As of today, the Cedar stack is now in general availability.
Cedar features a streamlined HTTP stack allowing for advanced HTTP capabilities, heroku run for execution of arbitrary one-off dynos, Procfile and the process model for execution of any type of worker process. Most importantly, Cedar is a polyglot platform with official support for Clojure, Java, Node.js, Python, Ruby, and Scala, and extensibility for unlimited others via buildpacks.
The Dev Center team has spent the last few months “Cedar-izing” our developer documentation, so now most articles describe use of Heroku on the Cedar platform.
Cedar is the most powerful, performant, and reliable of the three Heroku runtime stacks. In a few weeks, we'll be making it the default. But don't wait for that; even in the meantime we recommend using heroku create --stack cedar for all new apps, especially those in production. If you have applications under active development running on Aspen or Bamboo, we recommend migrating to Cedar.