The Heroku Cedar stack went public beta last year with a series of blog posts. Since then, over 80,000 developers have deployed over 4.5 million times, to apps written in dozens of different programming languages and frameworks. Today, over 75 percent of Heroku app development activity is on the Cedar stack. Production apps like Banjo, Rapportive, PageLever, do.com, and Project Zebra run on Cedar; some of these serve hundreds of millions or even billions of requests per month.
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.
You can still create applications on one of our other stacks using heroku create --stack, but we recommend Cedar for all new apps. If you have applications under active development running on Aspen or Bamboo, we recommend migrating to Cedar.