At the Forge - Blosxom

If this were all that Blosxom provides, I would not be too impressed. But after examining it a bit more closely, I see that it contains a great deal of power. That power is there thanks to the combination of display templates (known as flavours, using the British spelling) and the ability to accept any number of plugin programs. The combination of these two features makes Blosxom quite extensible.

Blosxom comes with two flavours built-in, the default HTML flavour and the optional RSS flavour for the RSS syndication feed. You can view the RSS feed yourself by tacking ?flav=rss onto the end of your blog's URL. So, if you normally view your Weblog at http://localhost/cgi-bin/blosxom.cgi, you can view the RSS feed for the site at http://localhost/cgi-bin/blosxom.cgi?flav=rss. Alternatively, you can specify your preferred flavour by changing the suffix of the page you retrieve. Thus, we can see RSS with http://localhost/cgi-bin/blosxom.cgi/index.rss.

A complete flavour registry is available on the Blosxom Web site. But the basic idea is easy to grasp: in your data directory, alongside your Weblog entries, you create an HTML file whose name reflects the part of Blosxom's output you want to change.

The filename's suffix is the same as the flavour you want to modify. Thus, the file header.html changes the way the Weblog's header is displayed in the HTML flavour, and date.blah changes Blosxom's display of dates in the blah flavour. Users can set the flavour in the URL by adding the flav name-value pair (as we saw before), and the default is set in blosxom.cgi itself, with the variable $default_flavour. Because blog entries have a .txt suffix, you cannot have a txt flavour.

Each flavour file consists of an HTML snippet, along with Perl variable names that might be instantiated into the particular file. For example, story flavour files receive the variables $title and $body, among others. (A full list is available on the Blosxom Web site.) I thus can change my blog's output such that headlines are huge and right-aligned, followed by the body:

<p>
<H1 align="right">$title</h1>
<br />
$body
</p>

The above flavour inserts the $body variable, the contents of our blog story, verbatim into the HTML. This is fine if the blog author knows HTML and is willing to enter paragraph tags manually. But if we want to let people separate paragraphs with blank lines, we need to run a program on our story. Luckily, Blosxom makes it easy to write such programs with an extensible plugin architecture.

Each plugin is a Perl program loaded with the require function, which reads and evaluates code in a particular file. So require foo.pl opens foo.pl and evaluates the code it contains. I normally suggest that people avoid require in favor of use, which executes a number of commands, including require. However, because require executes at runtime, whereas use executes during the compilation phase, it is far easier to work with it here.

Blosxom assumes that any file in the plugin directory, defined by the optional $plugin_dir variable, is a plugin. Plugins are both loaded and applied in alphabetical order, which means if you want to make sure a particular plugin is applied first or last, you might need to rename it.

Each plugin is nothing more than a simple Perl program that defines one or more subroutines. Every plugin must define the start subroutine, which simply returns 1. This allows Blosxom to determine that the plugin is alive, ready and willing to be invoked. A number of other plugin subroutines are available that each plugin optionally may define, ranging from entries (which returns a list of entries) to story (which allows you to modify the contents of a story). By breaking things down in this way, Blosxom allows for a tremendous amount of customization and sophistication, while keeping the core code small and compact.

So, what sorts of features can plugins provide? There seems to be only a few restrictions. You can change the source from which Weblog entries are retrieved, the way in which this list of entries is filtered, the templates used to display the entries and the contents of the entries themselves.

A large number of plugins are available from the Blosxom Web site. Some of them depend on other plugins, while others, such as the calendar, appear only if you are using a flavour that supports the plugin. Other plugins work immediately and merely need to be dropped into your plugin directory.

A simple example of a plugin that works out of the box is atomfeed, which provides an Atom syndication feed. Atom is a competitor to RSS that has been promoted by a number of heavy-hitting bloggers and programmers, in no small part because of the competing standards now evident in the RSS world. To get an Atom feed, simply copy the atomfeed plugin to your plugins directory. You then can retrieve your Atom feed with http://localhost/cgi-bin/blosxom.cgi?flav=atom or http://localhost/cgi-bin/blosxom.cgi/index.atom.