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

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

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 CWhttpd -t) and checking virtual host settings (with CWhttpd -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 CWperl -cx).

For those interested, Tie::IxHash 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.

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.

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.

Kevin Ruscoe