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Re: [xml] EXI

On Tue, 2007-12-25 at 23:35 -0500, Daniel Veillard wrote:


Now that I have expresed  my concerns about the content of the spec we can
look spearately about any libxml2 implementation. I have a few more concerns
there:
 - those are first working draft specifications, I know how long it takes
   to finish such spec when there is no controversy about them, for something
   like EXI it may take a couple of years before you get a finished version
   (if any), and being an early implementor usually brings you just more pain
   e.g XPointer where I implemented the full early spec and only a tiny, 
   near useless fraction ended up as a REC.


I would agree and disagree at same time. Yes, it is way of trials and
error, but if nobody will try to follow new standard It will never
becomes perfect enough.


 - who would use it ? I mean EXI target very specialized domain spaces
   like embedded or specific processing, would those people actually use
   a libxml2 version where the point is more genericity of usage and
   the size and portability designs of the library probably don't match
   the specific requirements of those use cases.


I can't say for wide range of ppl, but I definitely going to use EXI.
I've use libxml for several years (in fact since libxml2 appearance).
And was forced to develop simple internal implementation of something
like EXI to address similar problems that was addressed by EXI.


 An implementation just for the sake of being able to claim existence of
a widely distributed early implementor doesn't sound to me a good reason 
to put EXI in libxml2.


It sounds like a bad news for me.


So now that I'm done expressing my doubts about it, let's see the technical
points which an implementation in libxml2 would raise :-)


issues that I came across are:

First, the EXI implementation should be an independent parser (and
generator) front-end. It should emmanate SAX events, so that we can
seamlessly use SAX, DOM, and xmlReader for EXI documents. Hopefully
this will also allow us to use all the other XML technologies (XPath,
XML Schema, XSLT, etc.) that libxml supports. I do not know the details
of libxml well enough to evaluate if this is indeed the case.


  Yes like for the HTML parser the right thing to do is to plug at the
SAX level to allow a flow of event, possibly connecting to tree and
reader APIs. Note also that a read-only interface is really not sufficient
you want to be able to save, if you can't round-trip it's really of
limited use or indication of a serious problem.


Second, EXI has some built-in datatypes that are like the XML Schema
datatypes. Obviously, some code should be reused here.


  Assuming the types are really compatible, yes. I just find crazy 
to mix layers like this, but again it's a spec concern, less of an implemtnation
one ... except for the fact that if you compile Schemas support in the 
library size grows a lot.


Third, EXI supports a schema-informed grammar, which means that it can
use information found in XML Schemas, RELAX NG schemas, or DTDs to
create a more compact EXI document. Although the schema-informed grammar
is independent on the various schemas (XML Schema, RELAX NG, DTD), it
eventually has to be populated by those schemas, so it will create some
kind of dependency to these parts of libxml.


  yes. Also note that the validations parts of libxml2 and espcially
the regexp/automata support is really built for validation far less for
introspection, this may present a challenge (but I'm not sure).


Fourth, EXI allows (but does not mandate) the support of user-defined
CODECS for encoding and decoding contents. As this is optional, I have
not looked further into that, but obviously it should be considered if
and how this should be supported by libxml.


  That I have a big grief against as previously explained, it's probably
too early to look at this from a technical viewpoint as I think it will
take time to settle down from a standard/political one ;-)

  I hope I don't sound too negative, but I have a hard time to be convinced
by EXI myself. On the other hand libxml2 development should be user demand
driven, and to some extend my participation in the XML Core group itself
is as representative of the libxml2 community. So if others could voice in
it would be a good idea. Also we have IMHO plenty of time, it's not like
EXI is about to become a REC, this is just a first draft with all the 
associated uncertainties about its content or schedule.

  Thanks Bjorn for raising the issue, even if this may not be a very
simple one :-)

Daniel


-- 
Vladimir B. Grebenschikov
vova fbsd ru
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