diff --git a/.rubocop.yml b/.rubocop.yml
new file mode 100644
index 0000000..aa78bbc
--- /dev/null
+++ b/.rubocop.yml
@@ -0,0 +1,19 @@
+Layout/SpaceInsideParens:
+  Enabled: false
+
+Metrics/LineLength:
+  Max: 120
+
+Metrics/ModuleLength:
+  Exclude:
+    - "**/*_spec.rb"
+
+Metrics/BlockLength:
+  Exclude:
+    - "**/*_spec.rb"
+
+Layout/TrailingWhitespace:
+  Enabled: false
+
+Style/Semicolon:
+  Enabled: false
diff --git a/Gemfile b/Gemfile
index adcf261..a334033 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,7 +1,9 @@
-source "https://rubygems.org"
+# frozen_string_literal: true
+
+source 'https://rubygems.org'
 
 # Specify your gem's dependencies in roda-proxy.gemspec
 gemspec
 
-gem "rake", "~> 12.0"
-gem "rspec", "~> 3.0"
+gem 'rake', '~> 12.0'
+gem 'rspec', '~> 3.0'
diff --git a/Rakefile b/Rakefile
index b7e9ed5..82bb534 100644
--- a/Rakefile
+++ b/Rakefile
@@ -1,6 +1,8 @@
-require "bundler/gem_tasks"
-require "rspec/core/rake_task"
+# frozen_string_literal: true
+
+require 'bundler/gem_tasks'
+require 'rspec/core/rake_task'
 
 RSpec::Core::RakeTask.new(:spec)
 
-task :default => :spec
+task default: :spec
diff --git a/bin/console b/bin/console
index b9b7536..3efd5ff 100755
--- a/bin/console
+++ b/bin/console
@@ -1,7 +1,8 @@
 #!/usr/bin/env ruby
+# frozen_string_literal: true
 
-require "bundler/setup"
-require "roda/proxy"
+require 'bundler/setup'
+require 'roda/proxy'
 
 # You can add fixtures and/or initialization code here to make experimenting
 # with your gem easier. You can also use a different console, if you like.
@@ -10,5 +11,5 @@ require "roda/proxy"
 # require "pry"
 # Pry.start
 
-require "irb"
+require 'irb'
 IRB.start(__FILE__)
diff --git a/config.ru b/config.ru
index 4a37b60..95bea64 100644
--- a/config.ru
+++ b/config.ru
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require './addressing'
 
 run App
diff --git a/lib/roda/proxy.rb b/lib/roda/proxy.rb
index 4efada2..35488e6 100644
--- a/lib/roda/proxy.rb
+++ b/lib/roda/proxy.rb
@@ -1,10 +1,22 @@
-require "faraday"
-require "roda/proxy/version"
+# frozen_string_literal: true
 
+require 'faraday'
+require 'roda/proxy/version'
+
+# :nodoc:
 class Roda
+  # :nodoc:
   module RodaPlugins
+    # Roda plugin for simple API proxying
     module Proxy
     
+      # Respond to the configure method to set the destination when proxying
+      # Expects the following options:
+      # [to] Required. The scheme and host of the proxy. Should not end with a slash.
+      # [path] Optional. The path to append to the above for proxying.
+      #        Should begin with a +/+. Defaults to +/+.
+      # Example:
+      #   plugin :proxy, to: 'https://foo.bar', path: '/my/api'
       def self.configure(app, opts = {})
         app.opts[:proxy_to] = opts.fetch(:to, nil) 
         app.opts[:proxy_path] = opts.fetch(:path, '/')
@@ -12,45 +24,67 @@ class Roda
         raise 'Proxy host not set, use "plugin :proxy, to: http://example.com"' unless app.opts[:proxy_to]
       end
 
-    
+      # :nodoc:
       module RequestMethods
+        
+        # Proxies the request, forwarding all headers except +Host+ which is 
+        # rewritten to be the destination host. The response headers, body and
+        # status are returned to the client.
         def proxy
-          #pp @_request.env
-          #pp @_request.body.read
           method = Faraday.method(env['REQUEST_METHOD'].downcase.to_sym)
-          pp _proxy_url
-          pp _proxy_headers
-          pp roda_class
           f_response = method.call(_proxy_url) { |req| _proxy_request(req) }
-          response.status = f_response.status
-          f_response.headers.each { |k,v| response[k] = v }
-          response.write(f_response.body)
-          halt
+          _respond(f_response)
+        end
+        
+        # Conditionally proxies when +condition+ is true and with selective probability.
+        # For instance, to proxy 50% of the time:
+        #   r.proxy_when(probability: 0.5)
+        # Condition can be a truthy value or a block / lambda, in which case
+        # the result from the +#call+ is expected to be truthy.
+        #   r.proxy_when( r.env['HTTP_PROXY_ME'] == 'true' )
+        # The two parameters can be combined, the probability is evaluated first.
+        #   r.proxy_when( r.env['HTTP_PROXY_ME'] == 'true', probability: 0.8 )
+        # If and only if this method choses not to proxy is the block evaluated.
+        # The block is then expected to return a meaningful response to Roda.
+        def proxy_when(condition = true, probability: 1.0)
+          shall_proxy = Random.rand(0.0..1.0) <= probability
+                    
+          if shall_proxy && ( condition.respond_to?(:call) ? condition.call : condition )
+            yield(self)
+          else
+            proxy
+          end
         end
         
         private
         
         def _proxy_url
-          @_proxy_uri ||= URI(roda_class.opts[:proxy_to])
-            .then { |uri| uri.path = roda_class.opts[:proxy_path] ; uri }
-            .then { |uri| uri.query = env['QUERY_STRING'] ; uri }
+          @_proxy_url ||= URI(roda_class.opts[:proxy_to])
+                          .then { |uri| uri.path = roda_class.opts[:proxy_path]; uri }
+                          .then { |uri| uri.query = env['QUERY_STRING']; uri }
         end
         
         def _proxy_headers
           env
-            .select { |k,_v| k.start_with? 'HTTP_'}
-            .reject { |k,_v| k == 'HTTP_HOST' }
+            .select { |k, _v| k.start_with? 'HTTP_' }
+            .reject { |k, _v| k == 'HTTP_HOST' }
             .transform_keys do |k| 
               k.sub(/^HTTP_/, '')
-                .split('_')
-                .map(&:capitalize)
-                .join('-')
+               .split('_')
+               .map(&:capitalize)
+               .join('-')
             end.merge({ 'Host' => "#{_proxy_url.host}:#{_proxy_url.port}" })
         end
         
         def _proxy_request(req)
           req.headers = _proxy_headers
         end
+        
+        def _respond(proxied_response)
+          response.status = proxied_response.status
+          proxied_response.headers.each { |k, v| response[k] = v }
+          response.write(proxied_response.body)
+        end
       end
     end
     
diff --git a/lib/roda/proxy/version.rb b/lib/roda/proxy/version.rb
index 62b281c..c38094e 100644
--- a/lib/roda/proxy/version.rb
+++ b/lib/roda/proxy/version.rb
@@ -1,5 +1,7 @@
+# frozen_string_literal: true
+
 class Roda
   module Proxy
-    VERSION = "0.1.0"
+    VERSION = '0.1.0'
   end
 end
diff --git a/roda-proxy.gemspec b/roda-proxy.gemspec
index 3f9421a..0d22232 100644
--- a/roda-proxy.gemspec
+++ b/roda-proxy.gemspec
@@ -1,34 +1,37 @@
+# frozen_string_literal: true
+
 require_relative 'lib/roda/proxy/version'
 
 Gem::Specification.new do |spec|
-  spec.name          = "roda-proxy"
+  spec.name          = 'roda-proxy'
   spec.version       = Roda::Proxy::VERSION
-  spec.authors       = ["Nigel Brookes-Thomas"]
-  spec.email         = ["nigel@brookes-thomas.co.uk"]
+  spec.authors       = ['Nigel Brookes-Thomas']
+  spec.email         = ['nigel@brookes-thomas.co.uk']
 
   spec.summary       = 'Proxy service for Roda'
   spec.description   = 'Roda proxy service'
-  spec.homepage      = "http://foo.bar"
-  spec.license       = "MIT"
-  spec.required_ruby_version = Gem::Requirement.new(">= 2.3.0")
+  spec.homepage      = 'http://foo.bar'
+  spec.license       = 'MIT'
+  spec.required_ruby_version = Gem::Requirement.new('>= 2.3.0')
 
-  spec.metadata["allowed_push_host"] = "TODO: Set to 'http://mygemserver.com'"
+  spec.metadata['allowed_push_host'] = "TODO: Set to 'http://mygemserver.com'"
 
-  spec.metadata["homepage_uri"] = spec.homepage
-  spec.metadata["source_code_uri"] = "http://foo.bar"
-  spec.metadata["changelog_uri"] = "http://foo.bar"
+  spec.metadata['homepage_uri'] = spec.homepage
+  spec.metadata['source_code_uri'] = 'http://foo.bar'
+  spec.metadata['changelog_uri'] = 'http://foo.bar'
 
   # Specify which files should be added to the gem when it is released.
   # The `git ls-files -z` loads the files in the RubyGem that have been added into git.
-  spec.files         = Dir.chdir(File.expand_path('..', __FILE__)) do
+  spec.files = Dir.chdir(File.expand_path(__dir__)) do
     `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
   end
-  spec.bindir        = "exe"
+  spec.bindir        = 'exe'
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
-  spec.require_paths = ["lib"]
+  spec.require_paths = ['lib']
   
   spec.add_dependency 'faraday', '~> 1.0'
   spec.add_dependency 'roda', '~> 3.0'
   
   spec.add_development_dependency 'rerun', '~> 0.13'
+  spec.add_development_dependency 'rubocop', '~> 0.80'
 end
diff --git a/spec/roda/proxy_spec.rb b/spec/roda/proxy_spec.rb
index c666fec..8baf6dc 100644
--- a/spec/roda/proxy_spec.rb
+++ b/spec/roda/proxy_spec.rb
@@ -1,9 +1,11 @@
+# frozen_string_literal: true
+
 RSpec.describe Roda::Proxy do
-  it "has a version number" do
+  it 'has a version number' do
     expect(Roda::Proxy::VERSION).not_to be nil
   end
 
-  it "does something useful" do
+  it 'does something useful' do
     expect(false).to eq(true)
   end
 end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index e519d25..b624ad1 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,9 +1,11 @@
-require "bundler/setup"
-require "roda/proxy"
+# frozen_string_literal: true
+
+require 'bundler/setup'
+require 'roda/proxy'
 
 RSpec.configure do |config|
   # Enable flags like --only-failures and --next-failure
-  config.example_status_persistence_file_path = ".rspec_status"
+  config.example_status_persistence_file_path = '.rspec_status'
 
   # Disable RSpec exposing methods globally on `Module` and `main`
   config.disable_monkey_patching!