gistfile1.rb

# config/environments/development.rb

Hello::Application.configure do
  # ...

  # https://rails.lighthouseapp.com/projects/8994/tickets/4676-reload-routes-on-each-request-in-dev-mode
  service_reloader = ActiveSupport::FileUpdateChecker.new(Dir["app/services/**"]) { Rails.application.reload_routes! }

  config.to_prepare do
    service_reloader.execute_if_updated
  end
end

# app/services/main_service.rb

require 'sinatra/base'
require 'sinatra/reloader'

class MainService < Sinatra::Base
  configure :development do
    register Sinatra::Reloader
  end

  get "/" do
    "hi everybody!"
  end
end

# config/routes.rb

Hello::Application.routes.draw do
  mount MainService, at: "/api"
end