3 ms·

Why would a browser use HEAD instead of conditional GET to re-request cached content? It would require one more round-trip if a refresh is actually required.

• vsync 13y ago
  The best is to use GET with an If-Modified-Since header: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.25 http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14... This acts like HEAD if the resource has not been modified and like GET if it has. In practice this seems to be the go-to technique for browsers as well.
• apendleton 13y ago
  I've definitely observed both behaviors in Firefox, and to be honest, I'm not entirely sure why. I'll see what I can dig up...
  • mbreese 13y ago
    HEAD just returns the headers, not the content. So, it is a faster way to see if content has been updated. If it has, then the client can request the content with a GET.
    • apendleton 13y ago
      Right, the question was why not use a conditional get, which either returns content if the thing has been modified, or a 304 Not Modified and no content if it hasn't -- this accomplishes the same thing as HEAD with only one request. Above, though, I wasn't sure why one or the other wasn't always used... I looked it up, though. It looks like you have to have an ETag to do a conditional GET, so browsers use HEAD if the original response (the cached one) didn't include one.
      • vsync 13y ago
        > I looked it up, though. It looks like you have to have an ETag to do a conditional GET Nope, not the case. May I ask where you read that? It differs from my reading of RFC2616 and my experience. If the server gave you an ETag though you're supposed to include it in future cache-conditional requests. Section 13.3.4 (http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.3.4 http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13...) describes the interaction between ETags and modification dates.
        • apendleton 13y ago
          A blog post that's probably not worth linking to, since apparently it was wrong. Am I correctly inferring that you need at least a Last-Modified in the response to do a cache-conditional request, though?
          • sluukkonen 13y ago
            Either Last-Modified or Etag is sufficient. Most servers will add both, though. Edit: The difference is that Last-Modified allows clients to use a heuristic to determine if the response should be cached for a certain duration (unless explicit Cache-Control or Expires headers are used). The heuristic isn't specified, but a 10% fraction of Date - Last-Modified is suggested, and in fact, that is what e.g. Internet Explorer does.