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Re: Service discovery redux - endpoint-based mechanisms

From: Steve Harris <steve.harris@garlik.com>
Date: Fri, 14 Aug 2009 16:57:54 +0100
Cc: "public-rdf-dawg@w3.org Group" <public-rdf-dawg@w3.org>
Message-Id: <A2DCBAA9-404D-499E-8C27-CB3B415D9950@garlik.com>
To: Lee Feigenbaum <lee@thefigtrees.net>
On 14 Aug 2009, at 14:34, Lee Feigenbaum wrote:

> Steve Harris wrote:
>>> Option 8 - New protocol operation (?serviceDescription)
>>> A request to a URI such as /endpoint/sparql?serviceDescription  
>>> returns the service description. Argument for: Simple extension of  
>>> SPARQL protocol that maps nicely to non-HTTP instantiations of the  
>>> protocol. Easy to implement and invoke. A well-known pattern from  
>>> Web Services stacks. Argument against: ?? Not sure, other than  
>>> Steve is ok with it but not thrilled. :-)
>> There is an annoyance with it, which only just occurred to me.  
>> There's no really easy way to reference the actual endpoint with a  
>> relative URI, which is sadly necessary in many situations.
>> You can use </endpoint/sparql>, in the example above, but that  
>> won't work in the general case if the endpoint is being reverse  
>> proxied.
> Steve,
> I'm feeling a bit dense today, but don't totally understand what  
> you're saying here. Mind giving an example?

Sorry, the message was a bit terse.

Imagine you have a SPARQL server running on http://localhost:8080/sparql/ 
, internally to a server, firewalled, and you want to reflect that  
externally as https://machine.example.com/myapp/sparql/ using a  
reverse proxy (this might sound a bit odd, but I know of at least  
three companies that are doing exactly this).

If the service description if fetched from the exact endpoint URI, and  
the description looks like:

<> a sparql:Endpoint ;
    sparql:has sparql:featureA ;

Then it will all work fine, from both the native endpoint and via the  

However, if the description of http://localhost:8080/sparql/ lives at http://localhost:8080/sparql/?serviceDescription 
, then you can't use <>, so you have to write something like:

</sparql> a sparql:Endpoint ;
    sparql:has sparql:featureA ;

And that will give you an incorrect and unhelpful answer if you  
request it as https://machine.example.com/myapp/sparql/?serviceDescription

- Steve

Steve Harris
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+44(0)20 8973 2465  http://www.garlik.com/
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Received on Friday, 14 August 2009 15:58:33 UTC

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