Introducing AxKit
Bootstrapping AxKit
by Barrie SlaymakerMarch 13, 2002
Series Introduction
This article is the first in a series of articles introducing the AxKit web application platform. Starting from the basics, this series explains how to install a basic AxKit server and then explores AxKit's more powerful capabilities. Basic familiarity with (or ability to google for) XML, Apache, and mod_perl is assumed but expertise is not required. Some References and Helpful Resources are provided at the end of this article to help get you started as well.
AxKit: What is it?
If you already know about AxKit and the wonders of server side XML processing, you may wish to skip to the Basic Installation section.
AxKit is an application and document server that uses XML processing pipelines to generate and process content and deliver it to clients in a wide variety of formats. It's important to note that AxKit is not limited to XML source documents; non-XML documents and data sources can be converted to XML as needed. A basic AxKit pipeline looks like:
The source document may be an article, a data set, data returned by a database query, the output from a Perl routine, mod_perl handler, CGI script, etc., etc. This document is fed in to the first processor ("Transform 1") which alters it according to a "stylesheet", which specifies a set of transforms to apply to the document. The output from the first processor is fed to the second, the second to the third, and so on until the final document is passed to the browser.
Processing techniques available conventional XML processing like XSLT, advanced processing more suited to dynamic content (such as Perl versions of XSP and tag libraries), and low-level processing in Perl for those occasions where high level abstractions merely get in the way.
AxKit provides seamless caching (of both code for generating dynamic content and for transformed documents), compression, and character set conversion. AxKit also allows other technologies (CGI scripts, session management tools, data converters, Perl modules, Inline::C, etc) to be used to extend its capabilities.
The current version (v1.5) is tightly coupled with Apache/mod_perl and leverages the exceptional configurability and performance of that platform; work is underway to enable use in other environments like offline processing, cgi-bin, and standalone servers.
Why AxKit?
All the hype surrounding in-browser XML processing makes it seem like there should be little or no need for server-side XML processing. However, for a variety of reasons, XML in the browser is just not available everywhere and, when available, has limitations that server side processing addresses.
Server-side XML processing allows content to be queried, reorganized, translated, styled, and so on, before sending the "final" HTML, XML, text, SVG, or other output to the browser. The server can implement heavy duty data processing using the most effective and appropriate tools (as opposed to the rather more limited tools available client-side, even when you can control the client configuration). The server can then decide to deliver a formatted document ready for presentation and display or to delegate presentation formatting (and its attendant processing overhead) to the client. In some sense, that's an ideal approach: use the server to apply heavyweight or unusual transforms and offload the presentation formatting to the browser.
Some of the advantages of using XML on the server are:
- XML can be transformed on the server in to a wide variety of content delivery languages: XML, WML, XHTML, HTML, RTF, PDF, graphics formats, plain text, etc.
- Presentation can be separated from content and logic so that transforming the presentation does not mean altering the XML documents or settings on the authoring tools. This article demonstrates separating logic, content and presentation.
- Even using the same delivery language (HTML, say), documents can be formatted in different ways for differing display media, screen vs. printer, for instance.
- XML offers support for specifying the character encodings. Source documents can be transcoded into different character sets depending on the browser.
- XML documents can be expressed in a natural order for their "primary" display mode and reordered differently to provide different views of the data. Terms in a document's glossary or index are not in document order, for instance.
- As the content and capabilities of a web site evolves, new XML tags can be introduced in new content without having to "upgrade" older articles. We'll touch on tag libraries (taglibs) in this article and examine them in more depth in later articles.
- XML related technologies are becoming well known; hiring XML literate personnel is becomming easier. With in-house proprietary formats, you always have to train them yourself. With third party proprietary formats, you hope you can get good enough training to do what you need.
- XML processing tools (XSLT, editors, etc) are becoming commonly available, so adopting XML outside of the IT department is feasible. There's still a gap in support for WYSIWYG authoring tools, though some are now commercially available.
- XML can optionally be transformed on the client for browsers that offer sufficient features, thus reducing server workload. Be prepared for slow client-side transforms on larger pages, and you will probably want to do reordering and subsetting operations on the server.
- XML can provide descriptions of the structure of your web site and its content in multiple formats. The "Semantic Web" is coming to a browser near you (real soon now ;-). One of the biggest academic concerns about the web is the lack of semantic content. When looking at a page from a book in HTML format, who's to say whether a heading enclosed by <H1> is a book's author, title, publisher, or even a chapter or section heading? XML can be used to clarify such issues, and standards like the Resource Description Framework (RDF, of course) are gaining ground here.
- XML-based technologies like the RDF Site Summary (RSS) and the more general Resource Description Framework can be leveraged to allow automated navigation, syndication, and summarization of a web site by other web sites. This is especially applicable to portions of a web site containing current news and press releases; these are often reproduced on other sites' news summaries and "headlines" pages.
AxKit enables all of this and far more. Unlike some more insular environments, AxKit happily bolts up to almost any other technology that you can load in to an Apache web server or reach using external requests.
