- From: Booth, David (HP Software - Boston) <dbooth@hp.com>
- Date: Fri, 5 May 2006 00:11:28 -0400
- To: "Dan Connolly" <connolly@w3.org>
- Cc: <public-swbp-wg@w3.org>
Hi Dan, > From: Dan Connolly [mailto:connolly@w3.org] > On Thu, 2006-05-04 at 01:04 -0400, Booth, David (HP Software - Boston) > > > From: Dan Connolly [mailto:connolly@w3.org] > > > > From: David Booth > > > . . . > > > > Because "information resources" can return different > > > > "representations" > > > > at different times (even if some happen to return the same > > > > representation every time), it seems to me that "information > > > > resources" are by their very nature abstract. > > > > > > Please be careful with your quantifiers. Your argument seems to go > > > from: > > > There are some information that have more than one > > > representation and hence are abstract to > > > All information resources have more than one representation. > > > > Almost. My argument goes from "Some information resources > > have more > > than one representation and hence are abstract" to "All information > > resources are abstract". Here is the justification. (For clarity, > > I'll avoid the term "abstract" below, and instead speak of > > "functions from time to data", since that is more precise.) > > > > 1. Given: A URI identifies a *single* resource. > > > > 2. Any "information resource" that is intended to be time varying > > (such as the "current weather report in Oaxaca") is obviously a > > function from time to data, as illustrated above. Thus, we > > know that > > some "information resources" are functions from time to data. > > Actually, in the general case, they may be functions of more > that just time: preferred media type, language, > authentication credentials, even user agent, in some cases. Yes, those are different inputs from the client. I omitted that detail because it is not relevant to this discussion. The time-varying nature of the "current weather report in Oaxaca" is independent of client input. > > > 3. For other "information resources" that are plain Web pages, if > > those Web pages ever change, then those "information resource" must > > also be functions from time to data. > > Well, they must have functions from time to data related to > them. I don't see how you conclude that they are necessarily > identical to those functions. Are you suggesting that http://example.org/doc.html might identify one thing, d, which is not a function from time to data, but d has a function, fd, from time to data, associated with it, and fd determines what representation should be returned at what time? Unless fd were also used for some other purpose, I don't see the utility in distinguishing d from fd. It seems to complicate the model. What value does it add? > > > 4. The HTTP protocol and the URI resolution mechanism are such that > > the content associated with a URI *always* has the *potential* of > > changing. Thus, the content associated with a URI is *inherently* > > changeable over time, even if by policy some Web pages are > > intended to remain constant. > > I don't agree. Wow, I am really puzzled. I don't understand how paragraph 4 above could be disputable. If I register a domain, then for any URI under that domain, I can change the content that is served from that URI at any time. Right? I don't understand what is disputable about that. > > If the IETF says http://www.ietf.org/rfc/rfc822.txt > identifies a piece of text, and not a function from time to > data, that's not just a statement of policy; we have > delegated to them the right to say what the resource _is_. Well, not quite. When IETF registered ietf.org, what we *really* delegated to them was the right to serve content from URIs under that domain. You are proposing that we *also* interpret this delegation as giving them the right to authoritatively declare what "resource" is associated with each URI under their domain. That's fine too (and I support that proposal), but the httpRange-14 decision says if a URI dereference yields a 2xx status, then the URI's resource *should* be an "information resource". So I think that gives the TAG a responsibility to be clear about what it means for something to be an "information resource", which is what I am trying to figure out. > And I don't think they're contradicting any established norms > when they say that it identifies one piece of text. That depends on the definition of "information resource". > > > 5. I haven't a clue what utility there would be in calling > > something > > an "information resource" if that thing is never ever intended to > > return some data in a 2xx response to an HTTP GET. > > > > Therefore, by Occam's Razor I conclude: > > > > All "information resources" are functions from time to data. > > Occam's Razor isn't a valid logical inference. I'm not making a logical inference. I'm proposing a *definition*. That's exactly what Occam's Razor is for: When two explanations both satisfy the observed phenomena, prefer the simpler one. I'm proposing a simpler one. > It's sometimes > appealing, but never compelling. In this case, I don't find > it even appealing. > > The more relevant principle is that of minimal constraint. If > a resource owner says their resource is a piece of data, then > we should not constrain them otherwise unless we have really > compelling reasons to do so. That's fine, I certainly agree with that principle also. I don't think my proposed definition is adding any additional constraints. Oh . . . wait. Maybe I'm now understanding your concern with adopting a simpler definition of "information resource". Are you saying that, even though a definition of "information resource" as "a function from time to data" may be simpler, adopting such a definition would prohibit URI owners from claiming that their "information resources" are pieces of data? Well, yes I guess it would be adding that constraint. Is that a problem? Hmm. It's hard for me to evaluate that since: (a) I don't have a clear enough understanding of your (or the TAG's) definition of "information resource"; and (b) I have not seen a lot of people claiming "this URI identfies both an information resource and a piece of data". More on this below. > > > . . . I think the IETF has made it pretty > > > clear that http://www.ietf.org/rfc/rfc822.txt has just > > > one representation. And they haven't done anything to > > > make the resource itself distinguishable from its > > > representation, so if they said the 2 are identical, that > > > would be coherent. > > > > > > Likewise, W3C has bound the URI > > > > http://www.w3.org/TR/2002/REC-xhtml1-20020801/DTD/xhtml1-strict.dtd > > > to a particular sequence of bytes/characters. > > > . . . > > > > In fact, it is not even possible on the Web to create a > > > > URI that is > > > > permanently bound to a single document instance that can > > > > never change: > > > > > > I gave 2 counter-examples above. > > > > No, you gave examples of URIs that are bound to content that, by > > today's policy, is not *intended* to change. The fact is, > > the content *can* be changed, even intentionally, by the owners. > > I gave 2 examples where the statement that the resource *is* > a piece of data does not logically contradict any established norms. Sure. And the statement that "the resource is a function from time to data" also does not logically contradict any established norms. So? > > Whether the resource _is_ a piece of data or is a > time-varying abstraction is not something we can observe from > HTTP interactions with the resource itself. True, not for those two examples, but for many examples (such as the Oaxaca weather report) we can. > But in both > cases, the resource owner has published information that > strongly suggests that the resource _is_ a piece of data. Whoa! Where? Can you please point me to that "published information"? The only relevant evidence I have seen is that: - An HTTP GET on the URI returns a 2xx status with some data; and - The URI owner has stated that they will not change the content that is served. and that evidence does *not* suggest that the resource is a piece of data any more than it suggests that the resource is a constant *function* from time to data. Furthermore: (a) we *know* that the content served from those URIs *could* in fact change if the URI owners ever decide to do so; and (b) defining "information resource" as "a function from time to data" provides a simpler explanation for the observed evidence than defining an "information resource" as "either a function from time to data or a piece of data". Thus, it seems much more sensible to me to say that the resource is a function, from time to data, which for the foreseeable future is *likely* to be constant, but could in fact be non-constant. > Now > they haven't published those actual logical assertions, but > we can suppose that they did and explore the consequences. > And I don't find any contradictions when I do that exploration. > > > > > it is *always* possible to change the server configuration > > > > or domain > > > > IP mapping to cause a different document instance to be served. > > > > > > That would be a bug, in the 2 cases above. > > > > What I meant was, if the domain owners' policies change, then the > > documents may be changed *intentionally*. That's a feature, not a > > bug. > > > > > [[ > > > > In other words, an http URI on the real Web > > > > identifies a logical *location* whose > > > > content *always* has the potential of changing. ]] > > > > > > I don't agree. > > > > I don't understand how this statement could be subject to dispute. > > Can you explain? > > I explained by example above. Are we in the same universe? Help me out here. The statement in [[...]] above is a simple restatement of how HTTP works. I am at a loss to understand why you disagree with it. David Booth
Received on Friday, 5 May 2006 04:12:19 UTC