bionic (3) Tie::DxHash.3pm.gz

NAME

       Tie::DxHash - keeps insertion order; allows duplicate keys

VERSION

       This document describes Tie::DxHash version 1.03

SYNOPSIS

           use Tie::DxHash;
           my(%vhost);
           tie %vhost, 'Tie::DxHash' [, LIST];
           %vhost = (
               ServerName  => 'foo',
               RewriteCond => 'bar',
               RewriteRule => 'bletch',
               RewriteCond => 'phooey',
               RewriteRule => 'squelch',
           );

DESCRIPTION

       This  module was  written to     allow the  use of    rewrite  rules in   Apache configuration  files
       written with Perl Sections.   However, a potential user has stated that he  needs it to support  the use
       of  multiple ScriptAlias directives within a single Virtual Host  (which is required by  FrontPage,
       apparently).  If you find a completely different use for it, great.

       The original purpose of this  module is not quite  so obscure as it might sound.  Perl Sections   bring
       the power   of a general-purpose  programming  language to Apache configuration files and,  having  used
       them  once,  many people use  them throughout.  (I take this approach since, even  in sections of the
       configuration where  I do  not need  the  flexibility, I find  it  easier to use  a consistent syntax.
       This also makes the code easier for XEmacs to  colour in ;-) Similarly, mod_rewrite is easily the most
       powerful way to  perform URL rewriting and I tend to use it  exclusively, even when a  simpler directive
       would  do the  trick, in order to group my redirections together and keep them consistent.  So, I came up
       against the following problem quite early on.

       The synopsis  shows  some syntax which  might  be needed when using  mod_rewrite within a  Perl Section.
       Clearly,  using an ordinary hash will   not do what you want.  The two additional features we  need are
       to  preserve insertion order and to allow  duplicate keys.   When retrieving an  element from  the hash
       by  name, successive requests for the same name must iterate through the duplicate entries (and,
       presumably, wrap around when  the end of  the chain is reached).  This is where Tie::DxHash  comes   in.
       Simply  by  tying   the  offending   hash,  the corresponding configuration directives work as expected.

       Running an Apache syntax  check (with docroot check)  on your configuration file (with "httpd -t") and
       checking virtual host settings (with "httpd -S") succeed without complaint.   Incidentally,  I  strongly
       recommend building   your Apache configuration files with make (or equivalent) in  order to enforce the
       above two checks, preceded by a Perl syntax check (with "perl -cx").

SUBROUTINES/METHODS

       This module   is  intended to be   called  through Perl's   tie  interface.  For reference, the following
       methods have been defined:

           CLEAR
           DELETE
           EXISTS
           FETCH
           FIRSTKEY
           NEXTKEY
           SCALAR
           STORE
           TIEHASH

DIAGNOSTICS

       None.

CONFIGURATION AND ENVIRONMENT

       Tie::DxHash requires no configuration files or environment variables.

DEPENDENCIES

       None.

INCOMPATIBILITIES

       None reported.

INTERNALS

       For those interested, Tie::DxHash works by storing the  hash data in an array of hash references
       (containing  the key/value  pairs).  This  preserves  insertion order.  A separate set  of iterators (one
       per  distinct key) keeps track of  the last retrieved value for a given key, thus  allowing the
       successive retrieval of multiple values for the same key to work as expected.

BUGS AND LIMITATIONS

       The algorithms used to retrieve and delete elements by  key run in O(N) time, so do not expect  this
       module to work well   on large data  sets.   This is not  a problem for the module's intended  use.  If
       you find  another use for the module which involves larger quantities of data, let me know and I will put
       some effort into optimising for speed.

       The  mod_rewrite  directives for  which   this module  was   written  (primarily RewriteCond and
       RewriteRule) can  occur in all  four configuration file contexts (i.e. server config,  virtual host,
       directory, .htaccess).  However, Tie::DxHash only helps when  you are using  a directive which  is mapped
       onto a  Perl hash.  This limits you to  directives which are block  sections with begin and end tags
       (like  <VirtualHost>  and  <Directory>).   I  get  round  this  by   sticking my mod_rewrite directives
       in  a name-based virtual host container  (as shown in the synopsis) even in the degenerate case where the
       web server only has one virtual host.

SEE ALSO

       perltie(1), for information on ties generally.

       Tie::IxHash(3), by Gurusamy Sarathy, if you need to preserve insertion order but not allow duplicate
       keys.

       For   information  on  Ralf S.  Engelschall's   powerful  URL  rewriting module, mod_rewrite,      check
       out     the      reference      documentation   at "http://httpd.apache.org/docs/mod/mod_rewrite.html"
       and  the URL Rewriting Guide at "http://httpd.apache.org/docs/misc/rewriteguide.html".

       For help in using Perl Sections to configure Apache,  take a look at the section called           "Apache
       Configuration      in            Perl"        at
       "http://perl.apache.org/guide/config.html#Apache_Configuration_in_Perl", part of the mod_perl    guide,
       by Stas Bekman.    Alternatively,  buy the  O'Reilly book Writing Apache Modules with Perl and C, by
       Lincoln  Stein & Doug MacEachern, and study Chapter 8: Customizing the Apache Configuration Process.

AUTHOR

       Kevin Ruscoe  "<kevin@sapphireoflondon.org>"

LICENSE AND COPYRIGHT

       Copyright (c) 2006, Kevin Ruscoe "<kevin@sapphireoflondon.org>". All rights reserved.

       This module is free software; you can redistribute it and/or modify it under the same terms as Perl
       itself. See perlartistic.

DISCLAIMER OF WARRANTY

       BECAUSE THIS SOFTWARE IS LICENSED FREE  OF CHARGE, THERE IS  NO WARRANTY FOR THE SOFTWARE,  TO THE EXTENT
       PERMITTED  BY  APPLICABLE LAW.  EXCEPT WHEN  OTHERWISE STATED IN  WRITING  THE  COPYRIGHT  HOLDERS
       AND/OR  OTHER  PARTIES  PROVIDE THE SOFTWARE "AS  IS" WITHOUT WARRANTY  OF ANY  KIND,  EITHER EXPRESSED
       OR  IMPLIED, INCLUDING, BUT NOT  LIMITED TO, THE   IMPLIED WARRANTIES OF MERCHANTABILITY  AND FITNESS
       FOR  A  PARTICULAR PURPOSE.  THE  ENTIRE  RISK AS  TO THE  QUALITY  AND PERFORMANCE OF THE  SOFTWARE IS
       WITH  YOU. SHOULD THE SOFTWARE  PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR,
       OR CORRECTION.

       IN NO EVENT UNLESS REQUIRED  BY APPLICABLE LAW OR AGREED  TO IN WRITING WILL ANY COPYRIGHT HOLDER,   OR
       ANY OTHER PARTY  WHO  MAY MODIFY  AND/OR REDISTRIBUTE THE SOFTWARE  AS  PERMITTED BY THE  ABOVE  LICENCE,
       BE LIABLE   TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES
       ARISING OUT OF THE USE OR INABILITY TO  USE THE SOFTWARE  (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR
       DATA  BEING RENDERED INACCURATE OR LOSSES  SUSTAINED BY YOU  OR THIRD PARTIES OR A FAILURE OF THE
       SOFTWARE  TO OPERATE WITH  ANY OTHER SOFTWARE), EVEN IF SUCH HOLDER  OR  OTHER PARTY HAS   BEEN  ADVISED
       OF THE POSSIBILITY   OF SUCH DAMAGES.