XML database

An XML database is a data persistence software system that allows data to be stored in XML format. This data can then be queried, exported and serialized into the desired format.

Two major classes of XML database exist:

1. XML-enabled: these map all XML to a traditional database (such as a relational database^[1]), accepting XML as input and rendering XML as output. This term implies that the database does the conversion itself (as opposed to relying on middleware).
2. Native XML (NXD): the internal model of such databases depends on XML and uses XML documents as the fundamental unit of storage, which are, however, not necessarily stored in the form of text files.

Rationale for XML in databases

O'Connell (2005, 9.2) gives one reason for the use of XML in databases: the increasingly common use of XML for data transport, which has meant that "data is extracted from databases and put into XML documents and vice-versa". It may prove more efficient (in terms of conversion costs) and easier to store the data in XML format.

Native XML databases

The term "native XML database" (NXD) can lead to confusion. Many NXDs do not function as standalone databases at all, and do not really store the native (text) form.

The formal definition from the XML:DB initiative (which appears to be inactive since 2003^[2]) states that a native XML database:

• Defines a (logical) model for an XML document — as opposed to the data in that document — and stores and retrieves documents according to that model. At a minimum, the model must include elements, attributes, PCDATA, and document order. Examples of such models include the XPath data model, the XML Infoset, and the models implied by the DOM and the events in SAX 1.0.

• Has an XML document as its fundamental unit of (logical) storage, just as a relational database has a row in a table as its fundamental unit of (logical) storage.

• Need not have any particular underlying physical storage model. For example, NXDs can use relational, hierarchical, or object-oriented database structures, or use a proprietary storage format (such as indexed, compressed files).

Additionally, many XML databases provide a logical model of grouping documents, called "collections". Databases can set up and manage many collections at one time. In some implementations, a hierarchy of collections can exist, much in the same way that an operating system's directory-structure works.

All XML databases now^[update] support at least one form of querying syntax. Minimally, just about all of them support XPath for performing queries against documents or collections of documents. XPath provides a simple pathing system that allows users to identify nodes that match a particular set of criteria.

In addition to XPath, many XML databases support XSLT as a method of transforming documents or query-results retrieved from the database. XSLT provides a declarative language written using an XML grammar. It aims to define a set of XPath filters that can transform documents (in part or in whole) into other formats including Plain text, XML, or HTML.

Many XML databases also support XQuery to perform querying. XQuery includes XPath as a node-selection method, but extends XPath to provide transformational capabilities. Users sometimes refer to its syntax as "FLWOR" (pronounced 'Flower') because the query may include the following clauses: 'for', 'let', 'where', 'order by' and 'return'. Traditional RDBMS vendors (who traditionally had SQL only engines), are now shipping with hybrid SQL and XQuery engines. Hybrid SQL/XQuery engines help to query XML data alongside the relational data, in the same query expression. This approach helps in combining relational and XML data.

Some XML databases support an API called the XML:DB API (or XAPI) as a form of implementation-independent access to the XML datastore. In XML databases, XAPI resembles ODBC and JDBC as used with relational databases. On the 24th of June 2009, The Java Community Process released the final version of the XQuery API for Java specification (XQJ) - "a common API that allows an application to submit queries conforming to the W3C XQuery 1.0 specification and to process the results of such queries".

XML Databases with database APIs (XQJ, XML:DB, RESTful)

XML Database	License	Language	XQJ API	XML:DB API	RESTful API
BaseX	BSD License	Java	Yes	Yes	Yes
eXist	LGPL License	Java	Yes	Yes	Yes
MarkLogic Server	Commercial	C++	Yes	No	Yes
MonetDB/XQuery	Proprietary	C++	No	Yes	No
Oracle	Commercial	C++	Yes	No	No
Sedna	Apache License	C++	Yes	Yes	No

References

1. ^ Mustafa Atay and Shiyong Lu, “Storing and Querying XML: An Efficient Approach Using Relational Databases”, ISBN 3-639-11581-3, VDM Verlag, 2009.
2. ^ http://xmldb-org.sourceforge.net/faqs.html

External references

• XML Databases - The Business Case, Charles Foster, June 2008 - Talks about the current state of Databases and data persistence, how the current Relational Database model is starting to crack at the seams and gives an insight into a strong alternative for today's requirements.
• An XML-based Database of Molecular Pathways (2005-06-02) Speed / Performance comparisons of eXist, X-Hive, Sedna and Qizx/open
• XML Native Database Systems: Review of Sedna, Ozone, NeoCoreXMS 2006
• XML Data Stores: Emerging Practices
• Bhargava, P.; Rajamani, H.; Thaker, S.; Agarwal, A. (2005) XML Enabled Relational Databases, Texas, The University of Texas at Austin.
• O'Connell, S. Advanced Databases Course Notes, Southampton, University of Southampton, 2005
• Initiative for XML Databases
• XML and Databases, Ronald Bourret, September 2005
• XML Database Products, Ronald Bourret, 2000–2009
• The State of Native XML Databases, Elliotte Rusty Harold, August 13, 2007