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Re: I-D ACTION:draft-nottingham-http-link-header-01.txt

From: Phil Archer <parcher@icra.org>
Date: Tue, 29 Apr 2008 12:07:55 +0100
Message-ID: <4817018B.8040307@icra.org>
To: Brian Smith <brian@briansmith.org>
CC: 'Mark Nottingham' <mnot@mnot.net>, 'atom-syntax Syntax' <atom-syntax@imc.org>, 'HTTP Working Group' <ietf-http-wg@w3.org>


I am sympathetic to your proposal for foo-Links - it seems clear and 
your experiments add weight to it. However, it seems to lose out on 
flexibility. With Link, anyone can create a new relationship type by 
providing an absolute URI, it's only relative ones that are tied to the 
IANA namespace.

Putting people in a tight corner will lead them to ignore some of the 
rules and I fear you'll soon see 'links' of the type


so that clashes become rather more likely.

Phil Archer

Brian Smith wrote:
> [CC'd to ietf-http-wg. I think it is better to continue the conversation
> over there as most of my comments are not specific to Atom.]
> Mark Nottingham wrote:
>> The draft does not advocate removing links from Atom 
>> documents to put them in headers; rather, the common
>> use case is repeating them in headers, so that they
>> can be easily discovered and processed.
> With HTML, I've never seen the Link header used that way; it has always been
> used to add new links to the document (usually style sheets that vary
> depending on the UA).
> When processing Atom documents, we are usually more interested in
> atom:title, atom:content/@src (if any), and atom:content/@type. In AtomPub
> we also often want atom:link/@rel='edit' and atom:link/@rel='edit-media'.
> Since the Link header can only store the atom:link elements, we are almost
> always going to have to parse the document anyway. If parsing Atom (or HTML)
> is problematic for the software application then the application shouldn't
> have chosen to store everything in Atom (or HTML) documents. :)
>>> For all those reasons, I actually think it makes a lot more 
>>> sense for the Link header registry to be mutually exclusive
>>> with the HTML and Atom registry, instead of attempting to
>>> merge them all together.
>> You're the first person to suggest that. I think we can get 
>> to a place where there's alignment between the specs without
>> abusing the semantics of existing relations. It's certainly
>> worth trying...
> It seems like a lot of effort just to (re-)define all the link relations in
> a format-agnostic way without being overly vague. It is probably even more
> work to convince everybody (especially the HTML WG) to agree to the result.
> I think it would be nice if the same link relation identifiers meant the
> same thing in Atom as they do in HTML. However, for most of the existing
> registrations, I don't see the advantage to also making them available in
> the HTTP message header. 
> Last Friday I implemented support for the Link header in a simple AtomPub
> application. Now I will take an even stronger stance: its use should not be
> encouraged at all. It is much simpler to process hyperlinks that use the
> "Relation: URI" syntax like Location and Content-Location than it is to
> process hyperlinks that use the Link header. For example, writing Python
> middleware or Apache mod_rewrite/mod_headers rules to filter/add/remove
> links is much harder using the Link header than when using the
> Location-header approach:
> 1. There is too much flexibility in the syntax of the "rel" parameter. For
> example, the following all mean the same thing:
>       rel=edit
>       rel="edit"
>       rel="\e\d\i\t"
>       rel="http://www.iana.org/assignments/link-relations.html#edit"
>       ....
> If you want to be able to catch all variations, then you have to write a
> pretty nasty regular expression.
> 2. The Link header mixes unrelated information into the same header field.
> Consequently, in order to process specific types of links, you have to parse
> the Link header field into parts, process the parts that you are interested
> in, and put it all back together.
> 3. The "rev" mechanism makes processing unnecessarily difficult. You have to
> be careful to note whenever rev=A means the same thing of rel=B when you are
> attempting to process the header.
> I think a better alternative to a single "Link" header is to define a
> standard for multiple Link-like headers:
> [Relation]-Links: #(URI-Reference LWS *(; param=value LWS))
> For example, an "edit" link would be:
> Edit-Links: http://foo.org
> This could be done by changing the registration rules for HTTP headers so
> that header fields with a "-Links" suffix must have the above syntax, with
> the definitions of the "media", "type", and "title" parameters to be the
> fixed to be the same as in HTML 4 (or 5) and Atom 1.0. Each link header
> would have to define the processing rules for when multiple links are
> provided, and applications must be prepared to handle multiple links of the
> same type, even when they are not expected (that is why I chose "-Links"
> instead of "-Link").
> Try to write a mod_headers rule or Python WSGI middleware that filters out
> all the links with a particular type. Using the "-Links" header syntax, it
> is just "del environ[HTTP_RELATION_NAME_LINKS]" in Python and "unset
> RELATION-NAME-LINKS" in mod_headers. The Link header version requires a some
> tricky parsing in Python. I think it is actually impossible to process the
> Link header correctly using Apache's mod_headers.
> I think the "-Links" header idea allows for uniform syntax (like the Link
> header) while still being extremely easy to process.
> Thoughts?
> - Brian
Received on Tuesday, 29 April 2008 11:08:33 UTC

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