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is a standard language used to create extensible data structures (XML
schemas) and documents compatible with these structures (XML documents).
The scope for applying XML is almost unlimited: it can be used anywhere
that data is found - that is, anywhere in an information system, and particularly
play an essential role within information systems. They fall into three
main categories: document, object, and relational databases. After the
advent of XML technology came the arrival of native XML databases, along
with a new topic of debate: should there be a new type of database to
this article we shall look at the key points that will enable us to determine
the role of XML with regard to databases, and to pinpoint the opportunities
for acquiring a native XML database.
might be useful to approach this issue from the point of view of the type
of data that is required in an application. This will make it easier to
analyze the frame of use of XML and how appropriate it is to opt for a
native XML database. To do this, we will need to distinguish between the
data-oriented approach, and the document-oriented approach.
data-oriented XML file follows a precise structure, with fine-grained
elements, which are often subject to strict constraints. These elements
might be invoices, accounting entries or orders, for example. For an application
handling this type of data, the advantage of representing the data in
XML is that this will facilitate communication with other applications.
will be necessary to perform conversions between the internal format of
the data (relational, object, etc.) and the hierarchical structure of
the XML file used. The life cycle of the XML file will therefore be limited
to the transfer time between applications. Using XML as a storage format
is not highly useful, and can even compromise the design and architecture
are frameworks on the market that enable object and relational data to
be transformed into XML. Some are marketed by database vendors (IBM DB2
XML extender, Sybase ASE or Microsoft SQL Server XML extensions, etc.),
while others come from transactional middleware vendors (Exolab Castor
JDO, JAXB & JAXP implementations, Apache Axis, BEA WebLogic Workshop,
and so on).
XML databases also have their advantages (for new applications) as they
avoid the efforts required to convert XML files, and lead to good performances.
document-oriented XML file has little structure and the elements are more
coarse-grained. Unlike the data-oriented approach, a document-oriented
XML file must be conserved in its original form and has a long lifecycle.
Its existence may be governed by a workflow. It must be indexed, categorized
or even semanticized in order to be found and used. It should also be
possible to associate access security attributes with its structure.
Management Systems (CMS) fulfill these functions particularly well, and
offer operational XML extensions. A CMS infrastructure is generally based
on a file system, relational or object database or a proprietary tool.
This infrastructure is then "hidden" by a framework that provides
the notion of document and related aspects (workflow, security, semantics,
and so on).
XML databases may also be a solution, offering the advantages of native
support for XML document standards (DOM, SAX, XML Query and Xpath) and
the resulting good performances.
So is it worth acquiring a native XML database?
is not simply the use of the XML language that should guide the choice
of database, but the way in which it is used.
did not wait for XML to appear on the scene in order to implement structured
data storage systems. Object and relational databases are comfortably
established in this field and do not suffer from any shortcomings that
would justify their being replaced. Application servers and database XML
extensions enable applications to be opened up via XML interfaces - the
notion of Web Services.
argument of enhanced performances put forward by native XML database vendors
is a fragile one, as the stakes are rarely high: applications are often
uncoupled and communicate in asynchronous mode. Furthermore, although
it is valid to argue the importance of avoiding XML conversion stages,
this does not appear to justify the overheads and risks incurred by the
technological leap required.
and relational/XML conversion tools are well positioned to resolve the
issue and their impact on architectures is less significant. Indeed, the
market is a dynamic one and the productivity of tools is increasing.
content management issue is also well covered by CMS tools, which offer
additional XML extensions. Native XML databases do offer enhanced support
for the XQuery and Xpath standards, although these are still relatively
young. Native database vendors offer workflow or indexing capabilities
that tend to be less well-proven than CMSs.
the advantages of a native XML database is not an easy task, given the
number of tools that allow XML to be integrated with minimum impact on
existing databases. The native XML database market is struggling to take
off, and it seems likely to become a niche market.
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