The Revalidate Tag

The <revalidate> tag indicates a timestamp after which any content in cache with an older timestamp will be considered stale. The timestamp in the tag is normally the current time, to effect a revalidation on the next request from “now” and thus ensure that every edge server will revalidate its copy of the file.

The timestamp may be a time in the past, though it should never be a time before the content was last changed on the origin server. It cannot be a future time. Otherwise, old versions of the content may persist in cache. The tag has the following syntax:

<revalidate>1113551040</revalidate>
<revalidate>now</revalidate>
The content of the tag may be:
  • The key word, now, telling the system to process the request as soon as possible, with now meaning the time that the edge network receives the request.
  • An integer representing an epoch time. In the above example, the 1113551040 statement means, revalidate the file with an If-Modified-Since request to the origin if it was last validated before “Fri Apr 15 07:44:00 2005 UTC.” The time may not be a time in the future, but it may be a time in the past.