Andy Henderson wrote:
You can do that with the "Expires" response header. Everytime the
resource is requestef, serve it with a value of "now + minimumrefreshinterval".<<<
Ah. I see what you mean. Thank you.
The problem is that when you say "You can do that now with the "Expires" response header" - I can't. It's a theoretical capability I have, but not a practical one.
I am directly responsible for three feeds. One is a feed associated with my aggregator. It's a simple xml file stored on a shared server along with the rest of my web site. I have no access to any HTML headers. When aggregators access my feed, no code of mine runs - the transaction is handled by the server alone. The other two are feeds generated on the fly from a back-end database; again they are running on a shared server and, again, my development tool (IBM's Domino) gives me no access to set the Expires header.
I just used Microsoft's Fiddler tool to check all the feeds I subscribe to (not a scientific sample, I admit, but it's a pretty broad mix and includes blog sites and blogging tools) and just two provide Expires headers. One is the BBC, the other is Wired. Both set Expires to expire immediately. I'm guessing they have good reason to do that. I re-subscribed to Slashdot (which has implemented draconian bandwidth throttling measures) and it doesn't use Expires headers.
So, Expires is a measure that I could use in theory but is not available to me either directly or, apparently, via third party blogging sites/tools. When I look at best practice, I find Expires is either not used or is used in a different way.
Both from a provider viewpoint and from an aggregator viewpoint, Expires does not seem a practical option.
Well,
we're designing a feed format here. When this feed is served through HTTP, (re-)using the caching features of HTTP will ensure that any standard HTTP client will take advantage of it. For instance, if you use an HTTP client component that maintains it's own cache, it will automatically do the right thing. Also, when you're accessing the feed through an HTTP proxy, you will get copies from the proxy's cache when available.
I just checked and Apache allows you to set the "Expires" header through "mod_expires" (<http://httpd.apache.org/docs-2.0/mod/mod_expires.html>). Lotus Domino seems to do it though a thing called "Web Site Rule" (<http://www-128.ibm.com/developerworks/lotus/library/dwa-clientperf/>). I'm sure you can do it with other packages as well.
On the other hand, of the feeds you checked, how many did actually implement the corresponding RSS feature (<http://blogs.law.harvard.edu/tech/rss#ltttlgtSubelementOfLtchannelgt>)?
If you can demonstrate that lots of feeds use thos feature, and that aggregators indeed pay attention to it, you may be able to convince the WG that Atom needs this to achive feature parity.
Best regards, Julian
