On: Things I set on new servers

[mbreese]

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 → mbreese]

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 → apendleton]

> 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...) describes the interaction between ETags and modification dates.

[apendleton → vsync]

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 → apendleton]

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.