Module Versioning with Apache 1.x and mod_perl 1.x

1. Module Versioning Theron Stanford The Challenge Module Versioning The Idea The Keys to with Apache 1.x and mod_perl 1.x the Solution Declaring Packages Loading Modules Conﬁguring Mason Modifying httpd.conf Theron Stanford Demonstration Time! Oversee.net April 17, 2008

2. Software Framework Module Versioning Apache 1.3 Theron Stanford Perl 5.8 The Challenge mod_perl 1.29 The Idea HTML::Mason 1.32 The Keys to the Solution Declaring Packages Loading Modules Conﬁguring Mason Modifying httpd.conf Demonstration Time!

6. Original Directory Tree Module Versioning Theron /usr/local/perlmongers Stanford The Challenge lib www The Idea perl5 mason The Keys to the Solution Declaring Packages PerlMongers Loading Modules Conﬁguring Mason Modifying httpd.conf Demonstration The standard setup is like this. Apache looks for the Time! Mason ﬁles in www (www/mason is used for the cache) and the modules in lib/perl5/PerlMongers.

7. Desired Directory Tree Module Versioning Theron /usr/local/perlmongers Stanford The Challenge path1 path2 The Idea lib www lib www The Keys to the Solution Declaring Packages perl5 mason perl5 mason Loading Modules Conﬁguring Mason Modifying httpd.conf PerlMongers PerlMongers Demonstration Time! We want Apache and mod_perl to be able to load both branches and serve the content depending on the URL. We’d like to do this without having to change the names of the ﬁles or making any code changes within the modules.

8. Desired Directory Tree Module Versioning Theron /usr/local/perlmongers Stanford The Challenge path1 path2 The Idea lib www lib www The Keys to the Solution Declaring Packages perl5 mason perl5 mason Loading Modules Conﬁguring Mason Modifying httpd.conf PerlMongers PerlMongers Demonstration Time! We want Apache and mod_perl to be able to load both branches and serve the content depending on the URL. We’d like to do this without having to change the names of the ﬁles or making any code changes within the modules.

9. The Difficulty Module Versioning With Apache 1.x and mod_perl 1.x, there is only one Perl Theron Stanford interpreter that all the code goes into. This means that we can’t load two modules that declare The Challenge the same package without one clobbering the other. The Idea The Keys to It also means we can’t (easily) use two modules with the the Solution Declaring Packages same name, even if they’re in different directories — Perl’s Loading Modules Configuring Mason internal mechanism for managing modules is set up to Modifying httpd.conf prevent duplicate loading. Demonstration Time!

12. The Idea Module Versioning Modify the module code on the fly at load time so that Theron Stanford modules from different branches end up in non-conflicting packages. The Challenge (think of C++ function name mangling — but not so The Idea ugly!) The Keys to the Solution Set up the Apache configuration and Perl handlers to use Declaring Packages Loading Modules the correct module versions and Mason files. Configuring Mason Modifying httpd.conf Demonstration Time!

13. The Idea Module Versioning Modify the module code on the fly at load time so that Theron Stanford modules from different branches end up in non-conflicting packages. The Challenge (think of C++ function name mangling — but not so The Idea ugly!) The Keys to the Solution Set up the Apache configuration and Perl handlers to use Declaring Packages Loading Modules the correct module versions and Mason files. Configuring Mason Modifying httpd.conf Demonstration Time!

14. The Keys to the Solution Module Versioning Declaring Packages Theron Stanford Loading Modules The Challenge Conﬁguring Mason The Idea Modifying httpd.conf The Keys to the Solution Declaring Packages Loading Modules Conﬁguring Mason Modifying httpd.conf Demonstration Time!

15. Packages and Namespaces Module Versioning Every variable belongs to a package whose name is a Theron Stanford namespace. In simple scripts with no package declarations, every The Challenge variable belongs to package main. The symbols $var, The Idea The Keys to $::var, and $main::var all refer to the same variable. the Solution Declaring Packages One changes packages with a Loading Modules Conﬁguring Mason package NAMESPACE Modifying httpd.conf Demonstration statement. Time! A typical use of this is in modules. For instance, the module PerlMongers/Greeting.pm would begin with the statement package PerlMongers::Greeting.

19. How Packages Help Module Versioning The plan is to modify the module namespaces according Theron Stanford to which subdirectory they come from. If a module is in perlmongers/pathn/lib/perl5, then The Challenge its package is changed from PerlMongers::* to The Idea The Keys to pathnPerlMongers::*, where n is 1 or 2. the Solution Declaring Packages All references to PerlMongers::* inside any module Loading Modules Conﬁguring Mason must also be changed to pathnPerlMongers::*. Modifying httpd.conf If a module uses another PerlMongers::* module, then Demonstration Time! this second module must also have all of its references to PerlMongers::* changed to pathnPerlMongers::*. We must take into account circular uses of use. We must make sure we don’t accidentally do something like changing PerlMongers::* to path1path1PerlMongers::*.

25. Loading Modules Module Versioning Perl uses two arrays, @INC and %INC, to keep track of Theron Stanford module loading. @INC is a list of directories Perl searches in order to ﬁnd The Challenge the module. The Idea The Keys to %INC is a hash that stores the complete path of the loaded the Solution Declaring Packages module. This prevents reloading. Loading Modules Conﬁguring Mason Because these variables are in the global namespace, users Modifying httpd.conf can modify them at will: Demonstration Time! Example: adding a directory to the front of @INC (this is essentially how use lib works) Example: deleting an entry in %INC to force reloading of a module

31. Manipulating @INC Module Versioning @INC is not limited to directory paths. Theron Stanford For our purposes, it is enough to know that @INC can contain a subroutine reference. The Challenge Arguments: a reference to itself and the name of the file The Idea to be loaded. The Keys to the Solution Return values: a list, whose first element is a filehandle to Declaring Packages read the file from Loading Modules Configuring Mason (If an empty list or undef is returned, the next value in Modifying httpd.conf @INC is considered.) Demonstration Time! In this case, the corresponding entry in %INC is set to the subroutine reference, though the subroutine could change this

36. How @INC Helps Module Versioning We unshift a subroutine reference onto the front of Theron Stanford @INC. This subroutine will inspect the ﬁlename and act according The Challenge to one of three cases. The Idea The Keys to the Solution Declaring Packages Loading Modules Conﬁguring Mason Modifying httpd.conf Demonstration Time!

37. How @INC Helps Module Versioning We unshift a subroutine reference onto the front of Theron Stanford @INC. This subroutine will inspect the ﬁlename and act according The Challenge to one of three cases. The Idea The Keys to the Solution Declaring Packages Loading Modules Conﬁguring Mason Modifying httpd.conf Demonstration Time!

38. Filename Begins with PerlMongers/ Module Versioning If the filename begins with PerlMongers/: Theron Slurp in the modules residing in Stanford perlmongers/pathn/lib/perl5 for each n. The Challenge Replace references to PerlMongers:: with The Idea pathnPerlMongers:: as appropriate. The Keys to Concatenate the text of these two modified modules and the Solution Declaring Packages place it in a lexical string variable, $source. Loading Modules Open the string variable as a filehandle using the syntax Configuring Mason Modifying httpd.conf open $fh, ’<’, $source; Demonstration and return the filehandle. Time!

43. Code for Filenames Perlmongers/* Module Versioning open $fh, "/usr/local/perlmongers/path1/lib/perl5/$filename"; Theron Stanford $s = <$fh>; close $fh; The Challenge $s =~ s"bPerlMongers::"path1PerlMongers::"g; The Idea $source .= $s; The Keys to $INC{"path1$filename"} the Solution = "/usr/local/perlmongers/path1/lib/perl5/$filename"; Declaring Packages Loading Modules open $fh, "/usr/local/perlmongers/path2/lib/perl5/$filename"; Conﬁguring Mason $s = <$fh>; Modifying httpd.conf close $fh; Demonstration Time! $s =~ s"bPerlMongers::"path2PerlMongers::"g; $source .= $s; $INC{"path2$filename"} = "/usr/local/perlmongers/path2/lib/perl5/$filename"; open $fh, ’<’, $source; return $fh;

44. Filename Begins with pathnPerlMongers/ Module Versioning If the filename begins with pathnPerlMongers/, then we Theron know that we’ve hit a use statement in a modified Stanford PerlMongers::* module: The Challenge Given pathnPerlMongers/Module.pm, slurp in The Idea perlmongers/pathn/lib/perl5/PerlMongers/ The Keys to Module.pm. the Solution Declaring Packages Replace references to PerlMongers:: with Loading Modules pathnPerlMongers:: as appropriate. Configuring Mason Modifying httpd.conf Place the module code in a lexical string variable, Demonstration $source. Time! Open the string variable as a filehandle using the syntax open $fh, ’<’, $source; and return the filehandle.

49. Code for Filenames path1PerlMongers/* Module Versioning $filename =~ s"^path1""; Theron Stanford open $fh, "/usr/local/perlmongers/path1/lib/perl5/$filename"; $s = <$fh>; The Challenge close $fh; The Idea $s =~ s"bPerlMongers::"path1PerlMongers::"g; The Keys to $source .= $s; the Solution $INC{"path1$filename"} Declaring Packages Loading Modules = "/usr/local/perlmongers/path1/lib/perl5/$filename"; Conﬁguring Mason $INC{"$filename"} = $coderef; Modifying httpd.conf open $fh, ’<’, $source; Demonstration Time! return $fh;

50. Other Filenames Module Versioning In all other cases, the module is not one of ours, and the Theron Stanford subroutine returns undef. Perl then continues to work its way through @INC to ﬁnd The Challenge the module. The Idea The Keys to the Solution Declaring Packages Loading Modules Conﬁguring Mason Modifying httpd.conf Demonstration Time!

51. Other Filenames Module Versioning In all other cases, the module is not one of ours, and the Theron Stanford subroutine returns undef. Perl then continues to work its way through @INC to ﬁnd The Challenge the module. The Idea The Keys to the Solution Declaring Packages Loading Modules Conﬁguring Mason Modifying httpd.conf Demonstration Time!

52. Handler Specifics Module Versioning Each path needs its own Mason handler so that it will pick Theron Stanford up the components from the right directory. Once the handler is written, it need never be modified again. The Challenge The comp_root and data_dir parameters sent to The Idea The Keys to HTML::Mason::ApacheHandler->new() will take care of the Solution this. Declaring Packages Loading Modules Configuring Mason Modifying httpd.conf Demonstration Time!

53. Handler Specifics Module Versioning Each path needs its own Mason handler so that it will pick Theron Stanford up the components from the right directory. Once the handler is written, it need never be modified again. The Challenge The comp_root and data_dir parameters sent to The Idea The Keys to HTML::Mason::ApacheHandler->new() will take care of the Solution this. Declaring Packages Loading Modules Configuring Mason Modifying httpd.conf Demonstration Time!

54. Munging Mason Files Module Versioning The Mason files are filled with use statements for the Theron Stanford originally named Perl modules; thus, these files must be changed to use the renamed modules. The Challenge Fortunately, Mason has a way to do this: the preprocess The Idea The Keys to parameter sent to the Solution HTML::Mason::ApacheHandler->new(). Declaring Packages Loading Modules Configuring Mason The value of preprocess is a subroutine reference. The Modifying httpd.conf subroutine takes a reference to a string containing the Demonstration Time! contents of the Mason file; the subroutine can then modify the contents in place before they are loaded by Mason.

57. Munging Mason Files: The Code Module Versioning package PerlMongers::Handler; Theron Stanford use strict; use HTML::Mason::ApacheHandler; The Challenge The Idea my $ah = HTML::Mason::ApacheHandler->new( The Keys to comp_root => ’/usr/local/perlmongers/path1/www’, the Solution data_dir => ’/usr/local/perlmongers/path1/www/mason’, Declaring Packages Loading Modules static_source => 1, Conﬁguring Mason error_mode => ’fatal’, Modifying httpd.conf preprocess => sub { Demonstration Time! my $ref = shift; $$ref =~ s"bPerlMongers::"path1PerlMongers::"g; }, ); sub handler { my $r = shift; return $ah->handle_request($r); }

58. Configuring Apache Module Versioning The Apache httpd.conf file must be configured to handle the Theron Stanford following: Inserting the subroutine reference at the beginning of @INC The Challenge to load the PerlMongers::* modules correctly. This is The Idea The Keys to done by putting this code in a small startup script, the Solution startup.pl, and using a PerlRequire directive to Declaring Packages Loading Modules include it. Configuring Mason Modifying httpd.conf Note: The order of code compilation is not always what Demonstration you expect in Perl and mod_perl. Thus, you should put Time! the code inside startup.pl inside a BEGIN block. (I speak from direct experience.) Pointing the URLs to the correct directories, including the correct handlers for the Mason code.

59. Configuring Apache Module Versioning The Apache httpd.conf file must be configured to handle the Theron Stanford following: Inserting the subroutine reference at the beginning of @INC The Challenge to load the PerlMongers::* modules correctly. This is The Idea The Keys to done by putting this code in a small startup script, the Solution startup.pl, and using a PerlRequire directive to Declaring Packages Loading Modules include it. Configuring Mason Modifying httpd.conf Note: The order of code compilation is not always what Demonstration you expect in Perl and mod_perl. Thus, you should put Time! the code inside startup.pl inside a BEGIN block. (I speak from direct experience.) Pointing the URLs to the correct directories, including the correct handlers for the Mason code.

60. Code for Loading Modules Module Versioning PerlRequire /usr/local/perlmongers/lib/perl5/startup.pl Theron Stanford <Perl> The Challenge use lib qw(/usr/local/perlmongers/lib/perl5); The Idea </Perl> The Keys to the Solution ## Reversing the order of these statements Declaring Packages Loading Modules ## gives the following error: Conﬁguring Mason ## Modifying httpd.conf ## Subroutine requiresub redefined Demonstration Time! ## at /usr/local/perlmongers/lib/perl5/startup.pl line 2. ## ## Any insights as to why would be greatly appreciated!

61. Code for Path-Speciﬁc Content Module Versioning PerlModule PerlMongers::Handler Theron Stanford Alias /path1 /usr/local/perlmongers/path1/www The Challenge Alias /path2 /usr/local/perlmongers/path2/www The Idea The Keys to <Location /path1> the Solution SetHandler perl-script Declaring Packages Loading Modules PerlHandler path1PerlMongers::Handler Conﬁguring Mason </Location> Modifying httpd.conf <Location /path2> Demonstration Time! SetHandler perl-script PerlHandler path2PerlMongers::Handler </Location>

62. Demonstration Time! Module Versioning Theron Stanford The Challenge The Idea The Keys to the Solution Declaring Packages Loading Modules Conﬁguring Mason Modifying httpd.conf Demonstration Time!

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