High Performance Web Sites

Conditional GET Requests

If the browser has a copy of the component in its cache, but isn't sure whether it's still valid, a conditional GET request is made. If the cached copy is still valid, the browser uses the copy from its cache, resulting in a smaller response and a faster user experience.

Typically, the validity of the cached copy is derived from the date it was last modified. The browser knows when the component was last modified based on the Last-Modified header in the response (refer to the previous sample responses). It uses the If-Modified-Since header to send the last modified date back to the server. The browser is essentially saying, "I have a version of this resource with the following last modified date. May I just use it?"

GET /us.js.yimg.com/lib/common/utils/2/yahoo_2.0.0-b2.js HTTP/1.1
Host: us.js2.yimg.com
User-Agent: Mozilla/5.0 (...) Gecko/20061206 Firefox/1.5.0.9
Accept-Encoding: gzip,deflate
If-Modified-Since: Wed, 22 Feb 2006 04:15:54 GMT

HTTP/1.1 304 Not Modified
Content-Type: application/x-javascript
Last-Modified: Wed, 22 Feb 2006 04:15:54 GM

If the component has not been modified since the specified date, the server returns a "304 Not Modified" status code and skips sending the body of the response, resulting in a smaller and faster response. In HTTP/1.1 the ETag and If-None-Match headers are another way to make conditional GET requests. Both approaches are discussed in Chapter 15.