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hierarchies fail, and this is my struggle with xml at the moment, when
they have to support multiple hierarchies simultaneously. and they
largely fail because of a) the update problem, and b) the new hierarchy
problem. reverse bill of materials is a case in point.
having said that xml works really well where neither of these are an
issue - documents where the "semantics" don't change only the contents;
and as i said before moving transactions between systems.
even relational systems have problems because the semantics is embedded
in the sql select statements. most so called post relational systems
(not really sure that's a legitimate term, even though it's used a lot)
basically embed semantics back into the structure.
things like owl and to a lesser extent name spaces try to express the
semantics as a meta model. imho a far superior approach. i just don't
like naming relationships - prefer to acknowledge they exist and what it
takes to define them, but not necessarily name them.
now to xml and the cinderella id tag. the same effect as the
hierarchical xml could be achieved by allowing a name/value pairing to
store the structure as attributes in the xml tag and they should be
treated as elements as well.
the id tag is the required unique key, while special associate elements
store structure. this has the advantage of flatenning the xml and
allowing the parsers to create structure on the fly to suit the translators.
which would be approximately
early days, but something like this would be much better for data
modelling. perhaps we can have post-xml? ;)
Michael Champion wrote:
> On May 19, 2004, at 10:37 AM, Dare Obasanjo wrote:
>> Hierarchical databases failed for a reason.
> Just to be pedantic, the hierarchical model failed, hierarchical
> databases are still chugging along. IMS (a hierarchical DBMS that is
> the meanest, nastiest, ugliest mainframe dinosaur) still quietly
> manages an awfully big chunk of the world's data: "More than
> ninety-percent of the Fortune 1000 companies use IMS. IMS serves 200
> million end users, managing over 15 billion Gigabytes of production
> data and processing over 50 billion transactions every day."
> 25 years or so ago, Codd conclusively demonstrated the superiority of
> the relational model, but the world seems to keep reinventing
> hierarchical databases, AKA post-relational DBMS, Object-Relational
> RDBMS with an XML column type, native XML DBMS, even OODBMS (note that
> Progress has given ObjectStore new life since they bought Excelon,
> presumably because it has a lucrative niche). This happens because an
> awful lot of real-world relationships are hierarchical -- "contains /
> part-of", "parent / descendent", "manages / managed-by" .... -- and it
> is pragmatic to use tools that natively understand hierarchy to deal
> with them. Likewise, lots of things are intrinsically ordered (time
> being the obvious one) and as best I understand it, temporal
> relationships are one of the weaker aspects of the relational model
> even in theory, nevermind practice; again, XML treats order as a
> first-class citizen and is often a pragmatic tool.
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