YSlow的性能测试提示

Add an Expires or a Cache-Control Header

tag: server

There are two aspects to this rule:

• For static components: implement "Never expire" policy by setting far future Expires header
• For dynamic components: use an appropriate Cache-Control header to help the browser with conditional requests

Web page designs are getting richer and richer, which means more
scripts, stylesheets, images, and Flash in the page. A first-time
visitor to your page may have to make several HTTP requests, but by
using the Expires header you make those components cacheable. This
avoids unnecessary HTTP requests on subsequent page views. Expires
headers are most often used with images, but they should be used on all components including scripts, stylesheets, and Flash components.

Browsers (and proxies) use a cache to reduce the number and size of
HTTP requests, making web pages load faster. A web server uses the
Expires header in the HTTP response to tell the client how long a
component can be cached. This is a far future Expires header, telling
the browser that this response won't be stale until April 15, 2010.

      Expires: Thu, 15 Apr 2010 20:00:00 GMT

If your server is Apache, use the ExpiresDefault directive to set an
expiration date relative to the current date. This example of the
ExpiresDefault directive sets the Expires date 10 years out from the
time of the request.

      ExpiresDefault "access plus 10 years"

Keep in mind, if you use a far future Expires header you have to
change the component's filename whenever the component changes. At
Yahoo! we often make this step part of the build process: a version
number is embedded in the component's filename, for example,
yahoo_2.0.6.js.

Using a far future Expires header affects page views only after a
user has already visited your site. It has no effect on the number of
HTTP requests when a user visits your site for the first time and the
browser's cache is empty. Therefore the impact of this performance
improvement depends on how often users hit your pages with a primed
cache. (A "primed cache" already contains all of the components in the
page.) We measured this at Yahoo!
and found the number of page views with a primed cache is 75-85%. By
using a far future Expires header, you increase the number of components
that are cached by the browser and re-used on subsequent page views
without sending a single byte over the user's Internet connection.