Maximizing parity between development and production environments is a best practice for minimizing surprises at deployment time. The version of language VM you're using is no exception. One approach to this is to specify it using the same dependency management tool used to specify the versions of libraries your app uses. Clojure uses this technique with Leinigen, Scala with SBT, and Node.js with NPM. In each case, Heroku reads the dependency file during slug compile and uses the version of the language that you specify.
Today, we're pleased to announce that we've added support for specifying a Ruby version to Gem Bundler, the dependency management tool for Ruby. This will allow you to specify a version of Ruby to be used in your Ruby app on Heroku.
Try it out:
$ gem install bundler --pre
In your Gemfile:
source 'http://rubygems.org'
ruby '1.9.3'
gem 'rails', '3.2.3'
Then:
$ bundle install
$ git add Gemfile
$ git commit -m 'use Ruby 1.9.3'
$ git push heroku master
Prove that you're running 1.9.3:
$ heroku run 'ruby -v'
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-linux]
$ heroku run 'ruby -e "puts RUBY_VERSION"'
1.9.3
Patch Versions
While you can specify the version of Ruby for you app, you can't specify a patch version, such as Ruby 1.9.2-p290. Ruby patches often include important bug and security fixes and are extremely compatible. Heroku will provide the most secure patch level of whatever minor version number you request.
Thanks
Thanks to Terence Lee Heroku Ruby team member and bundler maintainer for the additional support of ruby versions to the Heroku Ruby Buildpack and orchestrated the release of Bundler 1.2.0. Also thanks to Yehuda Katz and the entire Bundler team for helping get this release out the door.