- From: Frank Manola <fmanola@mitre.org>
- Date: Wed, 09 May 2001 14:45:06 -0400
- To: w3c-rdfcore-wg@w3.org
As I said in an earlier message, I think we need to clarify what "entity" means in our discussions. In particular, we need to distinguish between the definition of "entity" in the HTTP spec (where it is some payload that can be returned) and the definition in Brian's model, where "Entities are things like web pages, numbers and trees in the park". (If the Web develops to the point where accessing what's identified by a URI can return a tree, I hope I get adequate warning before doing it!) I take Brian's model as being something like this (although Brian is the ultimate source of wisdom for *his* model): There are things like numbers (conceptual things), trees in the park, and people (physical things), that we think about or perceive [complicated philosophical interlude omitted]. Call these things "entities". We want to talk about some of these things (that seem to be distinct, or "have identity") on the Web. Call those things "resources". We give the resources URIs in order to identify them . The mapping between a resource (identified by a URI) and the entity it denotes is, at least to start, in the mind of the denoter (whoever decided the entity needed to be a resource, and assigned a URI to it). Since there can be multiple denoters and assigners of URIs, multiple resources can be mapped to the same entity (each denoter is independently thinking of the same entity as a resource). (NB1: it helps NOT to think about entities/resources that are HTML files within filespaces controlled by the URI assigner; this is an important special case, but it IS a special case; think of entities/resources that are trees or people instead. NB2: At this point, it's well to also point out that I'm assuming a 1-1 correspondence between URIs and resources, and what varies between denoters is the way entities are associated with resources/URIs). How many resources get "created" for a given piece of physical reality or situation depends on how the denoter wants to look at things. For example, we're reminded that accessing a resource like a Web page can return different content at different times, even though the "resource" remains the same. That's a particular view that says that what the resource denotes is the concept of the page as a container of varying content. There are lots of examples of this idea, e.g., "my car" can be considered the same resource, even though various parts have been replaced at various times. We certainly want to support views like that. On the other hand, the denoter might want to actually define separate resources to represent the different states or representations of what, at another level, could be considered a single resource (if they have distinct identity for that denoter's purposes). For example, I might have multiple replicas of a database or server distributed around a network to improve performance. At one level (e.g., that of an end user), there's one resource (and one URI). At another level (e.g., that of the administrator), there are (also) multiple resources and URIs that identify the specific replicas. We want to support that kind of situation as well. Finally, even though you can imagine two resources being mapped to the same entity, if different people are doing the mappings, it isn't necessarily going to be easy to decide when two resources are really equivalent. --Frank jos.deroo.jd@belgium.agfa.com wrote: > > > > Two resources r1 and r2 can be mapped by D to the same entity. In that > case > > > we say they are equivalent. > > > > I think this is a mistake -- there are many pages which return the same > > entity (the same set of text) but we cannot assume from this that they have > > equivalent resources. To do so would be a grave mistake. > > The entity returned may vary over time while the resource stays the same. > > Just because at one point in time they return the same entity does not mean > > that they always will. > > maybe not if we talk about set-of-entities > which could go like: > > there is a set of resources called R > there is a set of uris called U > there is a function f:U->R > for all u element of U there exists a r element of R such that f(u) = r > (or we could represent a resource r as a skolem function of it's uri u) > so u1 = u2 => f(u1) = f(u2) > so u1 = u2 => r1 = r2 > > there is a set of set-of-entities called S > (s element of S is meant to represent the possible states of a resource) > there is a function g:R->S > for all r element of R there exists a s element of S such that g(r) = s > for all u element of U there exists a s element of S such that g(f(u)) = s > so u1 = u2 => s1 = s2 > > -- > Jos De Roo, AGFA http://www.agfa.com/w3c/jdroo/ -- Frank Manola The MITRE Corporation 202 Burlington Road, MS A345 Bedford, MA 01730-1420 mailto:fmanola@mitre.org voice: 781-271-8147 FAX: 781-271-8752
Received on Wednesday, 9 May 2001 14:46:15 UTC