Status: Alpha

This builder takes in a Ruby program that defines a singular HTTP handler and outputs it as a serverless function.

When to Use It

Whenever you want to expose an API or a function written in Ruby.

How to Use It

Define a index.rb file inside a /api directory as follows:

require 'cowsay'

Handler = do |req, res|
    res.status = 200
    res['Content-Type'] = 'text/plain'
    res.body = Cowsay.say('hello world', 'cow')
Note: If you want to use a directory name other than /api, read the Builds documentation.

Inside Gemfile define:

source ""

gem "cowsay", "~> 0.3.0"

Rack Interface

Many Ruby frameworks interface with Rack for forwarding HTTP requests to web applications written in Ruby. You can use Rack with frameworks such as Sinatra or Rails.

Instead of defining a handler, write the application as usual, like this Sinatra application:

require 'sinatra'

get '/*' do
    'Hello world'

Inside Gemfile define:

source ""

gem "sinatra", "~> 2.0"
gem "rack", "~> 2.0"

Add a Rack config file at to start the application:

require './app'

run Sinatra::Application

Most frameworks use their own implementation of routing. However, you can use a catch-all route to circumvent the framework and instead use the Now Routing Layer to match a route to a serverless function.

Technical Details


The entrypoint file must be a .rb source file with one of the following variables defined:

  • Handler proc that matches the do |req, res| signature
  • Handler class that inherits from the WEBrick::HTTPServlet::AbstractServlet class

Alternatively, a .ru Rack config file will serve the Ruby application it contains.


Ruby 2.5.3 is used.


This builder supports installing dependencies defined in the Gemfile. Alternatively, dependencies can be vendored with the bundler install --deployment command (useful for gems that require native extentions). In this case, dependencies are not built on deployment.