Ruby

Railpack builds and deploys Ruby applications with support for several language-specific tools and frameworks.

Detection

Your project is detected as a Ruby application if a Gemfile is present in the root directory.

Versions

The Ruby version is determined in the following order:

• Set via the RAILPACK_RUBY_VERSION environment variable
• Read from the .ruby-version file
• Read from the Gemfile file
• Read from mise-compatible version files (.tool-versions, mise.toml)
• Defaults to 3.4.6

Runtime Variables

These variables are available at runtime:

BUNDLE_GEMFILE="/app/Gemfile"
GEM_PATH="/usr/local/bundle"
GEM_HOME= "/usr/local/bundle"
MALLOC_ARENA_MAX="2"

Configuration

Railpack builds your Ruby application based on your project structure. The build process:

• Installs Ruby and required system dependencies
• Installs project dependencies
• Configures the Ruby environment for production

The start command is determined by:

1. Framework-specific start command (see below)
2. config/environment.rb file
3. config.ru file
4. Rakefile file

Config Variables

Variable	Description	Example
RAILPACK_RUBY_VERSION	Override the Ruby version	3.4.2

Framework Support

Railpack detects and configures caches and commands for popular frameworks:

Rails

Railpack detects Rails projects by:

• Presence of config/application.rb

Databases

Railpack automatically installs system dependencies for common databases:

• PostgreSQL: Installs libpq-dev
• MySQL: Installs default-libmysqlclient-dev
• Magick: Installs libmagickwand-dev
• Vips: Installs libvips-dev
• Charlock Holmes: Installs libicu-dev, libxml2-dev, libxslt-dev

Node.js Integration

If a package.json file is detected in your Ruby project, or if the execjs gem is used:

• Node.js will be installed automatically
• NPM dependencies will be installed if package.json exists
• Build scripts defined in package.json will be executed
• Development dependencies will be pruned in the final image

This is particularly useful for Rails applications with frontend assets.

Bootsnap

Railpack automatically detects and optimizes applications using Bootsnap:

• Runs bundle exec bootsnap precompile --gemfile during dependency installation
• Runs bundle exec bootsnap precompile app/ lib/ during the build phase for Rails applications

Asset Pipeline

Railpack detects Rails applications using asset pipeline gems:

• Sprockets: Runs bundle exec rake assets:precompile during build
• Propshaft: Runs bundle exec rake assets:precompile during build

Rails API-only applications without asset pipeline gems skip asset compilation.

Bundler Version

Railpack automatically detects the Bundler version from the BUNDLED WITH section in your Gemfile.lock and installs that specific version.

Performance Optimizations

Railpack includes several performance optimizations:

• jemalloc: Installs and configures libjemalloc for improved memory allocation performance
• YJIT: For Ruby 3.2+, installs rustc and cargo required for YJIT compilation support

Local Path Dependencies

If your Gemfile includes gems with local path: specifications, Railpack will automatically copy those local directories during the build process.