Tutorials

Tutorials, HowTos, and other instructions on how to do cool IT things.

Pluggable systems HowTo

I recently had a discussion with Peter Wolanin about pluggable subsystems. (You can tell this is going to be an exciting entry already, can't you?) Drupal has supported a few pluggable subsystems for a long time, namely the database and cache systems. In both cases, they work on a very simple principle: Conditionally include one of two (or more) files that defines the same set of functions but with different bodies.

That's all well and good and simple, but has some very serious limitations. Most notably, because the same function name is defined twice you can never load multiple versions at the same time. That becomes a problem if you want to, say, connect to a MySQL and PostgreSQL database in the same page request. In addition, Drupal 7 is on track to include a fully introspective code registry for conditional code loading, which, based on earlier benchmarks, should be a huge performance boost. The Registry, however, assumes that all code resources (functions, classes, and interfaces) are globally unique throughout Drupal. Having a given function name defined twice will confuse the poor thing.

That is not an insurmountable problem, or even, truth be told, a difficult one. It simply requires switching from a simple include to a more formal mechanism. There are, in fact, several ways that can be done, so to further the education of the world at large (and probably embarrass myself a bit in front of other architecture buffs) I decided to write a brief survey of simple pluggable mechanisms.

Emulating preprocess theme functions in Drupal 5

Drupal 6 introduced the concept of a theme preprocess function. In short, it allows themes, theme engines, and even modules to step into the theming process and manipulate the variables that get sent to a template. It replaces the older, clunkier _phptemplate_variables() function from Drupal 5's phptemplate templating engine, but serves much the same purpose.

While we can't backport all of that new functionality, it is possible to greatly simplify _phptemplate_variables() in Drupal 5 in a way that looks a lot like Drupal 6. Specifically, we can break up _phptemplate_variables() into separate functions that act like a theme's preprocess functions in Drupal 6.

Drupal development server configuration

The Drupal development list has been sharing Drupal development server setup secrets lately (try saying that five times fast), so I figured I'd toss mine out there. And since knowing me it wouldn't be short, I figured I'd blog it instead of just posting it in email. :-) Here, then, is the development environment we've set up for Palantir.net.

PHP Group By with Arrays

By far the most common idiom when using SQL from a web application (PHP or otherwise) is simply listing records. The standard logic looks something like this (give or take real templating):

<?php
$result
= mysql_query("SELECT tid, name, size, color FROM things ORDER BY name, size");
print
"<table>\n";
print
"<tr><th>Name</th> <th>Size</th> <th>Color</th></tr>\n";
while (
$record = mysql_fetch_object($result)) {
  print
"<tr>\n";
  print
"<td><a href='viewthing.php?tid={$record->tid}'>{$record->name}</a></td>\n";
  print
"<td>{$record->size}</td>\n";
  print
"<td>{$record->color}</td>\n";
  print
"</tr>\n";
}
print
"</table>\n";
?>

That's all well and good, but in practice can be quite limiting. Why? Because you can't then group records, that is, display not one but several tables, one for each color. SQL, of course, offers a GROUP BY clause. That doesn't do what we want, however. GROUP BY is an aggregate clause, and is used for creating totals and summaries of records. We want to cluster records by a field that is not the ordering field, or a value that is calculated off of the record itself.

I've generally used two different methods for PHP-side grouping, one of them much cleaner and more flexible at the cost of a little performance.

Simplifying SQL

Most PHP applications do fundamentally the same thing: Shuffle data from an SQL database to a web page and back again. The details vary with the application, but in general that's what most web apps do. That very quickly runs into the bane of most PHP developers' lives: SQL syntax.

It's not SQL syntax itself that is bad per se. The problem is that it is a string-serialized format, which means you have to take your nice clean data structures and serialize them out into a string that has no semantic meaning to your PHP application. That's boring, dull, and introduces all sorts of places to totally mess up your application with a typo, and that's without even touching on issues of security. And then there are the issues with SQL syntax itself, in particular the way in which INSERT and UPDATE statements, which seem like they should be similar, have no similarity whatsoever. That makes "replace" operations (insert if new or update if not) very tedious to write, particularly if you have a lot of fields.

Fortunately, with a little ingenuity and help from PHP's array handling, we can give ourselves a common syntax for INSERT and UPDATE operations that maintains semantic meaning, and then get DELETE statements free of charge. Let's see how.

Hacking DNS

At the end of my last entry, I hinted at some network restructuring I'd done in the fallout of Yahoo deciding to take its network and go home. To be more specific I was just putting the last finishing touches on what was already (I think) a cool use (or abuse, I'm sure some would claim) of the DNS network. While my setup has evolved a bit over time, I am going to explain its current, theoretically final incarnation in the hopes that it proves useful to others who wish to go nuts with DNS in order to run fancy servers on a dynamic IP address.

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