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Subject: Re: [ANN] XInclude processor for xml-commons
To: nicolaken@apache.org
Cc: cocoon-dev@xml.apache.org, commons-dev@xml.apache.org,
   xerces-j-dev@xml.apache.org
Message-ID: <OF2D6261D2.6AD7D071-ON85256D1D.007D1682@torolab.ibm.com>
From: "Neil Graham" <neilg@ca.ibm.com>
Date: Mon, 5 May 2003 18:46:25 -0400
MIME-Version: 1.0
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Hi Nicola Ken,

Some of this I covered in the note I just wrote to Stefano; but a couple of
brief comments:

> ...which brings us to the real catch22 problem, as Xerces needs XNI but

> all systems out use SAX. Why not only extend SAX? (ignorant mode)

Well, SAX is its own community and will evolve (or not...) as it will.  We
needed a new kind of entity resolver, for instance; we could have extended
SAX's, but then SAX decided to define EntityResolver2.  This would have
been a pretty ugly mess as soon as SAX 1.1 extensions become finalized!

Besides, with things like the introduction of Augmentation parameters
everywhere in XNI, I doubt we share many method signatures with SAX
anymore.

> Is it impossible to do validation of a document from a SAX stream? Or is
> XNI compulsory?

Well, it's compulsory for a component that lives in Xerces.  I'm not
entirely clear how you'd build a DTD grammar out of the information you get
from SAX, but I've not tried to do this so it well might be possible.  But
of course it would be easy (well, as easy as it ever is...) to build a
representation for an XML Schema grammar from a SAX stream.

> Is it possible to do XInclude from a SAX stream? Or is XNI compulsory?

Well, if you want to go by the letter of the spec, which Joerg tells us
assumes validation to happen after XInclude processing, then it's only
possible if you find a validator that takes SAX events as input.  You could
certainly build such a thing, but I'm not sure many exist ATM.

Anyway, before responding any further on these threads, I need to spend
some quality time with XInclude, and hopefully also have a look at Joerg's
code.

Cheers!
Neil
Neil Graham
XML Parser Development
IBM Toronto Lab
Phone:  905-413-3519, T/L 969-3519
E-mail:  neilg@ca.ibm.com


|---------+---------------------------->
|         |           Nicola Ken       |
|         |           Barozzi          |
|         |           <nicolaken@apache|
|         |           .org>            |
|         |                            |
|         |           04/30/2003 06:21 |
|         |           AM               |
|         |           Please respond to|
|         |           nicolaken        |
|         |                            |
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  |       cc:       cocoon-dev@xml.apache.org, commons-dev@xml.apache.org, xerces-j-dev@xml.apache.org                                          |
  |       Subject:  Re: [ANN] XInclude processor for xml-commons                                                                                |
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  >---------------------------------------------------------------------------------------------------------------------------------------------|


Neil Graham wrote, On 30/04/2003 1.41:
> Hi Nicola Ken,
>
>>>In your original post, you'd mentioned that you'd implemented this as a
>>>"SAX filter"; did you mean that it's an XMLFilter implementation?  If
>>> so,
>>>and you're using a standard XMLReader implementation of some sort, then
>>>how do you do XInclude processing before validation?
>
>>Ooooh, opening a can of worms ;-)
>
> Both commons-dev and xerces-j-dev have been really quiet lately; about
time
> for some activity!  :)

:-D

>>Yes having used XInclude I can solemly state that XInclude has to occur
>>before validation. The question is: should a parser validate? ;-)
>
>>The fact is that Processors like Cocoon will always have other methods
>>to use before a validation, so what we (Cocoon) need is instead a
>>validating transformer, or something like that.
>
>>Not that I'm defining solutions, just that Xerces is monolithic seen
>>from a SAX-adhering processor world.
>
>
> I see what you're saying, I think:  you like the idea of a componentized
> parser that operates in a pipeline, but you wish that SAX had been used
as
> the glue for such a beast, rather than its own API.  Right?
>
> And if SAX had seemed rich and complete enough, and had it supported the
> kind of configuration-management facilities that are needed for such an
> arrangement, I imagine that's the road we would have gone down.  But it
> isn't/doesn't, so what was there to do?  :)

Yeah, I know... I read about XNI, so I'm aware of the reasons it was
created...

> So I guess I'd turn the question around:  If using SAX means you're stuck
> with monolithic-looking parsers, but XNI would give you a parser with all
> kinds of flexibility, maybe using XNI might merit consideration?  :)
Sure
> it would bind you to a specific parser (until other parsers begin to use
> XNI :)) ), but if you write your own SAX-glued parser then you're stuck
to
> that anyway...

...which brings us to the real catch22 problem, as Xerces needs XNI but
all systems out use SAX. Why not only extend SAX? (ignorant mode)

>>Dumb proposal: why not a XNI2SAX Filter that uses an XNI processor for
>>SAX?
>
> We already have one:  org.apache.xerces.parsers.SAXParser; heck, it even
> understands both SAX1 and SAX2!  :)  The trouble is that, once you're
> emitting SAX, you're not in the Xerces pipeline world anymore; so this
guy
> is only useful at the end of such a pipeline (whatever XNI components
that
> pipeline has in it).  So there's no way of putting a Xerces validator
after
> that component, for instance (to do so would kind of defeat the point of
> XNI).  Does that explain anything?

Is it impossible to do validation of a document from a SAX stream? Or is
XNI compulsory?

Is it possible to do XInclude from a SAX stream? Or is XNI compulsory?

Is there a real possibility of hacking the SAX pipelines to make this
happen for *some* components like validation or XInclude?

> Here's hoping I haven't let too many more worms out of that can...

No, you have been very on-the-spot :-)

--
Nicola Ken Barozzi                   nicolaken@apache.org
             - verba volant, scripta manent -
    (discussions get forgotten, just code remains)
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