How Backward Compatibility Became a Strategic Strength for Drupal

In a reflective post, Dries Buytaert revisits his 2006 belief that maintaining backward compatibility would lead to Drupal's decline. At the time, Drupal lacked automated testing, making it difficult to support stable APIs across versions. However, the adoption of test-driven development in 2008 and semantic versioning in 2013 changed the equation. Today, Drupal has more than twice as much test code as production code, enabling smooth upgrades and responsible code deprecation. This shift has allowed the project to evolve without carrying technical debt, proving that backward compatibility can support innovation when backed by a solid testing foundation.

Dries draws a comparison with SQLite, which maintains an extensive test-to-code ratio, and suggests that Drupal’s commitment to testing has been transformative. He credits the community for building a culture of quality and continuous improvement, which now supports some of the smoothest upgrades in Drupal's history. What was once seen as a liability is now seen as a strength that could sustain Drupal for the next two decades. The post ends with a forward-looking question: What are the ideas today that may seem counterintuitive but could define the future of Drupal and open source?