Add a default health controller

Load balancers and uptime monitors all need a basic endpoint to tell whether the app is up. Here's a good starting point that'll work in many situations.
rails · dhh · Jan 10, 2023 · Jan 9, 2023 · Jan 9, 2023 · Jan 9, 2023
commit 9e093d124136b9e3ed0a9184407fdc3afc5d5c3b
diff --git a/railties/lib/rails.rb b/railties/lib/rails.rb
@@ -28,6 +28,7 @@ module Rails
   extend ActiveSupport::Autoload
   extend ActiveSupport::Benchmarkable
 
+  autoload :HealthController
   autoload :Info
   autoload :InfoController
   autoload :MailersController

diff --git a/railties/lib/rails/generators/rails/app/templates/config/routes.rb.tt b/railties/lib/rails/generators/rails/app/templates/config/routes.rb.tt
@@ -1,6 +1,10 @@
 Rails.application.routes.draw do
   # Define your application routes per the DSL in https://guides.rubyonrails.org/routing.html
 
+  # Reveal health status on /up that returns 200 if the app boots with no exceptions, otherwise 500.
+  # Can be used by load balancers and uptime monitors to verify that the app is live.
+  get "up" => "rails/health#show"
+
   # Defines the root path route ("/")
   # root "articles#index"
 end
diff --git a/railties/lib/rails/health_controller.rb b/railties/lib/rails/health_controller.rb
@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class Rails::HealthController < ActionController::Base # :nodoc:
+  rescue_from(Exception) { render_down }
+
+  def show
+    render_up
+  end
+
+  private
+    def render_up
+      render html: html_status(color: "green")
+    end
+
+    def render_down
+      render html: html_status(color: "red"), status: 500
+    end
+
+    def html_status(color:)
+      %(<html><body style="background-color: #{color}"></body></html>).html_safe
+    end
+end