[uf-rest] REST opacity and URL schemes.

Dr. Ernie Prabhakar drernie at opendarwin.org
Fri Apr 21 15:04:46 PDT 2006

Hi Breton,

Sounds good to me.   There is no "formal" meaning of semicolon, but  
since it is "newly discovered" we are free to define our own  
semantics, as long as they:
	a) Don't confuse anyone (including proxies)
	b) Are implemented using hypermedia (e.g., as links)
	    and thus don't require changes to servers or (uncaring) clients

In the context of this particular list, I'm leaning towards using  
semicolon to define "aspects" of a resource.  We'll see if we can do  
this consistently enough that at least we don't confuse ourselves...

-- Ernie P.

On Apr 21, 2006, at 10:27 AM, Breton Slivka wrote:

> It would seem that my discovery of a rarely used aspect of the HTTP  
> url scheme, namely the semicolon ; has led to perhaps a level of  
> unjustified excitement and advocacy. Intent is very difficult to  
> gleam from carefully reading a spec, despite, or perhaps because of  
> the high standards specs are held to for preciseness and lack of  
> ambiguity, which encourage a sort of unnatural writing style which  
> in all honesty is hard to read and easy to misinterpret. That  
> aside, I have a question after reading a set of documents by Tim  
> Berners-Lee, which are more geared toward the intentional side of  
> things rather than the specification side of things, which you will  
> find here: < http://www.w3.org/DesignIssues/Axioms.html >
> Regarding the issues I'm concerned with, this is my interpretation  
> of the document, and you can tell me whether it's incorrect or not:
> *For REST, it is mostly important that GET have no side effects,  
> and anything which does have side effects be implemented with POST  
> (or mail).
> *forward slashes, as mapped to unix heirarchical structures are  
> used merely as a matter of convenience, and due to the principals  
> of opacity, are not neccesarily significant to the resolution of a  
> URI,  except regarding the resolution of relative URI's in which  
> case, such interpretation should be defined per URI scheme, with  
> client support. Forward slashes do not indicate a resource, but the  
> entire URI indicates the resource, and it's entirely up to the  
> server how to find that resource based on the URL. No procedure for  
> internal URI resolution is explicitly defined.
> *That http defines such a scheme for resolving relative URI's for /  
> style urls, but not yet for semicolons as indicators of parameters  
> (though TBL provides such a scheme as a possibility for  
> implementation)
> *Therefore, virtually any symbol, including but not limited to  
> forward slashes and semicolons can be used for the purposes of  
> mapping to arbitrary data topologies, at the discretion of the URI  
> scheme designer, which can be interpreted and resolved by the  
> server in whatever way is appropriate. In addition, a great deal of  
> this flexibility is left in the http scheme to be used at the  
> discretion of server programmers. As long as opacity is observed,  
> and URI's remain idempotent nouns.
> *To this add the additional constraint given by roy fielding that  
> this can only be done as long as the nature of the URI scheme is  
> clearly communicated by the server to the client. In other words,  
> there is no assumption made that the client will be able to infer a  
> URI scheme from a base URI without explicit communication that such  
> a URI scheme is in use.
> *Notable exception is the # fragment identifier which is reserved  
> for use by clients, and to be defined when registering the mime  
> type for a document.
> *TBL then demonstrates the flexibility of URI's by laying out his  
> Matrix URI scheme, that is using the ; and = symbols to specify a  
> matrix information space, which is distinctly incompatible with a  
> heirarchical space. This does not imply that ; has any inherent  
> meaning relative to the REST or uri model, but as a scheme  
> designer, TBL is using it to represent a type of information  
> topology being made available by a server.
> If one is to take all the above statements as true and accurate  
> interpretations of REST, then solutions to certain problems become  
> available, which are not possible, or are awkward under a naive  
> surface impression that REST requires URLS to contain only forward  
> slashes / to represent information topologies. It is beginning to  
> look to me that this impression has more to do with Aesthetics, and  
> search engine technologies, than any specific requirements of REST.
> A problem this interpretation potentially solves, for instance, is  
> that of multiple views of the same information resource. Usually,  
> this would be accomplished through content negotiation. However  
> this is only effective if your multiple views all have distinct  
> mime types. If you have for example, a resource such as < http:// 
> example.com/blog/posts/53 > representing an article on a blog, you  
> may have a non editable view, and an editable view.
> In an ideal world this sort of thing could be specified using a  
> fragment identifier,  such as < http://example.com/blog/posts/ 
> 53#edit >, however for this to work on a practical level, it  
> requires client support which is not likely, and somewhat  
> incompatible with the fragment identifier scheme defined for html/xml
> Another option is to use the forward slash  < http://example.com/ 
> blog/posts/53/edit >
> However it seems logically innacurate to state that a view is a  
> hierarchical child of the resource, rather than something more  
> lateral, or directly related.
> If the bullet points above are accurate, then it becomes possible  
> and reasonable to use a semi colon, or any other appropriate symbol  
> to represent a lateral space, such as a view. Matrix notation seems  
> appropriate, since such a view system can be seen as a "matrix" (or  
> tensor) with 1 axis, and english words representing named points  
> along that axis.
> < http://example.com/blog/posts/53;view=edit > < http://example.com/ 
> blog/posts/53;view=display> < http://example.com/blog/posts/ 
> 53;view=hatom>. This makes sense to me as viewing a slice or  
> subsection of a single resource, which includes multiple possible  
> views.
> The caveats being how relative URL's are resolved by clients,  
> whether the server supports this level of flexibility, and whether  
> appropriate mechanisms are employed for informing the client of the  
> URI scheme in use (such as hrefs along all points on a matrix  
> allowing navigation)
> This may be overintellectualizing a simple problem, but the main  
> purpose of this post is to verify if I have my bulleted  
> interpretations correct, otherwise my conclusion may be false.
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