- From: <noah_mendelsohn@us.ibm.com>
- Date: Thu, 12 Dec 2002 14:39:46 -0500
- To: Tim Berners-Lee <timbl@w3.org>
- Cc: Paul Prescod <paul@prescod.net>, www-tag@w3.org
This reminds me, I have missed in the arch. document an exposition and detailed clarification of URI opacity principles. As Tim points out in his note, URI's are not in general as opaque as, say, UUIDs. The very fact that RFC 2396 calls out not only schemes but hierarchies seems to be intended as more than a means of assigning the names...one assumes that at certain points in the chain the hierarchical name may, in fact, be navigated based on its structure (e.g. to find the resource during retrieval). On the other hand, the general principle of opacity is surely important. So, a suggestion: add to the arch. document a clear exposition of what really is implied by the opacity principle, including more suggestive guidelines as to when it might or might not be appropriate to either use structural means to construct a URI (e.g. build it up a piece at a time as a user constructs a reference in a hierarchical space) or to base processing on its internal structure. Thanks! ------------------------------------------------------------------ Noah Mendelsohn Voice: 1-617-693-4036 IBM Corporation Fax: 1-617-693-8676 One Rogers Street Cambridge, MA 02142 ------------------------------------------------------------------ Tim Berners-Lee <timbl@w3.org> Sent by: www-tag-request@w3.org 12/11/2002 10:53 PM To: Paul Prescod <paul@prescod.net> cc: www-tag@w3.org, (bcc: Noah Mendelsohn/Cambridge/IBM) Subject: Re: Determining what a URI identifies On Monday, Nov 4, 2002, at 15:21 US/Eastern, Paul Prescod wrote: > Tim Berners-Lee wrote: >> ... >> So when RDF talks about <#joe> as having >> a contact:mailbox of <mailto:joe@example.com> >> an RDF processor which is aware of the URI spec >> and the spec of mailto: knows that the object >> is an email mailbox according to the email specs. > > You want the processor to infer the type of an object from the syntax > of its URI? What happened to opacity of URIs? > Good point! Its a great general rule not to look inside a URI when you can do what you can without doing so. However, the opacity gradually reveals more and more information as more and more specifications are used, one after the other in the chain of normative reference which starts with the URI spec. The principle of opacity suggests that you do not in an application put constraints (or interpretations) on the stuff in a URI or you will limit the other specs it can be used with and thus not leverage the whole power of the web. Its important, for example, not to assume that anything whose URI ends "html" is an HTML document - it may had an md5 URI - or some as yet uninvented URI scheme, or the server may use the last few characters for something else. So principle of opacity does not say "the characters in a URI are quite arbitrary, put anything there." You can't use a mailto: URI to identify anything other than an RFC822 mailbox because the specs say how mailboxes are identified by these address@domain.things and how these can be used as URIs with the "mailto:" prefix. Tim > Paul Prescod
Received on Thursday, 12 December 2002 14:41:30 UTC