default_spec.rb

before(:all) do
  @mock_servers = []

  @mock_servers << MockHttpServer.new(9000, 'backend-service-1')
  @mock_servers << MockHttpServer.new(9001, 'backend-service-2')
end

it 'sends requests for /backend1 backend-service-1 backend' do
  backend1_uri = 'http://localhost/backend1'

  expect(get(backend1_uri)).to reach_backend('backend-service-1')
end

after(:all) do
  @mock_servers.each { |server| server.stop }
end

mock_http_server.rb

require 'rack'

class MockHttpServer
  def initialize(port, response_body)
    @response_body = response_body
    @thread = Thread.new do
      Rack::Handler::WEBrick.run(self, Host: '0.0.0.0', Port: port)
    end
  end

  def stop
    Thread.kill(@thread)
  end

  def call(env)
    [ 200, { 'Content-Type' => 'text/plain' }, [@response_body] ]
  end
end

spec_helper.rb

require 'serverspec'
require 'mock_http_server'

set :backend, :exec

RSpec::Matchers.define :reach_backend do |expected|
  match do |actual|
    actual.body == expected
  end
end

RSpec::Matchers.define :contain_header do |expected|
  match do |actual|
    expected_header = expected.to_a.flatten
    expected_header_name = expected_header[0]
    expected_header_value = expected_header[1]

    actual_headers = actual.to_hash

    actual_headers[expected_header_name][0] == expected_header_value
  end
end

def get(uri, headers = {})
  uri = URI(uri)

  Net::HTTP.start(uri.host, uri.port) do |http|
    req = Net::HTTP::Get.new(uri, headers)
    http.request(req)
  end
end