Sorry for the delay, finally off the road and had a chance to read.
Overall I like this document a lot, and I think it should be easy to
align its use of the 'monitor' link relation with the Atom-over-HTTP
protocol described in draft-nottingham-http-cache-channels.
A couple of comments / concerns:
* "...it MUST return a Link: header..." seems like it's too strong;
servers may want to be selective about when they offer monitoring, and
there may be other ways to advertise it that are more appropriate.
* Following on that thought, the idea behind the most recent Link
draft is to talk about link relations generically, and make Link
headers one use of them. Another use of them is in draft-nottingham-
site-meta (which is still under pretty active revision); this would
allow you to advertise a monitoring endpoint for your whole site, for
example.
* "It MAY return both." is ambiguous; think you mean SIP + SIPS.
* 3.4 Subscription Duration -- just a thought, is it worth tying the
default subscription duration to the freshness lifetime of the HTTP
response (as defined by HTTP)?
* 3.5.1 message/httpfrag -- did you consider using application/http
(defined in RFC2616)? I mention this because HTTP already defines how
to update a cache entry's headers based upon a HEAD or 304 response;
it would be nice to reuse that if possible. Also, what does making the
status-line optional bring? The semantics of the message would be
clearer if it were required...
* 3.5.2 - Why require an ETag or Content-MD5? Why recommend a Last-
Modified?
* 3.10 - limiting notifications to one a second makes sense also
because this is the granularity of time in HTTP.
Specific responses below;
On 22/11/2008, at 2:53 AM, Adam Roach wrote:
On 11/21/08 2:07 AM, Theo Zourzouvillys wrote:
On Fri, Nov 21, 2008 at 4:12 AM, Adam Roach <[EMAIL PROTECTED]> wrote:
Very interesting proposals. Admittedly, for the use cases SIP is
most
interested in, the results are not typically HTML, but the idea of
communicating these in the body may have merit. I'm curious about
which use
cases you have in mind -- do you imagine that there will be
circumstances
under which the HTTP server will have the ability to notify other
nodes of
changes, but not have the ability to include a Link: header?
absolutely - perhaps a resource hosted on a CDN, where you commonly
can not send custom headers.
You certainly can, in a cache-driven CDN (e.g., Akamai); the cases
where you can't aren't really CDNs as the industry understands them,
they're Web hosting farms (because they use replication; e.g.,
Amazon's new service, if I read it correctly). In that case, providing
a channel without coordination with them can be harmful, because you
can get into race conditions where you send the notification before
the content is fully replicated.
More to the point, if you're using a CDN (of either type), the most
likely value-add scenarios are using a monitoring channel to notify
the CDN itself of changes, and also the CDN providing a monitoring
server as a service.
or a blog, which may be hosted on some other platform (for example,
blogger.com) where you can edit the HTML but not the headers.
I'm getting somewhat exasperated at how often this argument comes up
in specification discussions; to date mostly at the W3C. If we follow
it to its conclusion, the only Internet protocols that we can develop
are those that work with free hosting sites that only provide FTP
access. Surely the Internet and the Web shouldn't be held hostage to
them.
That said, I agree that the draft shouldn't preclude other discovery
mechanisms, as mentioned above.
the Link header as defined in draft-notthingham-http-link-header
itself is only a way of representing the <link> header in HTML and
<atom:link/> header in atom entries outside of the content where it
is
not possible because the content-type does not have support. So it
already "just works" by creating a link registry entry, however would
be good to explicitly point out that it could be anywhere a link
relation is allowed.
That's interesting -- Mark, do you have any input on the topic?
I'm having trouble parsing the above -- is the concern that some
people might believe that any link in the headers *has* to be mirrored
in the content? If so, that's the first time I've heard this brought
up, and it certainly isn't the intent. We can clarify if necessary.
Cheers,
--
Mark Nottingham http://www.mnot.net/
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