W3C home > Mailing lists > Public > www-ws-arch@w3.org > January 2003

RE: Hypermedia vs. data (long)

From: Assaf Arkin <arkin@intalio.com>
Date: Wed, 1 Jan 2003 14:44:14 -0800
To: "Newcomer, Eric" <Eric.Newcomer@iona.com>, "Miles Sabin" <miles@milessabin.com>, <www-ws-arch@w3.org>

> To argue that a thesis published in 2000 that essentially
> summarizes experiences and work around HTTP 1.1 constitutes a
> kind of constraining architecture for all Web applications seems
> counter to the history of HTTP itself, which includes the ideas
> that HTTP will need to be extended.


In my opinion Roy's thesis is relevant for both HTTP 1.0 and HTTP 1.1.
Although HTTP 1.1 was released just a year before, it was a work in progress
for quite a while, and I would assume Roy had visibility into both the
emerging spec and early implementations of HTTP 1.1.

On the other hand, having read the thesis, I think the point of view that
Roy expresses is that REST is an optimal architecture for hypermedia
systems. There are other uses for HTTP that are outside the scope of REST,
in particular what the WSA working group is dealing with. To quote Roy:

"REST is not intended to capture all possible uses of the Web protocol
standards. There are applications of HTTP and URI that do not match the
application model of a distributed hypermedia system."


"The modern Web is one instance of a REST-style architecture. Although
Web-based applications can include access to other styles of interaction,
the central focus of its protocol and performance concerns is distributed
hypermedia. REST elaborates only those portions of the architecture that are
considered essential for Internet-scale distributed hypermedia interaction."

To repeat what I've said a few times before, REST is a killer architecture
and a valid one that should not be dismissed, it intersects with Web
services, but it doesn't cover the problem space that Web services are
addressing. It can apply to some problems in the Web service domain, but it
doesn't claim to cover the entire Web services domain.

At any rate, I believe that if this discussion is to move any forward, Roy -
as the author of the thesis - should be asked to present his opinion on
where/how REST and Web services intersect, and where they do, how the REST
semantics can be captured in specific Web services use cases.


> It seems to me there may be some valid argument over which
> approach to extension has the most merit, but since actual
> adoption and implementation of technology is not always aligned
> with theory, it seems much less valuable to continue to argue
> about whether SOAP and WSDL make sense as HTTP extensions in the
> absence of any other approach having gained comparable acceptance.
> A lot of this boils down, as we've discussed before, to whether
> or not semantic information can be contained in the documents
> being exchanged, or at least whether or not pointers to such
> semantic information can be contained within the documents being
> exchanged.  REST would say no, but extensions to HTTP might well
> say yes, as they have.
> Eric
> -----Original Message-----
> From: Miles Sabin [mailto:miles@milessabin.com]
> Sent: Tuesday, December 31, 2002 6:09 PM
> To: www-ws-arch@w3.org
> Subject: Re: Hypermedia vs. data (long)
> Christopher B Ferris wrote,
> > REST is based on the premise that the agent receiving the data has
> > but one responsibility; to render it for (typically) human
> > consumption. It is based on a limited set of standardized media types.
> > It is low-coordination because the function of the user agent is
> > simply to render these standardized media types.
> In fairness to the REST-heads, that isn't its premise, and I don't think
> claiming it is will help Mark see round his blind-spot wrt the semantic
> issues.
> The premise of REST, at least as far as I understand it, is that an
> important class of distributed applications can be modelled as state
> transition networks, with resources as nodes, URIs as edges, and the
> operation of the application implemented as transitions over and
> modifications of that network. Hypertext rendered for human consumption
> is one application of this model, certainly, but there's no reason
> whatsoever why it should be the only one.
> But the issue Mark seems unwilling to address is the fact that the model
> stated this abstractly says nothing at all about the semantics of the
> application. Those semantics are the _interpretation_ of the graph:
> what the nodes/resources, edges/URIs, transitions and modifications
> _mean_ in the context of the correct functioning of the application.
> That interpretation isn't present in the graph itself (unless you count
> not-currently-machine-digestable annotations in the resources) and has
> to come from somewhere else. It can't simply be inferred from the
> structure of the graph, for the simple reason that graphs have a nasty
> habit of being isomorphic to one another. If all you look at is the
> structure and don't have anything else to pin down the significance of
> the resources and URIs, many applications will end up being
> indistinguishable from each other: eg. a pair of apps identical other
> than having "buy" and "sell" transposed might have exactly the same
> structure in REST terms ... pretty clearly a problem on the not
> unreasonable assumption that "buy" and "sell" have rather different
> real-world significance.
> Interpretation is the bit that REST leaves out (or, rather, simply rules
> out of scope), quite rightly IMO. But it's pretty clearly something
> that needs to be added if you want something that actually does
> something useful. In the case of hypertext it's added by a mixture of
> hard-coded behaviour in user-agents and intelligent behaviour on the
> part of users. If human intervention is ruled out for Web services,
> then that just leaves hard-coded behaviour of a greater or lesser
> degree of sophistication ... or verb inflation from Marks POV. I think
> that's both unavoidable and not in any obvious way damaging to REST as
> an architectural style.
> In fairness to Mark tho', I think it's still a wide open question
> whether or not WSDL or DAML-S or whatever are up to the job either. But
> I don't think that justifies ostrich-like behaviour.
> Cheers,
> Miles
Received on Wednesday, 1 January 2003 17:45:25 UTC

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