A simple rack to log api performance.

api_profiler.rb

class APIProfiler
  def initialize(app, config = {})
    @app = app
    # default print out log to STDOUT, otherwise you can inject rails logger
    @config = config
    @config[:logger] = Logger.new(STDOUT) unless config[:logger]
    @config[:log_level] = :info unless config[:logger_level]
    @config[:filter] = lambda { false } unless config[:filter]  #turn off by default
  end
=begin
  def initialize(app, config = {})
    @app = app
    @config = config
#    @config[:size_threshold] = 20000 unless config[:size_threshold]
    @config[:time_threshold] = 5000 unless config[:time_threshold]
    # default print out log to STDOUT, otherwise you can inject rails logger
    @config[:logger] = Logger.new(STDOUT) unless config[:logger]
    @config[:log_level] = :info unless config[:logger_level]
  end
=end

  def call(env)
    dup._call(env)
  end

  def _call(env)
    @status, @headers, @response = @app.call(env)
    # only time is default setted
    _log(env) if @config[:filter].call(env, @status, @headers, @response)
    [@status, @headers, self]
  end

  def _log(env)
    @config[:logger].send @config[:log_level], "!! SLOW | #{@headers['X-Runtime'].to_i} | #{env['REQUEST_METHOD']} | #{env['PATH_INFO']} | #{env['rack.request.form_vars']}" 
  end

  def each(&block)
    @response.each(&block)
  end

  def to_s
  end
end

environment.rb

  config.middleware.insert_after ActionController::Failsafe, 'APIProfiler', {
    :logger => config.logger,
    :filter => lambda { |env, status, headers, response| headers['X-Runtime'].to_i > 500 }
  } unless RAILS_ENV == 'production'