Ruby Application Behavior on Heroku

Last updated February 12, 2025

Auto-Detection
Config Vars
Add-ons
Process Types
Additional Reading

Heroku fully supports pure Ruby applications, such as headless processes and evented web frameworks like Goliath. This article describes their behavior.

Auto-Detection

When Heroku recognizes a deployed app as a pure Ruby application, we respond with -----> Ruby app detected at deploy time.

$ git push heroku main
-----> Ruby app detected

Config Vars

We set the following environment variables on Ruby apps, depending on if you use Cloud Native Buildpacks or not:

Classic Ruby Buildpack Config Vars

GEM_PATH => vendor/bundle/#{RUBY_ENGINE}/#{RUBY_ABI_VERSION}
LANG => en-us
PATH => bin:vendor/bundle/#{RUBY_ENGINE}/#{RUBY_ABI_VERSION}/bin:/usr/local/bin:/usr/bin:/bin
DISABLE_SPRING => 1

GEM_PATH is set to the Bundler gem vendor directory.

Ruby Cloud Native Buildpack Config Vars

We set defaults for the following environment variables:

JRUBY_OPTS="-Xcompile.invokedynamic=false" - Invoke dynamic is a feature of the JVM intended to enhance support for dynamically typed languages, such as Ruby. This setting caused issues with Physion Passenger 4.0.16 and was disabled details. You can override this value.
RACK_ENV=${RACK_ENV:-"production"} - An environment variable that can affect Rack-based web servers and web apps behavior. You can override this value.
RAILS_ENV=${RAILS_ENV:-"production"} - A value used by all Rails apps. By default, Rails ships with three environments: development, test, and production. We recommend that all apps being deployed use production and against using a custom env such as staging details. You can override this value.
SECRET_KEY_BASE=${SECRET_KEY_BASE:-<generate a secret key>} - In Rails, 4.1+ apps, a value for generating cryptographic tokens for various things. Notably, this value is used in generating user sessions, so modifying it between builds has the effect of logging out all users. This buildpack provides a default generated value. You can override this value.
BUNDLE_WITHOUT=development:test - Tells Bundler not to install development or test groups during bundle install. You can override this value.

Environment variables modified - In addition to the default list, the buildpack modifies these environment variables:

BUNDLE_BIN=<bundle-path-dir>/bin - Install executables for all gems into the specified path.
BUNDLE_CLEAN=1 - After a successful bundle install, the Bundler automatically runs bundle clean to remove all stale gems from previous builds that are no longer specified in the Gemfile.lock.
BUNDLE_DEPLOYMENT=1 - Requires Gemfile.lock to be in sync with the current Gemfile.
BUNDLE_GEMFILE=<app-dir>/Gemfile - Tells Bundler where to find the Gemfile.
BUNDLE_PATH=<bundle-path-dir> - Directs Bundler to install gems to this path
DISABLE_SPRING= "1" - Spring is a library that tries to cache application state by forking and manipulating processes with the goal of decreasing development boot time. Disabling it in production removes significant problems. See details.
GEM_PATH=<bundle-path-dir> - Tells Ruby where gems are.
MALLOC_ARENA_MAX=2 - Controls glibc memory allocation behavior with the goal of decreasing overall memory allocated by Ruby. See details.
PATH - Various executables are installed and the PATH env var will be modified so they can be executed at the system level. This is mostly done via interfaces provided by libcnb and CNB layers rather than directly.
RAILS_LOG_TO_STDOUT=" enabled" - Sets the default logging target to STDOUT for Rails 5+ apps. details
RAILS_SERVE_STATIC_FILES=" enabled" - Enables the ActionDispatch::Static middleware for Rails 5+ apps so that the Ruby webserver, such as Puma, serves static files such as the ones in public/assets. See details.