Simple rack middleware to provide a health monitor for rails app using sidekiq, memcache, and activerecord.

heartbeat.rb

module Rack
  class Heartbeat

    def initialize(app)
      @app = app
    end

    def call(env)
      if env['PATH_INFO'] == "/pulse"
        @messages = []
        alive?(env)
      else
        @app.call(env)
      end
    end

  private

    def alive_response
      if @messages.length == 0
        [200, {'Content-Type' => 'text/plain'}, ["OK"]]
      else
        [500, {'Content-Type' => 'text/plain'}, ["DEAD!\n\n#{@messages.join "\n"}"]]
      end
    end

    def alive?(env)
      database_is_up?
      if redis_is_up?
        sidekiq_latency_is_good?
        sidekiq_queue_is_managable?
      end
      cache_is_up?
      alive_response
    end

    def database_is_up?
      begin
        if ActiveRecord::Base.connection && ActiveRecord::Base.connection.active?
          return true
        else
          @messages << 'DB: database is down'
          return false
        end
      rescue
        @messages << 'DB: database is down'
        return false
      end
    end

    def sidekiq_latency_is_good?
      begin
        if Sidekiq::Queue.new.latency < 30
          return true
        else
          @messages << "Sidekiq: Down! Latency is #{Sidekiq::Queue.new.latency}"
          return false
        end
      rescue
        @messages << "Sidekiq: Can't get queue latency, is redis down?"
        return false
      end
    end

    def sidekiq_queue_is_managable?
      begin
        if Sidekiq::Queue.new.size < 100
          return true
        else
          @messages << "Sidekiq: Queue is huge: #{Sidekiq::Queue.new.size}"
          return false
        end
      rescue
        @messages << "Sidekiq: Can't access the queue, is redis down?"
        return false
      end
    end

    def redis_is_up?
      begin
        if Sidekiq.redis { |conn| conn.info }
          return true
        else
          @messages << "Redis: down!"
          return false
        end
      rescue
        @messages << "Redis: down!"
        return false
      end
    end

    def cache_is_up?
      begin
        if Rails.cache.write('__heartbeat_cache_test__', 'ok', :expires_in => 1.second)
          return true
        else
          @messages << 'Cache: Memcache is disconnected.'
          return false
        end
      rescue
        @messages << 'Cache: Memcache is disconnected.'
        return false
      end
    end

  end
end