Rails Metal是Edge Rails的新特性。最近的Rails core team的工作是用rack取代了Rails老的request处理代码，并且集成了中间件的支持。Rails Meta允许你的Rails 应用使用Rack中间件来创建一个超级快的action。 


无耻的转载 国外同行 写的简单的Rails Metal的hello word例子：
class Poller < Rails::Rack::Metal
        def call(env)
             if env[ "PATH_INFO"] =~ /^\/poller/
                [[200], { "Content-Type" => "text/html"}, "Hello, World!"]
             else
                [[404], { "Content-Type" => "text/html"}, "Not Found"]
            end
        end
    end

对比一下老的controller：
class OldPollerController < ApplicationController
            def poller
                render :text => "Hello World!"
            end
        end

看一下两者的差距：
# first, let's benchmark the traditional controller
    $ ab -n 1000 http: //127.0.0.1:3000/old_poller/poller
    ... snip ...
    Requests per second:        408.45 [#/sec] (mean)
    Time per request:             2.448 [ms] (mean)

    # now for the Metal middleware
    $ ab -n 1000 http: //127.0.0.1:3000/poller
    ... snip ...
    Requests per second:        1154.66 [#/sec] (mean)
    Time per request:             0.866 [ms] (mean)

看吧，Rails Metal 的性能要比老版本的controller快2.8倍 。。。