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Re: SPARQL 1.1 Protocol: Format of fault messages

From: Richard Cyganiak <richard@cyganiak.de>
Date: Sun, 3 Oct 2010 21:40:44 +0100
Cc: public-rdf-dawg-comments@w3.org
Message-Id: <C7DC541F-AD1F-4698-A2ED-D8F83AE5A1E2@cyganiak.de>
To: Eric Prud'hommeaux <eric@w3.org>
Eric,

If you write in the spec that they should serve XHTML, then they will  
serve you tag soup. This is not a realistic option.

Richard


On 3 Oct 2010, at 21:08, Eric Prud'hommeaux wrote:

> * Richard Cyganiak <richard@cyganiak.de> [2010-10-03 19:36+0100]
>> On 3 Oct 2010, at 17:03, Eric Prud'hommeaux wrote:
>>> I read that as you're OK with the SOAP binding specifying a SOAP  
>>> fault
>>> which is different from the HTTP bindings, so <malformed-query/>  
>>> stays
>>> as is.
>>
>> I don't know what a SOAP fault is. I'm just concerned with the HTTP
>> bindings.
>>
>>>> Saying that you SHOULD use a certain format leaves enough wiggle
>>>> room for implementors to innovate, IMO.
>>>
>>> Sure, but we'd also like to encourage a backwards-compatible
>>> extensibility model which allows me to have a great new idea without
>>> breaking all the old code which e.g. reports errors to humans.
>>>>
>>>
>>>> I have no objection against also proposing alternative response
>>>> formats, as long as one option is clearly highlighted as the “no-
>>>> brainer” option.
>>>
>>> And I'm interested in making sure that the alternatives subsume the
>>> no-brainer in a way that the no-brainer parser can use.
>>
>> Fair points.
>>
>>>> And RDFa would be better than a magic class name.
>>>
>>> I'm afraid of demanding a generic RDFa parser for consumers of error
>>> messages.
>>
>> I see your point. But I'm afraid of even demanding an XHTML parser.
>> In reality, too much XHTML ends up as tag soup.
>
> I think the XML parser they use for the SPARQL Results will be fine.
> If they were using DOM to parse, they can getElementById; if they're
> using SAX, they can:
>
> public void startElement(String namespaceURI, String localName,
>                         String qName, Attributes atts) throws  
> SAXException {
>    userMessage = false;
>    for (int att = 0; att < atts.getLength(); att++) {
> 	if (atts.getLocalName(att) == "id" &&
> 	    atts.getValue(att) == "malformedQueryFault") {
> 	    userMessage = true;
> 	}
>    }
> }
>
> public void characters(char[] ch, int start, int length) throws  
> SAXException {
>    if (userMessage)
> 	...
>
> Either way, I think that the additional client code required to get
> the user message is between 1 and 10 lines. For user code, they
> already have some way to entity-encode text so it's basically
>    print(template.top + entityEncode(errorMessage) + template.bottom);
>
>
>> How about this:
>>
>> [[
>> A malformedQueryFault response SHOULD be a text/plain document
>> containing a human-readable representation of the error. For
>> example:
>>
>>   HTTP/1.1 400 Bad Request
>>   Content-Type: text/plain
>>   Content-Length: 37
>>
>>   Parse error at [5:17], unexpected ' '
>>
>> Conformant implementations MAY convey the error message using
>> other formats than text/plain. If they do so, or include more
>> content besides the one-line error message in a text/plain
>> response, then they MUST include a one-line human-readable
>> representation of the error as an X-Malformed-Query-Fault HTTP
>> header:
>>
>>   HTTP/1.1 400 Bad Request
>>   Content-Type: application/xhtml+xml
>
> +1 to inlcuding the header in the example. We can just use text/html
> and every browser on the planet will handle it.
>
>
>>   X-Malformed-Query-Fault: Parse error at [5:17], unexpected ' '
>
> Interesting, I could see doing this if the client's error-parsing
> burden were higher, but I've pretty much convinced myself (above)
> that it's easy (especially compared to getting at headers when I'm
> running as a CGI, servlet, webkit client, etc.).
>
>
>>   <?xml version="1.0" encoding="UTF-8"?>
>>   <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0//EN"
>> "http://www.w3.org/MarkUp/DTD/xhtml.dtd">
>>   <html xmlns="http://www.w3.org/1999/xhtml">
>>     <head>
>>       <title>Malformed Query</title>
>>     </head>
>>     <body>
>>       <h1>Malformed Query</h1>
>>       <pre id="malformedQueryFault">Parse error at [5:17],
>> unexpected ' '</pre>
>>     </body>
>>   </html>
>> ]]
>>
>>
>> Advantages:
>>
>> 1. Anyone can still do anything, as long as they include one HTTP  
>> header
>> 2. Machine-readability is *always* given
>
> but via heuristics; I have to have two handlers.
>
>
>> 3. Backwards compatible: If a client checks for presence of the
>> header, and if absent checks for a text/plain body, then it will
>> always get the message.
>
> My first interpretation was that you should give plain text, but if
> you weren't content with that, maybe something like this HTML here...
> I now understand that this is to encourage exactly one of these two
> alternatives.
>
> While this does allow me to convey structured info while still giving
> nice-to-read info to the primitive clients, I don't think that the
> text/plain avoids much effort. The clients will still need to
> implement both and the servers have a trivial amount of work to do to
> implement the XHTML. I think the net work is lower, and the protocol
> is simpler, if the only SHOULD is the XHTML alternative.
>
>
>
>> Best,
>> Richard
>>
>>
>>
>>
>>> (Of course, I'm happy to see that practice flourish on its
>>> own.) Class was a poor choice, how about ID? New proposal:
>>>
>>> [[
>>> A malformedQueryFault response SHOULD be an XHTML document in  
>>> which an
>>> element with an XML ID of <code>malformedQueryFault</code> conveys a
>>> human-readable representation of the error. For example:
>>>
>>> <?xml version="1.0" encoding="UTF-8"?>
>>> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0//EN"
>>> "http://www.w3.org/MarkUp/DTD/xhtml.dtd">
>>> <html xmlns="http://www.w3.org/1999/xhtml">
>>>  <head>
>>>    <title>Malformed Query</title>
>>>  </head>
>>>  <body>
>>>    <h1>Malformed Query</h1>
>>>    <pre id="malformedQueryFault">Parse error at [5:17],
>>> unexpected ' '</pre>
>>>  </body>
>>> </html>
>>>
>>> Other HTML elements in the response do not change the interpretation
>>> of the element with the XML ID of <code>malformedQueryFault</code>/
>>> ]]
>>>
>>>
>>>> Best,
>>>> Richard
>>>
>>> -- 
>>> -ericP
>>
>
> -- 
> -ericP
Received on Sunday, 3 October 2010 20:47:54 GMT

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