Health Endpoint using only Rack in Rails apps

config.ru

# This file is used by Rack-based servers to start the application.

require ::File.expand_path('config/environment', __dir__)

map "/health" do
  run Rack::HealthCheck.new
end

run Rails.application

health_check.rb

# lib/rack/health_check.rb
module Rack
  class HealthCheck
    def call(_env)
      status = {
        redis: {
          connected: redis_connected?
        },
        postgres: {
          connected: postgres_connected?,
          migrations_updated: postgres_migrations_updated?
        }
      }

      [200, {}, [status.to_json]]
    end

    protected

    def redis_connected?
      $redis.ping == 'PONG'
    rescue
      false
    end

    def postgres_connected?
      ApplicationRecord.establish_connection
      ApplicationRecord.connection
      ApplicationRecord.connected?
    rescue
      false
    end

    def postgres_migrations_updated?
      return false unless postgres_connected?

      !ApplicationRecord.connection.migration_context.needs_migration?
    end
  end
end