mod_perl 2.0

YAPC Europe / Amsterdam

Friday, 3 August 2001

Gerald Richter / ecos gmbh <>


mod_perl-2.0 - overview of mod_perl-2.0


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 diffcult 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 architechure and API.

Of the many changes happening in Apache 2.0, the one which has the most 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 verison 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 ``thread 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.x and Apache 2.0 are reaching stability, mod_perl-2.0 becomes 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.

This paper and talk assume basic knowlege of mod_perl 1.xx features and will focus only the differences mod_perl-2.00 will bring.

Note 1: The Apache and mod_perl APIs mentioned in this paper are both in an ``alpha'' state and subject to change.

Note 2: Some of the mod_perl APIs mentioned in this paper do not even exist and are subject to be implemented, in which case you would be redirected to ``Note 1''.

Apache 2.0 Summary

Note: This section will give you a brief overview of the changes in Apache 2.0, just enough to understand where mod_perl will fit in. For more details on Apache 2.0 consult the papers by Ryan Bloom.

MPMs - Multi-Processing Model Modules

In Apache 1.3.x concurrent requests were handled by multiple processes, and the logic to manage these processes lived in one place, http_main.c, 7200 some odd lines of code. If Apache 1.3.x 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.x processes requests, say, into a DCE RPC listener. This is possible only by slicing and dicing http_main.c into more pieces or by redefining the standalone_main function, with a -DSTANDALONE_MAIN=your_function compile time flag. Neither of which is a clean, modular mechanism.

Apache-2.0 solves this problem by intoducing Multi Processing Model modules, better known as MPMs. The task of managing incoming requests is left to the MPMs, shrinking http_main.c to less than 500 lines of code. Several MPMs are included with Apache 2.0 in the src/modules/mpm directory:

The prefork module emulates 1.3.x's preforking model, where each request is handled by a different process.

This MPMs implements a hybrid multi-process multi-threaded approach based on the pthreads standard.

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

The perchild MPM is similar to the threaded 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 suexec mechanism.

APR - Apache Portable Runtime

Apache 1.3.x 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.x 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. For mod_perl, APR will assist a great deal with 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 will be provided for certain areas of APR, such as the shared memory abstraction, but the majority of APR will be used by mod_perl ``under the covers''.

New Hook Scheme

In Apache 1.3, modules were registered using the module structure, normally static to mod_foo.c. This structure contains pointers to the command table, config create/merge functions, response handler table and function pointers for all of the other hooks, such as child_init and check_user_id. In 2.0, this structure has been pruned down to the first three items mention and a new function pointer added called register_hooks. It is the job of register_hooks to register functions for all other hooks (such as child_init and check_user_id). Not only is hook registration now dynamic, it is also possible for modules to register more than one function per hook, unlike 1.3. The new hook mechanism also makes it possible to sort registered functions, unlike 1.3 with function pointers hardwired into the module structure, and each module structure into a linked list. Order in 1.3 depended on this list, which was possible to order using compile-time and configuration-time configuration, but that was left to the user. Whereas in 2.0, the add_hook functions accept an order preference parameter, those commonly used are:


For mod_perl, dynamic registration provides a cleaner way to bypass the Perl*Handler configuration. By simply adding this configuration:

 PerlModule Apache::Foo

Apache/ can register hooks itself at server startup:

 Apache::Hook->add(PerlAuthenHandler => \&authenticate, Apache::Hook::MIDDLE);
 Apache::Hook->add(PerlLogHandler => \&logger, Apache::Hook::LAST);

However, this means that Perl subroutines registered via this mechanism will be called for *every* request. It will be left to that subroutine to decide if it was to handle or decline the given phase. As there is overhead in entering the Perl runtime, it will most likely be to your advantage to continue using Perl*Handler configuration to reduce this overhead. If it is the case that your Perl*Handler should be invoked for every request, the hook registration mechanism will save some configuration keystrokes.

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 it's 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 it's information.

If a mod_perl 1.xx 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 DECLINE_CMD so the other modules are also handed the info. Or, the $Apache::Server::SaveConfig variable can be set to save <Perl> configuration in the %Apache::ReadConfig:: namespace. Both methods are rather kludgy, version 2.0 will provide a Perl interface to the Apache configuration tree.


Filtering of Perl modules output has been possible for years since tied filehandle support was added to Perl. There are several modules, such as Apache::Filter and Apache::OutputChain which have been written to provide mechanisms for filtering the STDOUT ``stream''. There are several of these modules because no one 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.

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.

mod_perl and Threaded MPMs

Perl 5.6

Thread safe Perl interpreters, also known as ``ithreads'' (Intepreter Threads) provide the mechanism need for mod_perl to adapt to the Apache 2.0 thread architecture. This mechanism is a compile time option which encapsulates the Perl runtime inside of a single PerlInterpreter structure. With each interpreter instance containing its own symbol tables, stacks and other Perl runtime mechanisms, it is possible for any number of threads in the same process to concurrently callback into Perl. This of course requires each thread to have it's own PerlInterpreter object, or at least that each instance is only access by one thread at any given time.

mod_perl-1.xx has only a single PerlInterpreter, which is contructed by the parent process, then inherited across the forks to child processes. mod_perl-2.0 has a configurable number of PerlInterpreters and two classes of interpreters, parent and clone. A parent is like that in 1.xx, the main interpreter created at startup time which compiles any pre-loaded Perl code. A clone is created from the parent using the Perl API perl_clone() function. At request time, parent interpreters are only used for making more clones, as they 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 parent.

New mod_perl Directives for Threaded MPMs

Rather than create a PerlInterperter 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 reuse. That is, 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 the string needs to hold a larger than it did before, or an array more elements than in the past. As an optimization, Perl hangs onto these allocations, even though their values ``go out of scope''. With the 1.xx model, random children would be hit with these allocations. With 2.0, mod_perl has much better control over which PerlInterpreters are used for incoming requests. The intepreters are stored in two linked lists, one for available interpreters one for busy. When needed to handle a request, one is taken from the head of the available list and put back into the head of the list when done. This means if you have, say, 10 interpreters configured to be cloned at startup time, but no more than 5 are ever used concurrently, those 5 continue to reuse Perls allocations, while the other 5 remain much smaller, but ready to go if the need arises.

Various attributes of the pools are configurable with the following configuration directives:

The number of intepreters to clone at startup time.

If all running interpreters are in use, mod_perl will clone new interpreters to handle the request, up until this number of interpreters is reached. When Max is reached, mod_perl will block until one becomes available.

The minimum number of available interpreters this parameter will clone interpreters up to Max, before a request comes in.

mod_perl will throttle down the number of interpreters to this number as those in use become available.

The maximum number of requests an interpreter should serve, the interpreter is destroyed when the number is reached and replaced with a fresh clone.

As mentioned, when a request in a threaded mpm is handled by mod_perl, an interpreter must be pulled from the interpreter pool. The interpreter is then only available to the thread that selected it, until it is released back into the interpreter pool. By default, an interpreter will be held for the lifetime of the request, equivalent to this configuration:
 PerlInterpLifetime request

For example, if a PerlAccessHandler is configured, an interpreter will selected before it is run and not released until after the logging phase.

Intepreters will be shared across subrequests by default, however, it is possible configure the intepreter lifetime to be per-subrequest on a per-directory basis:

 PerlInterpLifetime subrequest

With this configuration, an autoindex generated page for example would select an interpreter for each item in the listing that is configured with a Perl*Handler.

It is also possible to configure the lifetime to be per-handler:

 PerlInterpLifetime handler

With this configuration, an interpreter will be selected before PerlAccessHandlers are run, and putback immediately afterwards, before Apache moves onto the authentication phase. If a PerlFixupHandler is configured further down the chain, another interpreter will be selected and again putback afterwards, before PerlResponseHandler is run.

For protocol handlers, the interpreter is held for the lifetime of the connection. However, a C protocol module might hook into mod_perl (e.g. mod_ftp) and provide a request_rec. In this case, the default lifetime is that of the request. Should a mod_perl handler want to maintain state for the lifetime of an ftp connection, it is possible to do so on a per-virtualhost basis:

 PerlInterpLifetime connection

Issues with Threading

The Perl ``ithreads'' implementation ensures that Perl code is thread safe, at least with respect to the Apache threads in which it is running. However, it does not ensure that extensions which call into third-party C/C++ libraries are thread safe. In the case of non-threadsafe extensions, if it is not possible to fix those routines, care will need to be taken to serialize calls into such functions (either at the xs or Perl level).

Another issue is that ``global'' variables are only global to the interpreter in which they are created. Some research has been done on the concept of solar variables which are global across all interpreter instances. It has not been decided if this feature would best fit built into the Perl core or as an extension, but fear not, the feature will be provided in one form or another.

Thread Item Pool API

As we discussed, mod_perl implements a pool mechanism to manage PerlInterpreters between threads. This mechanism has been abstracted into an API known as ``tipool'', 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. A good example of such a data structure is a database connection handle. The Apache::DBI module implements persisent connections for 1.xx, but may result in each child maintaining its own connection, when it is most often the case that number of connections is never needed concurrently. The TIPool API provides a mechanism to solve this problem, consisting of the following methods:

Create a new thread item pool. This constructor is passed an Apache::Pool object, a hash reference to pool configuration parameters, a hash reference to pool callbacks and an optional userdata variable which is passed to callbacks:
 my $tip = Apache::TIPool->new($p,
                               {Start => 3, Max => 6},
                               {grow => \&new_connection,
                                shrink => \&close_connection},

The configuration parameters, Start, Max, MinSpare, MaxSpare and MaxRequests configure the pool for your items, just as the PerlInterp* directives do for PerlInterpreters.

The grow callback is called to create new items to be added to the pool, shrink is called when an item is removed from the pool.

This method will return an item from the pool, from the head of the available list. If the current number of items are all busy, and that number is less than the configured maximum, a new item will be created by calling the configured grow callback. Otherwise, the pop method will block until an item is available.
 my $item = $tip->pop;

This method gives an item (returned from pop) back to the pool, which is pushed into the head of the available list:

Future improvements will be made to the TIPool API, such as the ability to sort the available and busy lists and specify if items should be popped and putback to/from the head or tail of the list.


Now we will take a look at how to make DBI take advantage of TIPool API with the Apache::DBIPool module. The module configuration in httpd.conf will look something like so:

 PerlModule Apache::DBIPool
 <DBIPool dbi:mysql:db_name>
   DBIPoolStart 10
   DBIPoolMax   20
   DBIPoolMaxSpare 10
   DBIPoolMinSpare 5
   DBIUserName dougm
   DBIPassWord XxXx

The module is loaded using the PerlModule directive just as with other modules. TIPools are then configured using DBIPool configuration sections. The argument given to the container is the dsn and within are the pool directives Start, Max, MaxSpare and MinSpare. The UserName and PassWord directives will be passed to the DBI connect method. There can be any number of DBIPool containers, provided each dsn is different, and/or each container is inside a different VirtualHost container.

Now let's examine the source code, keeping in mind this module contains the basics and the official release (tbd) will likely contain more details, such as how it hooks into to provide transparency the way Apache::DBI currently does.

After pulling in the modules needed Apache::TIPool, Apache::ModuleConfig and DBI, we setup a callback table. The new_connection function will be called with the TIP needs to add a new item and close_connection when an item is being removed from the pool. The Apache::Hook add method registers a PerlPostConfigHandler which will be called after Apache has read the configuration files.

This handler (our init function) is passed 3 Apache::Pool objects and one Apache::Server object. Each Apache::Pool has a different lifetime, the first will be alive until configuration is read again, such as during restarts. The second will be alive until logs are re-opened and the third is a temporary pool which is cleared before Apache starts serving requests. Since the DBI connection pool is associated with configuration in httpd.conf, we will use that pool.

The Apache::ModuleConfig get method is called with the Apache::Server object to give us the configuration associated with the given server. Next is a while loop which iterates over the configuration parsed by the DBIPool directive handler. The keys of this hash are the configured dsn, of which there is one per DBIPool configuration section. The values will be a hash reference to the pool configuration, Start, Max, MinSpare, MaxSpare and MaxRequests.

A new Apache::TIPool is then contructed, passing it the $pconf Apache::Pool, configuration $params, the $callbacks table and $conn hash ref. The TIPool is then saved into the $cfg object, indexed by the dsn.

At the time Apache::TIPool::new is called, the new_connection callback will be called the number of time to which Start is configured. This callback localizes Apache::DBIPool::connect to a code reference which makes the real database connection.

At request time Apache::DBIPool::connect will fetch a database handle from the TIPool. It does so by digging into the configuration object associated with the current virtual host to obtain a reference to the TIPool object. It then calls the pop method, which will immediatly return a database handle if one is available. If all opened connection are in used and the current number of connections is less than the configured Max, the call to pop will result in a call to new_connection. If Max has already been reached, then pop will block until a handle is putback into the pool.

Finally, the handle is blessed into the Apache::DBIPool::db class which will override the dbd class disconnect method. The overridden disconnect method obtains a reference to the TIPool object and passes it to the putback method, making it available for use by other threads. Should the Perl code using this handle neglect to call the disconnect method, the overridden connect method has already registered a cleanup function to make sure it is putback.

Apache::DBIPool Source

 package Apache::DBIPool;
 use strict;
 use Apache::TIPool ();
 use Apache::ModuleConfig ();
 use DBI ();
 my $callbacks = {
    grow => \&new_connection,     #add new connection to the pool
    shrink => \&close_connection, #handle removed connection from pool
 Apache::Hook->add(PerlPostConfigHandler => \&init); #called at startup
 sub init {
     my($pconf, $plog, $ptemp, $s) = @_;
     my $cfg = Apache::ModuleConfig->get($s, __PACKAGE__);
     #create a TIPool for each dsn
     while (my($conn, $params) = each %{ $cfg->{DBIPool} }) {
         my $tip = Apache::TIPool->new($pconf, $params, $callbacks, $conn);
         $cfg->{TIPool}->{ $conn->{dsn} } = $tip;
 sub new_connection {
     my($tip, $conn) = @_;
     #make actual connection to the database
     local *Apache::DBIPool::connect = sub {
         my($class, $drh) = (shift, shift);
         $drh->connect($dbname, @_);
     return DBI->connect(@{$conn}{qw(dsn username password attr)});
 sub close_connection {
     my($tip, $conn, $dbh) = @_;
     my $driver = (split $conn->{dsn}, ':')[1];
     my $method = join '::', 'DBD', $driver, 'db', 'disconnect';
     $dbh->$method(); #call the real disconnect method
 my $EndToken = '</DBIPool>';
 #parse <DBIPool dbi:mysql:...>...
 sub DBIPool ($$$;*) {
     my($cfg, $parms, $dsn, $cfg_fh) = @_;
     $dsn =~ s/>$//;
     $cfg->{DBIPool}->{$dsn}->{dsn} = $dsn;
     while((my $line = <$cfg_fh>) !~ m:^$EndToken:o) {
         my($name, $value) = split $line, /\s+/, 2;
         $name =~ s/^DBIPool(\w+)/lc $1/ei;
         $cfg->{DBIPool}->{$dsn}->{$name} = $value;
 sub config {
     my $r = Apache->request;
     return Apache::ModuleConfig->get($r, __PACKAGE__);
 #called from DBI::connect
 sub connect {
     my($class, $drh) = (shift, shift);
     $drh->{DSN} = join ':', 'dbi', $drh->{Name}, $_[0];
     my $cfg = config();
     my $tip = $cfg->{TIPool}->{ $drh->{DSN} };
     unless ($tip) {
         #XXX: do a real connect or fallback to Apache::DBI
     my $item = $tip->pop; #select a connection from the pool
     $r->register_cleanup(sub { #incase disconnect() is not called
     return bless 'Apache::DBIPool::db', $item->data; #the dbh
 package Apache::DBIPool::db;
 our @ISA = qw(DBI::db);
 #override disconnect, puts database handle back in the pool
 sub disconnect {
     my $dbh = shift;
     my $tip = config()->{TIPool}->{ $dbh->{DSN} };

PerlOptions Directive

A new configuration directive to mod_perl-2.0, PerlOptions, provides fine-grained configuration for what were compile-time only options in mod_perl-1.xx. In addition, this directive provides control over what class of PerlInterpreter is used for a VirtualHost or location configured with Location, Directory, etc.

These are all best explained with examples, first here's how to disable mod_perl for a certain host:

 <VirtualHost ...>
    PerlOptions -Enable

Suppose a one of the hosts does not want to allow users to configure PerlAuthenHandler, PerlAuthzHandler or PerlAccessHandler or <Perl> sections:

 <VirtualHost ...>
    PerlOptions -Authen -Authz -Access -Sections

Or maybe everything but the response handler:

 <VirtualHost ...>
    PerlOptions None +Response

A common problem with mod_perl-1.xx was the shared namespace between all code within the process. Consider two developers using the same server and each which to run a different version of a module with the same name. This example will create two parent Perls, one for each VirtualHost, each with its own namespace and pointing to a different paths in @INC:

 <VirtualHost ...>
    ServerName dev1
    PerlOptions +Parent
    PerlSwitches -Mblib=/home/dev1/lib/perl
 <VirtualHost ...>
    ServerName dev2
    PerlOptions +Parent
    PerlSwitches -Mblib=/home/dev2/lib/perl

Or even for a given location, for something like ``dirty'' cgi scripts:

 <Location /cgi-bin>
    PerlOptions +Parent
    PerlInterpMaxRequests 1
    PerlInterpStart 1
    PerlInterpMax 1
    PerlHandler Apache::Registry

Will use a fresh interpreter with its own namespace to handle each request.

Should you wish to fine tune Interpreter pools for a given host:

 <VirtualHost ...>
    PerlOptions +Clone
    PerlInterpStart 2
    PerlInterpMax 2

This might be worthwhile in the case where certain hosts have their own sets of large-ish modules, used only in each host. By tuning each host to have it's own pool, that host will continue to reuse the Perl allocations in their specific modules.

In 1.x versions of mod_perl, configured Perl*Handlers which are not a fully qualified subroutine name are resolved at request time, loading the handler module from disk if needed. In 2.x, configured Perl*Handlers are resolved at startup time. By default, modules are not auto-loaded during startup-time resolution. It is possible to configure this feature with:

 PerlOptions +Autoload

Consider this configuration:

 PerlResponseHandler Apache::Magick

In this case, Apache::Magick is the package name, and the subroutine name will default to handler. If the Apache::Magick module is not already loaded, PerlOptions +Autoload will attempt to pull it in at startup time.

Integration with 2.0 Filtering

The mod_perl-2.0 interface to the Apache filter API is much simpler than the C API, hiding most of the details underneath. Perl filters are configured using the PerlFilterHandler directive, for example:

 PerlFilterHandler Apache::ReverseFilter

This simply registers the filter, which can then be turned on using the core AddOutputFilter directive:

 <Location /foo>
    AddOutputFilter Apache::ReverseFilter

The Apache::ReverseFilter handler will now be called for anything accessed in the /foo url space. The AddOutputFilter directive takes any number of filters, for example, this configuration will first send the output to mod_include, which will in turn pass its output down to Apache::ReverseFilter:

 AddOutputFilter INCLUDE Apache::ReverseFilter

For our example, Apache::ReverseFilter simply reverses all of the output characters and then sends them downstream. The first argument to a filter handler is an Apache::Filter object, which at the moment provides two methods read and write. The read method pulls down a chunk of the output stream into the given buffer, returning the length read into the buffer. An optional size argument may be given to specify the maximum size to read into the buffer. If omitted, an arbitrary size will fill the buffer, depending on the upstream filter. The write method passes data down to the next filter. In our case scalar reverse takes advantage of Perl's builtins to reverse the upstream buffer:

 package Apache::ReverseFilter;
 use strict;
 sub handler {
     my $filter = shift;
     while ($filter->read(my $buffer, 1024)) {
         $filter->write(scalar reverse $buffer);
     return Apache::OK;

Perl interface to the APR and Apache API

In 1.x, 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.

The goal for 2.0 is to generate the majority of xs code and provide thin wrappers were 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 will covered from the get-go. Certain functions and structures which are considered ``private'' to Apache or otherwise un-useful to Perl will not be glued. The API behaves just as it did in 1.x, sosers of the API will not notice the difference, other than the addition of many new methods. And in the case of APR, it is possible to use APR modules outside of Apache, for example:

 % perl -MAPR -MAPR::UUID -le 'print APR::UUID->new->format'

The mod_perl generator is a custom suite of modules specifically tuned for gluing Apache and allows for complete control over everything, providing many possibilities none of xsubpp, swig nor are designed to do. Advantages to generating the glue code include:

Protocol Modules with mod_perl-2.0


Apache 2.0 ships with an example protocol module, mod_echo, which simply reads data from the client and echos it right back. Here we'll take a look at a Perl version of that module, called Apache::Echo. A protocol handler is configured using the PerlProcessConnectionHandler directive and we'll use the Listen and VirtualHost directives to bind to a non-standard port 8084:

 Listen 8084
 <VirtualHost _default_:8084>
     PerlProcessConnectionHandler Apache::Echo

Apache::Echo is then enabled when starting Apache:

 % httpd

And we give it a whirl:

 % telnet localhost 8084
 Connected to localhost (
 Escape character is '^]'.
 hello apachecon
 hello apachecon

The code is just a few lines of code, with the standard package declaration and of course, use strict;. As with all Perl*Handlers, the subroutine name defaults to handler. However, in the case of a protocol handler, the first argument is not a request_rec, but a conn_rec blessed into the Apache::Connection class. We have direct access to the client socket via Apache::Connection's client_socket method. This returns an object blessed into the APR::Socket class.

Inside the echo loop, we attempt to read BUFF_LEN bytes from the client socket into the $buff buffer. The $rlen parameter will be set to the number of bytes actually read. The APR::Socket recv method will return an apr_status_t value, be we need only check the read length to break out of the loop if it is less than or equal to 0 bytes. If we received some data, it is immediately echoed back to the client with the APR::Socket send method. If we were unable to echo back the same number of bytes read from the client, assume the connection was dropped and break out of the loop. Once the client has disconnected, the module returns Apache::OK, telling Apache we have handled the connection:

 package Apache::Echo;

 use strict;
 use Apache::Connection ();
 use APR::Socket ();

 use constant BUFF_LEN => 1024;

 sub handler {
     my Apache::Connection $c = shift;
     my APR::Socket $socket = $c->client_socket;

     my $buff;

     for (;;) {
         my($rlen, $wlen);
         my $rlen = BUFF_LEN;
         $socket->recv($buff, $rlen);
         last if $rlen <= 0;
         $wlen = $rlen;
         $socket->send($buff, $wlen);
         last if $wlen != $rlen;

     return Apache::OK;



Our first protocol handler example took advange of Apache's server framework, but did not tap into any other modules. The next example is based on the example in the ``TCP Servers with IO::Socket'' section of perlipc. Of course, we don't need IO::Socket since Apache takes care of those details for us. The rest of that example can still be used to illustrate implementing a simple text protocol. In this case, one where a command is sent by the client to be executed on the server side, with results sent back to the client.

The Apache::CommandServer handler will support four commands: motd, date, who and quit. These are probably not commands which can be exploited, but should we add such commands, we'll want to limit access based on ip address/hostname, authentication and authorization. Protocol handlers need to take care of these tasks themselves, since we bypass the HTTP protocol handler.

As with all PerlProcessConnectionHandlers, we are passed an Apache::Connection object as the first argument. Again, we will be directly accessing the client socket via the client_socket method. The login subroutine is called to check if access by this client should be allowed. This routine makes up for what we lost with the core HTTP protocol handler bypassed. First we call the Apache::RequestRec new method, which returns a request_rec object, just like that which is passed into request time Perl*Handlers and returned by the subrequest API methods, lookup_uri and lookup_file. However, this ``fake request'' does not run handlers for any of the phases, it simply returns an object which we can use to do that ourselves. The location_merge method is passed the ``location'' for this request, it will look up the <Location> section that matches the given name and merge it with the default server configuration. For example, should we only wish to allow access to this server from certain locations:

    <Location Apache::CommandServer>
        deny from all
        allow from 10.*

The location_merge method only looks up and merges the configuration, we still need to apply it. This is done in for loop, iterating over three methods: run_access_checker, run_check_user_id and run_auth_checker. These methods will call directly into the Apache functions that invoke module handlers for these phases and will return an integer status code, such as OK, DECLINED or FORBIDDEN. If run_access_check returns something other than OK or DECLINED, that status will be propagated up to the handler routine and then back up to Apache. Otherwise, the access check passed and the loop will break unless some_auth_required returns true. This would be false given the previous configuration example, but would be true in the presense of a require directive, such as:

    <Location Apache::CommandServer>
        deny from all
        allow from 10.*
        require user dougm

Given this configuration, some_auth_required will return true. The user method is then called, which will return false if we have not yet authenticated. A prompt utility is called to read the username and password, which are then injected into the headers_in table using the set_basic_credentials method. The Authenticate field in this table is set to a base64 encoded value of the username:password pair, exactly the same format a browser would send for Basic authentication. Next time through the loop run_check_user_id is called, which will in turn invoke any authentication handlers, such as mod_auth. When mod_auth calls the ap_get_basic_auth_pw() API function (as all Basic auth modules do), it will get back the username and password we injected. If we fail authentication a 401 status code is returned which we propagate up. Otherwise, authorization handlers are run via run_auth_checker. Authorization handlers normally need the user field of the request_rec for its checks and that field was filled in when mod_auth called ap_get_basic_auth_pw().

Provided login is a success, a welcome message is printed and main request loop entered. Inside the loop the getline function returns just one line of data, with newline characters stripped. If the string sent by the client is in our command table, the command is then invoked, otherwise a usage message is sent. If the command does not return a true value, we break out of the loop. Let's give it a try with this configuration:

 Listen 8085
 <VirtualHost _default_:8085>
     PerlProcessConnectionHandler Apache::CommandServer
     <Location Apache::CommandServer>
         allow from
         require user dougm
         satisfy any
         AuthUserFile /tmp/basic-auth
 % telnet localhost 8085
 Connected to localhost (
 Escape character is '^]'.
 Login: dougm
 Password: foo
 Welcome to Apache::CommandServer
 Available commands: motd date who quit
 Have a lot of fun...
 Mon Mar 12 19:20:10 PST 2001
 dougm    tty1     Mar 12 00:49
 dougm    pts/0    Mar 12 11:23
 dougm    pts/1    Mar 12 14:08
 dougm    pts/2    Mar 12 17:09
 Connection closed by foreign host.

Apache::CommandServer Source

 package Apache::CommandServer;

 use strict;
 use Apache::Connection ();
 use APR::Socket ();

 my @cmds = qw(motd date who quit);
 my %commands = map { $_, \&{$_} } @cmds;

 sub handler {
     my Apache::Connection $c = shift;
     my APR::Socket $socket = $c->client_socket;

     if ((my $rc = login($c)) != Apache::OK) {
         $socket->send("Access Denied\n");
         return $rc;

     $socket->send("Welcome to " . __PACKAGE__ .
                   "\nAvailable commands: @cmds\n");

     for (;;) {
         my $cmd;
         next unless $cmd = getline($socket);

         if (my $sub = $commands{$cmd}) {
             last unless $sub->($socket) == APR::SUCCESS;
         else {
             $socket->send("Commands: @cmds\n");

     return Apache::OK;

 sub login {
     my $c = shift;

     my $r = Apache::RequestRec->new($c);

     for my $method (qw(run_access_checker run_check_user_id run_auth_checker)) {
         my $rc = $r->$method();

         if ($rc != Apache::OK and $rc != Apache::DECLINED) {
             return $rc;

         last unless $r->some_auth_required;

         unless ($r->user) {
             my $socket = $c->client_socket;
             my $username = prompt($socket, "Login");
             my $password = prompt($socket, "Password");

             $r->set_basic_credentials($username, $password);

     return Apache::OK;

 sub getline {
     my $socket = shift;
     my $line;
     $socket->recv($line, 1024);
     return unless $line;
     $line =~ s/[\r\n]*$//;
     return $line;

 sub prompt {
     my($socket, $msg) = @_;
     $socket->send("$msg: ");

 sub motd {
     my $socket = shift;
     open my $fh, '/etc/motd' or return;
     local $/;
     my $status = $socket->send(scalar <$fh>);
     close $fh;
     return $status;

 sub date {
     my $socket = shift;
     $socket->send(scalar(localtime) . "\n");

 sub who {
     my $socket = shift;
     $socket->send(scalar `who`);

 sub quit {1}


mod_perl-2.0 Optimizations

As mentioned in the introduction, 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 will from the most part be hidden from mod_perl users, the exeception being that some will only be turned on with configuration directives. The explanation of these optimization ideas are best left for the live talk, a few which will be overviewed include:

The mod_perl homepage will announce mod_perl-2.0 developments as they become available.


this document: Doug MacEachern

talk on YAPC 2001 in Amsterdam: Gerald Richter <>