Developing for Drupal
The Drupal engine is open source. It is possible for each and every user to become a contributor. The fact remains that most Drupal users, even those skilled in programming arts, have never contributed to Drupal even though most of us had days where we thought to ourselves: “I wish Drupal could do this or that...”. Through this section, we hope to make Drupal more accessible to them.
The guide pages found here are collaborative, but not linked to particular Drupal versions. Because of this, documentation can become out of date. To combat this, we are moving most developer documentation into the Doxygen documentation that is versioned by CVS and generated from the source code. Look there for up-to-date and version-specific information.
What about upgrading and backwards compatability? For more details read this overview of the Drupal's philosophy on backwards compatibility