PageSpeed Service was turned off on August 3rd, 2015. Please see Turndown Information for PageSpeed Service.
Objective
Reduce payload size by utilizing ETags to send back a 304 Not Modified response.
PageSpeed rule
This rewriter implements the PageSpeed rule for minimizing payload size.
Description
All responses served by PageSpeed Service have an associated ETag that
identifies the content hash of the response.
If a browser uses the If-None-Match header
to pass this ETag value in a subsequent request to the PageSpeed Service
servers, the servers respond
with a 304 Not Modified response if the content has not changed.
In case of private responses, the PageSpeed Service servers will
contact the reference domain servers to get the fresh content and then compare
the computed ETag values to decide whether to serve a fresh response or a
304.
This feature significantly reduces the number of bytes downloaded in repeat views of a
page and in cases where the same resource is requested across different pages of
the same website.
Operation
PageSpeed Service adds the ETag header with the response content hash to all responses served by the service.
Example
The effect of this rewriter on an example resource can be observed by viewing
the headers for the downloaded image in this example page.
An Etag header with value
09bab05588df1fe22c73215e84f492ac can be found in the image
response. If you refresh the page (by pressing the Refresh button
on your browser), you will see a 304 Not Modified response in most
browsers.