需要说明的是，Scrapy其实已经提供了许多Downloader Middleware，比如负责失败重试、自动重定向等功能的Middleware，它们被DOWNLOADER_MIDDLEWARES_BASE变量所定义。
DOWNLOADER_MIDDLEWARES_BASE变量的内容如下所示：
{
'scrapy.downloadermiddlewares.robotstxt.RobotsTxtMiddleware': 100,
'scrapy.downloadermiddlewares.httpauth.HttpAuthMiddleware': 300,
'scrapy.downloadermiddlewares.downloadtimeout.DownloadTimeoutMiddleware': 350,
'scrapy.downloadermiddlewares.defaultheaders.DefaultHeadersMiddleware': 400,
'scrapy.downloadermiddlewares.useragent.UserAgentMiddleware': 500,
'scrapy.downloadermiddlewares.retry.RetryMiddleware': 550,
'scrapy.downloadermiddlewares.ajaxcrawl.AjaxCrawlMiddleware': 560,
'scrapy.downloadermiddlewares.redirect.MetaRefreshMiddleware': 580,
'scrapy.downloadermiddlewares.httpcompression.HttpCompressionMiddleware'