Drupal 9 is here , and there are already some useful modules available for the newest stable version. Currently, for Drupal 9 , there is Admin Toolbar, Redirect, Paragraphs, Metatag, Simple XML Sitemap, Field Group, Ctools, Entity Browser, Embed, Webform, Entity API, IMCE, Google Analytics, External Links, XML Sitemap, Focal Point, Acquia Connector, and the list appears to be growing. When searching for modules using the "Stable Release" filter, it seems that Drupal 9 is indeed forward compatible with Drupal 8!
Technical notes about past publications and work by Darrell Ulm including Apache Spark, software development work, computer programming, Parallel Computing, Algorithms, Koha, and Drupal. Source code snippets, like in Python for Spark. Retrospective of projects.