Checking size of each request

response_size.rb

require 'rack'

##
# Tries to calculate size of the complete response.
# This middleware should be chained in front of everything else.
# Middleware taking care of compression, etag handling etc could
# produce wrong results.
#
# Leaves streaming intact and does work file bodies from sockets
# and files.
#
# Assumes server supports keep-alive for HTTP 1.0 requests.
#
# This code has not been tested, I just wrote it down to
# demonstrate the general approach!
#
# Usage:
#
#   use(ResponseSize) { |size| puts "size is: #{size}" }
#
# Enjoy!
class ResponseSize
  # we need this to not sabotage streaming
  class Proxy < Rack::BodyProxy
    def initialize(body, size, &callback)
      super(body) { callback.call(@size) }
      @size = size
    end

    def each
      @body.each do |str|
        @size += str.bytesize
        yield str
      end
    end
  end

  BASE_OVERHEAD   = "HTTP/1.1 xxx \r\n\r\n".bytesize
  KEEP_ALIVE      = "Connection: Keep-Alive\r\n".bytesize
  HEADER_OVERHEAD = ": \r\n".bytesize

  def initialize(app, &callback)
    @app, @callback = app, callback
  end

  def call(env)
    status, headers, body = @app.call(env)

    # base overhead for every response
    size = BASE_OVERHEAD

    # size of the http messsage, like the "OK" in "HTTP/1.1 200 OK"
    size += Rack::Utils::HTTP_STATUS_CODES[status.to_i].bytesize

    # header size
    header.each do |key, values|
      values.split("\n").each do |value|
        size += key.bytesize + value.bytesize + HEADER_OVERHEAD
      end
    end

    # body size
    if headers['Content-Length'] or [204, 304].include?(status.to_i)
      size += KEEP_ALIVE if env['HTTP_VERSION'] == 'HTTP/1.0'
      size += Integer headers['Content-Length']
      callback.call(size)
    else
      body = Proxy.new(body, size, &callback)
    end

    # you still want to send that response, right?
    [status, headers, body]
  end
end

thanks, updated