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Table of Contents:

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The Writing Apache Modules with Perl and C book can be purchased online from O'Reilly and Amazon.com.
Your corrections of the technical and grammatical errors are very welcome. You are encouraged to help me improve this guide. If you have something to contribute please send it directly to me.
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Apache::Session - Maintain session state across HTTP requests

This module provides the Apache/mod_perl user with a mechanism for storing persistent user data in a global hash, which is independent of the underlying storage mechanism. Currently you can choose from these storage mechanisms Apache::Session::DBI, Apache::Session::Win32, Apache::Session::File, Apache::Session::IPC. Read the man page of the mechanism you want to use for a complete reference.

Apache::Session provides persistence to a data structure. The data structure has an ID number, and you can retrieve it by using the ID number. In the case of Apache, you would store the ID number in a cookie or the URL to associate it with one browser, but the method of dealing with the ID is completely up to you. The flow of things is generally:

  Tie a session to Apache::Session.
  Get the ID number.
  Store the ID number in a cookie.
  End of Request 1.

  (time passes)

  Get the cookie.
  Restore your hash using the ID number in the cookie.
  Use whatever data you put in the hash.
  End of Request 2.

Using Apache::Session is easy: simply tie a hash to the session object, stick any data structure into the hash, and the data you put in automatically persists until the next invocation. Here is an example which uses cookies to track the user's session.

  # pull in the required packages
  use Apache::Session::DBI;
  use Apache;
  use strict;
  # read in the cookie if this is an old session
  my $r = Apache->request;
  my $cookie = $r->header_in('Cookie');
  $cookie =~ s/SESSION_ID=(\w*)/$1/;
  # create a session object based on the cookie we got from the
  # browser, or a new session if we got no cookie
  my %session;
  tie %session, 'Apache::Session::DBI', $cookie,
      {DataSource => 'dbi:mysql:sessions',
       UserName   => $db_user,
       Password   => $db_pass
  # might be a new session, so lets give them their cookie back
  my $session_cookie = "SESSION_ID=$session{_session_id};";
  $r->header_out("Set-Cookie" => $session_cookie);

After setting this up, you can stick anything you want into %session (except file handles and code references and using _session_id), and it will still be there when the user invokes the next page.

It is possible to write an Apache authentication handler using Apache::Session. You can put your authentication token into the session. When a user invokes a page, you open their session, check to see if they have a valid token, and authenticate or forbid based on that.

By way of comparison note that IIS's sessions are only valid on the same web server as the one that issued the session. Apache::Session's session objects can be shared amongst a farm of many machines running different operating systems, including even Win32. IIS stores session information in RAM. Apache::Session stores sessions in databases, file systems, or RAM. IIS's sessions are only good for storing scalars or arrays. Apache::Session's sessions allow you to store arbitrarily complex objects. IIS sets up the session and automatically tracks it for you. With Apache::Session, you setup and track the session yourself. IIS is proprietary. Apache::Session is open-source. Apache::Session::DBI can issue 400+ session requests per second on light Celeron 300A running Linux. IIS?

An alternative to Apache::Session is Apache::ASP, which has session tracking abilities. HTML::Embperl hooks into Apache::Session for you.

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Apache::DBI - Initiate a persistent database connection

See mod_perl and relational Databases

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Apache::Watchdog::RunAway - Hanging Processes Monitor and Terminator

This module monitors hanging Apache/mod_perl processes. You define the time in seconds after which the process is to be counted as hanging or run away.

When the process is considered to be hanging it will be killed and the event logged in a log file.

Generally you should use the amprapmon program that is bundled with this module's distribution package, but you can write your own code using the module as well. See the amprapmon manpage for more information about it.

Note that it requires the Apache::Scoreboard module to work.

Refer to the Apache::Watchdog::RunAway manpage for the configuration details.

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Apache::VMonitor -- Visual System and Apache Server Monitor

Apache::VMonitor is the next generation of mod_status. It provides all the information mod_status provides and much more.

This module emulates the reporting functions of the top(), mount(), df() and ifconfig() utilities. There is a special mode for mod_perl processes. It has visual alert capabilities and a configurable automatic refresh mode. It provides a Web interface, which can be used to show or hide all the sections dynamically.

The are two main modes:

The main advantage of this module is that it reduces the need to telnet to the machine in order to monitor it. Indeed it provides information about mod_perl processes that cannot be acquired from telneting to the machine. =head3 Configuration

  # Configuration in httpd.conf
  <Location /sys-monitor>
    SetHandler perl-script
    PerlHandler Apache::VMonitor

  # startup file or <Perl> section:
  use Apache::VMonitor();
  $Apache::VMonitor::Config{BLINKING} = 0; # Blinking is evil
  $Apache::VMonitor::Config{REFRESH}  = 0;
  $Apache::VMonitor::Config{VERBOSE}  = 0;
  $Apache::VMonitor::Config{SYSTEM}   = 1;
  $Apache::VMonitor::Config{APACHE}   = 1;
  $Apache::VMonitor::Config{PROCS}    = 1;
  $Apache::VMonitor::Config{MOUNT}    = 1;
  $Apache::VMonitor::Config{FS_USAGE} = 1;
  $Apache::VMonitor::Config{NETLOAD}  = 1;
  @Apache::VMonitor::NETDEVS    = qw(lo eth0);
  $Apache::VMonitor::PROC_REGEX = join "\|", qw(httpd mysql squid);

More information is available in the module's extensive manpage.

It requires Apache::Scoreboard and GTop to work. GTop in turn requires the libgtop library but is not available for all platforms. Visit http://www.home-of-linux.org/gnome/libgtop/ to check whether your platform/flavor is supported.

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Apache::GTopLimit - Limit Apache httpd processes

This module allows you to kill off Apache processes if they grow too large or if they share too little of their memory. You can choose to set up the process size limiter to check the process size on every request:

The module is thoroughly explained in the sections: ``Keeping the Shared Memory Limit'' and ``Limiting the Size of the Processes''

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Apache::Request (libapreq) - Generic Apache Request Library

This package contains modules for manipulating client request data via the Apache API with Perl and C. Functionality includes:

parsing of application/x-www-form-urlencoded data

parsing of multipart/form-data

parsing of HTTP Cookies

The Perl modules are simply a thin xs layer on top of libapreq, making them a lighter and faster alternative to CGI.pm and CGI::Cookie. See the Apache::Request and Apache::Cookie documentation for more details and eg/perl/ for examples.

Apache::Request and libapreq are tied tightly to the Apache API, to which there is no access in a process running under mod_cgi.


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Apache::RequestNotes - Allow Easy, Consistent Access to Cookie and Form Data Across Each Request Phase

Apache::RequestNotes provides a simple interface allowing all phases of the request cycle access to cookie or form input parameters in a consistent manner. Behind the scenes, it uses libapreq Apache::Request>) functions to parse request data and puts references to the data in pnotes().

Once the request is past the PerlInit phase, all other phases can have access to form input and cookie data without parsing it themselves. This relieves some strain, especially when the GET or POST data is required by numerous handlers along the way.

See the Apache::RequestNotes manpage for more information.

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Apache::PerlRun - Run unaltered CGI scripts under mod_perl

See Apache::PerlRun - a closer look.

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Apache::RegistryNG -- Apache::Registry New Generation

Apache::RegistryNG is the same as Apache::Registry, aside from using filenames instead of URIs for namespaces. It also uses an Object Oriented interface.

  PerlModule Apache::RegistryNG
  <Location /perl>
    SetHandler perl-script
    PerlHandler Apache::RegistryNG->handler

Apache::RegistryNG inherits from Apache::PerlRun, but the handler() is overridden. Aside from the handler(), the rest of Apache::PerlRun contains all the functionality of Apache::Registry broken down into several subclass-able methods. These methods are used by Apache::RegistryNG to implement the exact same functionality of Apache::Registry, using the Apache::PerlRun methods.

There is no compelling reason to use Apache::RegistryNG over Apache::Registry, unless you want to do add or change the functionality of the existing Registry.pm. For example, Apache::RegistryBB (Bare-Bones) is another subclass that skips the stat() call performed by Apache::Registry on each request.

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Apache::RegistryBB -- Apache::Registry Bare Bones

It works just like Apache::Registry, but does not test the x bit (-x file test for executable mode), only compiles the file once (no stat() call is made per request), skips the OPT_EXECCGI checks and does not chdir() into the script parent directory. It uses the Object Oriented interface.


  PerlModule Apache::RegistryBB
  <Location /perl>
    SetHandler perl-script
    PerlHandler Apache::RegistryBB->handler

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Apache::OutputChain -- Chain Stacked Perl Handlers

Apache::OutputChain was written as a way of exploring the possibilities of stacked handlers in mod_perl. It ties STDOUT to an object which catches the output and makes it easy to build a chain of modules that work on output data stream.

Examples of modules that are build on this idea are Apache::SSIChain, Apache::GzipChain and Apache::EmbperlChain -- the first processes the SSI's in the stream, the second compresses the output on the fly, the last adds Embperl processing.

The syntax goes like this:

  <Files *.html>
    SetHandler perl-script
    PerlHandler Apache::OutputChain Apache::SSIChain Apache::PassHtml

The modules are listed in the reverse order of their execution -- here the Apache::PassHtml module simply picks a file's content and sends it to STDOUT, then it's processed by Apache::SSIChain, which sends its output to STDOUT again. Then it's processed by Apache::OutputChain, which finally sends the result to the browser.

An alternative to this approach is Apache::Filter, which has a more natural forward configuration order and is easier to interface with other modules.

It works with Apache::Registry as well, for example:

  Alias /foo /home/httpd/perl/foo
  <Location /foo>
    SetHandler "perl-script"
    Options +ExecCGI
    PerlHandler Apache::OutputChain Apache::GzipChain Apache::Registry

It's really a regular Apache::Registry setup, except for the added modules in the PerlHandler line.

(Apache::GzipChain allows to compress the output on the fly.)

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Apache::Filter - Alter the output of previous handlers

META: to be written (actually summarized the info from Apache::Filter manpage)

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Apache::GzipChain - compress HTML (or anything) in the OutputChain

Have you ever served a huge HTML file (e.g. a file bloated with JavaScript code) and wondered how could you send it compressed, thus dramatically cutting down the download times? After all Java applets can be compressed into a jar and benefit from faster download times. Why can't we do the same with plain ASCII (HTML, JS etc.)? ASCII text can often be compressed by a factor of 10.

Apache::GzipChain comes to help you with this task. If a client (browser) understands gzip encoding, this module compresses the output and sends it downstream. The client decompresses the data upon receipt and renders the HTML as if it were fetching plain HTML.

For example to compress all html files on the fly, do this:

  <Files *.html>
    SetHandler perl-script
    PerlHandler Apache::OutputChain Apache::GzipChain Apache::PassFile

Remember that it will work only if the browser claims to accept compressed input, by setting the Accept-Encoding header. Apache::GzipChain keeps a list of user-agents, thus it also looks at the User-Agent header to check for browsers known to accept compressed output.

For example if you want to return compressed files which will in addition pass through the Embperl module, you would write:

  <Location /test>
    SetHandler perl-script
    PerlHandler Apache::OutputChain Apache::GzipChain Apache::EmbperlChain Apache::PassFile

Hint: Watch the access_log file to see how many bytes were actually sent, and compare that with the bytes sent using a regular configuration.

(See also Apache::GzipChain).

Notice that the rightmost PerlHandler must be a content producer. Here we are using Apache::PassFile but you can use any module which creates output.

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Apache::Gzip - Auto-compress web files with Gzip

Similar to Apache::GzipChain but works with Apache::Filter.

This configuration:

  PerlModule Apache::Filter
  <Files ~ "*\.html">
    SetHandler perl-script
    PerlSetVar Filter On
    PerlHandler Apache::Gzip

will send all the *.html files compressed if the client accepts the compressed input.

And this one:

  PerlModule Apache::Filter
  Alias /home/http/perl /perl
  <Location /perl>
    SetHandler perl-script
    PerlSetVar Filter On
    PerlHandler Apache::RegistryFilter Apache::Gzip

will compress the output of the Apache::Registry scripts. Yes, you should use Apache::RegistryFilter instead of Apache::Registry for it to work.

You can use as many filters as you want:

  PerlModule Apache::Filter
  <Files ~ "*\.blah">
    SetHandler perl-script
    PerlSetVar Filter On
    PerlHandler Filter1 Filter2 Apache::Gzip

You can test that it works by either looking at the size of the response in the access.log or by telnet:

  panic% telnet localhost 8000
  Connected to
  Escape character is '^]'.
  GET /perl/test.pl HTTP 1.1
  Accept-Encoding: gzip
  User-Agent: Mozilla

And you will get the data compressed if configured correctly.

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Apache::PerlVINC - Allows Module Versioning in Location blocks and Virtual Hosts

With this module you can have different @INC for different virtual hosts, locations and equivalent configuration blocks.

Suppose two versions of Apache::Status are being hacked on the same server. In this configuration:

  PerlModule Apache::PerlVINC
  <Location /status-dev/perl>
    SetHandler       perl-script
    PerlHandler      Apache::Status
    PerlINC          /home/httpd/dev/lib
    PerlFixupHandler Apache::PerlVINC
    PerlVersion      Apache/Status.pm
  <Location /status/perl>
    SetHandler       perl-script
    PerlHandler      Apache::Status
    PerlINC          /home/httpd/prod/lib
    PerlFixupHandler Apache::PerlVINC
    PerlVersion      Apache/Status.pm

The Apache::PerlVINC is loaded and then two different locations are specified for the same handler Apache::Status, whose development version resides in /home/httpd/dev/lib and production version in /home/httpd/prod/lib.

In case the /status/perl request is issued (the latter configuration section), the fixup handler will internally do:

  delete $INC{Apache/Status.pm};
  unshift @INC, /home/httpd/prod/lib;
  require "Apache/Status.pm";

which will load the production version of the module and it'll be used to process the request. If on the other hand if the request to the /status-dev/perl location will be issued, as configured in the former configuration section, a similar thing will happen, but a different path (/home/httpd/dev/lib) will be prepended to @INC:

  delete $INC{Apache/Status.pm};
  unshift @INC, /home/httpd/dev/lib;
  require "Apache/Status.pm";

It's important to be aware that a changed @INC is effective only inside the <Location> or a similar configuration directive. Apache::PerlVINC subclasses the PerlRequire directive, marking the file to be reloaded by the fixup handler, using the value of PerlINC for @INC. That's local to the fixup handler, so you won't actually see @INC changed in your script.

In addition the modules with different versions can be unloaded at the end of request, using the PerlCleanupHandler handler:

  <Location /status/perl>
    SetHandler         perl-script
    PerlHandler        Apache::Status
    PerlINC            /home/httpd/prod/lib
    PerlFixupHandler   Apache::PerlVINC
    PerlCleanupHandler Apache::PerlVINC
    PerlVersion        Apache/Status.pm

Also notice that PerlVersion effect things differently depending on where it was placed. If it was placed inside a <Location> or a similar block section, the files will only be reloaded on requests to that location. If it was placed in a server section, all requests to the server or virtual hosts will have these files reloaded.

As you can guess, this module slows the response time down because it reloads some modules on a per-request basis. Hence, this module should only be used in a development environment, not a production one.

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When Apache's builtin syslog support is used, the stderr stream is redirected to /dev/null. This means that Perl warnings, any messages from die(), croak(), etc., will also end up in the black hole. The HookStderr directive will hook the stderr stream to a file of your choice, the default is shown in this example:

 PerlModule Apache::LogSTDERR
 HookStderr logs/stderr_log

[META: see http://forum.swarthmore.edu/epigone/modperl/vixquimwhen ]

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Because of the way mod_perl handles redirects, the status code is not properly logged. The Apache::RedirectLogFix module works around that bug until mod_perl can deal with this. All you have to do is to enable it in the httpd.conf file.

  PerlLogHandler Apache::RedirectLogFix

For example, you will have to use it when doing:


and do some manual header sending, like this:


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The output of system(), exec(), and open(PIPE,"|program") calls will not be sent to the browser unless your Perl was configured with sfio.

One workaround is to use backticks:

  print `command here`;

But a cleaner solution is provided by the Apache::SubProcess module. It overrides the exec() and system() calls with calls that work correctly under mod_perl.

Let's see a few examples:

  use Apache::SubProcess qw(system);
  my $r = shift;
  system "/bin/echo hi there";

overrides built-in system() function and sends the output to the browser.

  use Apache::SubProcess qw(exec);
  my $r = shift;
  exec "/usr/bin/cal"; 
  print "NOT REACHED\n";

overrides built-in exec() function and sends the output to the browser. As you can see the print statement after the exec() call will be never executed.

  use Apache::SubProcess ();
  my $r = shift;
  my $efh = $r->spawn_child(\&env);
  sub env {
      my $r = shift;
      $r->subprocess_env(HELLO => 'world');

env() is a function that sets an environment variable that can be seen by the main and sub-processes, then it executes /bin/env program via call_exec(). The main code spawn a process, and tells it to execute the env() function. This call returns an output filehandler from the spawned child process. Finally it takes the output generated by the child process and sends it to the browser via send_fd(), that expects the filehandler as an argument.

  use Apache::SubProcess ();
  my $r = shift;
  my $fh = $r->spawn_child(\&banner);
  sub banner {
      my $r = shift;
      # /usr/games/banner on many Unices

This example is very similar to the previous, but shows how can you pass arguments to the external process. It passes the string to print as a banner to via a subprocess.

  use Apache::SubProcess ();
  my $r = shift;
  use vars qw($String);
  $String = "hello world";
  my($out, $in, $err) = $r->spawn_child(\&echo);
  print $out $String;
  sub echo {
      my $r = shift;
      $r->subprocess_env(CONTENT_LENGTH => length $String);

The last example shows how you can have a full access to STDIN, STDOUT and STDERR streams of the spawned sub process, so you can pipe data to a program and send its output to the browser. The echo() function is similar to the earlier example's env() function. The /tmp/pecho is as follows:

  read STDIN, $buf, $ENV{CONTENT_LENGTH}; 
  print "STDIN: `$buf' ($ENV{CONTENT_LENGTH})\n";

So in the last example a string is defined as a global variable, so it's length could be calculated in the echo() function. The subprocess reads from STDIN, to which the main process writes the string (hello world). It reads only a number of bytes specified by CONTENT_LENGTH passed to the external program via environment variable. Finally the external program prints the data that it read to STDOUT, the main program intercepts it and sends to the client's socket (browser in most cases). [ TOC ]

Your corrections of the technical and grammatical errors are very welcome. You are encouraged to help me improve this guide. If you have something to contribute please send it directly to me.
The Writing Apache Modules with Perl and C book can be purchased online from O'Reilly and Amazon.com.

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Written by Stas Bekman.
Last Modified at 05/22/2001

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