation
is engaged any number of threads in the same process can safely
perform concurrent callbacks into Perl.  This of course requires each
thread to have its own C<PerlInterpreter> object, or at least that
each instance is only accessed by one thread at any given time.

The first mod_perl generation has only a single C<PerlInterpreter>,
which is constructed by the parent process, then inherited across the
forks to child processes.  mod_perl 2.0 has a configurable number of
C<PerlInterpreters> and two classes of interpreters, I<parent> and
I<clone>.  A I<parent> is like that in mod_perl 1.0, where the main
interpreter created at startup time compiles any pre-loaded Perl code.
A I<clone> is created from the parent using the Perl API
I<perl_clone()>
(I<http://perldoc.perl.org/perlapi.html#Cloning-an-interpreter>)
function.  At request time, I<parent> interpreters are only used for
making more I<clones>, as the I<clones> are the interpreters which
actually handle requests.  Care is taken by Perl to copy only mutable
data, which means that no runtime locking is required and read-only
data such as the syntax tree is shared from the I<parent>, which
should reduce the overall mod_perl memory footprint.

Rather than create a C<PerlInterpreter> per-thread by default,
mod_perl creates a pool of interpreters.  The pool mechanism helps cut
down memory usage a great deal.  As already mentioned, the syntax tree
is shared between all cloned interpreters.  If your server is serving
more than mod_perl requests, having a smaller number of
PerlInterpreters than the number of threads will clearly cut down on
memory usage. Finally and perhaps the biggest win is memory re-use: as
calls are made into Perl subroutines, memory allocations are made for
variables when they are used for the first time.  Subsequent use of
variables may allocate more memory, e.g. if a scalar variable needs to
hold a longer string than it did before, or an array has new elements
added.  As an optimization, Perl hangs onto these allocations, even
though their values "go out of scope".  mod_perl 2.0 has a much better
control over which PerlInterpreters are used for incoming requests.
The interpreters are stored in two linked lists, one for available
interpreters and another for busy ones.  When needed to handle a
request, one interpreter is taken from the head of the available list
and put back into the head of the same list when done.  This means if
for example you have 10 interpreters configured to be cloned at
startup time, but no more than 5 are ever used concurrently, those 5
continue to reuse Perl's allocations, while the other 5 remain much
smaller, but ready to go if the need arises.

Various attributes of the pools are configurable using L<threads mode
specific
directives|docs::2.0::user::config::config/Threads_Mode_Specific_Directives>.

The interpreters pool mechanism has been abstracted into an API known
as "tipool", I<Thread Item Pool>. This pool can be used to manage any
data structure, in which you wish to have a smaller number than the
number of configured threads. For example a replacement for
C<Apache::DBI> based on the I<tipool> will allow to reuse database
connections between multiple threads of the same process.

=head2 Thread-environment Issues

While mod_perl itself is thread-safe, you may have issues with the
thread-safety of your code. For more information refer to L<Threads
Coding Issues Under
mod_perl|docs::2.0::user::coding::coding/Threads_Coding_Issues_Under_mod_perl>.

Another issue is that "global" variables are only global to the
interpreter in which they are created. It's possible to share
variables between several threads running in the same process. For
more information see: L<Shared
Variables|docs::2.0::user::coding::coding/Shared_Variables>.

=head2 Perl Interface to the APR and Apache APIs

As we have mentioned earlier, Apache 2.0 uses two APIs:

=over

=item *

the Apache Portable APR (APR) API, which implements a portable and
efficient API to handle generically work with files, sockets, threads,
processes, shared memory, etc.

=item *

the Apache API, which handles issues specific to the web server.

=back

In mod_perl 1.0, the Perl interface back into the Apache API and data
structures was done piecemeal.  As functions and structure members
were found to be useful or new features were added to the Apache API,
the XS code was written for them here and there.

mod_perl 2.0 generates the majority of XS code and provides thin
wrappers where needed to make the API more Perlish.  As part of this
goal, nearly the entire APR and Apache API, along with their public
data structures are covered from the get-go.  Certain functions and
structures which are considered "private" to Apache or otherwise
un-useful to Perl aren't glued.  Most of the API behaves just as it
did in mod_perl 1.0, so users of the API will not notice the
difference, other than the addition of many new methods. Where API has
changed a special L<back compatibility
module|docs::2.0::user::porting::compat> can be used.

In mod_perl 2.0 the APR API resides in the C<APR::> namespace, and
obviously the C<Apache2::> namespace is mapped to the Apache API.

And in the case of C<APR>, it is possible to use C<APR> modules
outside of Apache, for example:

  % perl -MAPR -MAPR::UUID -le 'print APR::UUID->new->format'
  b059a4b2-d11d-b211-bc23-d644b8ce0981

The mod_perl 2.0 generator is a custom suite of modules specifically
tuned for gluing Apache and allows for complete control over
I<everything>, providing many possibilities none of I<xsubpp>, I<SWIG>
or I<Inline.pm> are designed to do.  Advantages to generating the glue
code include:

=over 4

=item *

Not tied tightly to xsubpp

=item *

Easy adjustment to Apache 2.0 API/structure changes

=item *

Easy adjustment to Perl changes (e.g., Perl 6)

=item *

Ability to "discover" hookable third-party C modules.

=item *

Cleanly take advantage of features in newer Perls

=item *

Optimizations can happen across-the-board with one-shot

=item *

Possible to AUTOLOAD XSUBs

=item *

Documentation can be generated from code

=item *

Code can be generated from documentation

=back

=head1 Integration with 2.0 Filtering

The mod_perl 2.0 interface to the Apache filter API comes in two
flavors. First, similar to the C API, where bucket brigades need to be
manipulated. Second, streaming filtering, is much simpler than the C
API, since it hides most of the details underneath. For a full
discussion on filters and implementation examples refer to the L<Input
and Output Filters|docs::2.0::user::handlers::filters> chapter.


=head2 Other New Features

In addition to the already mentioned new features, the following are
of a major importance:

=over

=item *

Apache 2.0 protocol modules are supported. Later we will see an
example of a protocol module running on top of mod_perl 2.0.

=item *

mod_perl 2.0 provides a very simply to use interface to the Apache
filtering API. We will present a filter module example later on.

=item *

A feature-full and flexible
C<L<Apache::Test|docs::general::testing::testing>> framework was
developed especially for mod_perl testing. While used to test the core
mod_perl features, it is used by third-party module writers to easily
test their modules. Moreover
C<L<Apache::Test|docs::general::testing::testing>> was adopted by
Apache and currently used to test both Apache 1.3, 2.0 and other ASF
projects.  Anything that runs top of Apache can be tested with
C<L<Apache::Test|docs::general::testing::testing>>, be the target
written in Perl, C, PHP, etc.

=item *

The support of the new MPMs model makes mod_perl 2.0 can scale better
on wider range of platforms. For example if you've happened to try
mod_perl 1.0 on Win32 you probably know that the requests had to be
serialized, i.e.  only a single request could be processed at a time,
rendering the Win32 platform unusable with mod_perl as a heavy
production service. Thanks to the new Apache MPM design, now mod_perl
2.0 can be used efficiently on Win32 platforms using its native
I<win32> MPM.


=back


=head2 Optimizations

The rewrite of mod_perl gives us the chances to build a smarter,
stronger and faster implementation based on lessons learned over the
4.5 years since mod_perl was introduced.  There are optimizations
which can be made in the mod_perl source code, some which can be made
in the Perl space by optimizing its syntax tree and some a combination
of both.  In this section we'll take a brief look at some of the
optimizations that are being considered.

The details of these optimizations from the most part are hidden from
mod_perl users, the exception being that some will only be turned on
with configuration directives.  A few of which include:

=over 4

=item *

"Compiled" C<Perl*Handlers>

=item *

Inlined C<Apache2::*.xs> calls

=item *

Use of Apache pools for memory allocations

=back


=head1 Maintainers

Maintainer is the person(s) you should contact with updates,
corrections and patches.

=over

=item *

Stas Bekman [http://stason.org/]

=back

=head1 Authors

=over

=item *

Doug MacEachern E<lt>dougm (at) covalent.netE<gt>

=item *

Stas Bekman [http://stason.org/]

=back

Only the major authors are listed above. For contributors see the
Changes file.

=cut

