Convert Your Drupal 8 Pantheon Site to Drupal 9 With Integrated Composer

Convert your Pantheon-hosted Drupal 8 sites to Drupal 9 with Integrated Composer, using the Drupal 8 to 9 Conversion Guide. Integrated Composer helps keep your Drupal 9 repository lean, avoid build artifacts on the main branch, and keep the one-click updates for both upstream commits & Composer dependencies that you have come to know and love with Pantheon.