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Introduction . This paper gives a high-level overview of how

to use XML with databases. It describes how the differences between

data-centric and document-centric documents affect their usage with

databases, how XML is commonly used with relational databases, and what

native XML databases are and when to use them. . NOTE:

Although the information discussed in this paper is (mostly)

up-to-date, the idea that the world of XML and databases can be seen

through the data-centric/document-centric divide is somewhat dated. At

the time this paper was originally written (1999), it was a convenient

metaphor for introducing native XML databases, which were then not

widely understood, even in the database community. However, it was

always somewhat unrealistic, as many XML documents are not strictly

data-centric or document-centric, but somewhere in between. So while

the data-centric/document-centric divide is a convenient starting

point, it is better to understand the differences between XML-enabled

databases and native XML databases and to choose the appropriate

database based on your processing needs. For a more modern look at the

difference between XML-enabled and native XML databases, see chapter 1

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Before we start talking about XML and databases, we need to answer a question that occurs to many people: "Is XML a database?" . An

XML document is a database only in the strictest sense of the term.

That is, it is a collection of data. In many ways, this makes it no

different from any other file -- after all, all files contain data of

some sort. As a "database" format, XML has some advantages. For

example, it is self-describing (the markup describes the structure and

type names of the data, although not the semantics), it is portable

(Unicode), and it can describe data in tree or graph structures. It

also has some disadvantages. For example, it is verbose and access to

the data is slow due to parsing and text conversion. . A more

useful question to ask is whether XML and its surrounding technologies

constitute a "database" in the looser sense of the term -- that is, a

database management system (DBMS). The answer to this question is,

"Sort of." On the plus side, XML provides many of the things found in

databases: storage (XML documents), schemas (DTDs, XML Schemas, RELAX

NG, and so on), query languages (XQuery, XPath, XQL, XML-QL, QUILT,

etc.), programming interfaces (SAX, DOM, JDOM), and so on. On the minus

side, it lacks many of the things found in real databases: efficient

storage, indexes, security, transactions and data integrity, multi-user

access, triggers, queries across multiple documents, and so on. . Thus,

while it may be possible to use an XML document or documents as a

database in environments with small amounts of data, few users, and

modest performance requirements, this will fail in most production

environments, which have many users, strict data integrity

requirements, and the need for good performance. . A good example

of the type of "database" for which an XML document is suitable is an

.ini file -- that is, a file that contains application configuration

information. It is much easier to invent a small XML language and write

a SAX application for interpreting that language than it is to write a

parser for comma-delimited files. In addition, XML allows you to have

nested entries, something that is harder to do in comma-delimited

files. However, this is hardly a database, since it is read and written

linearly, and then only when the application is started and ended. . Examples

of more sophisticated data sets for which an XML document might be

suitable as a database are personal contact lists (names, phone

numbers, addresses, etc.), browser bookmarks, and descriptions of the

MP3s you've stolen with the help of Napster. However, given the low

price and ease of use of databases like dBASE and Access, there seems

little reason to use an XML document as a database even in these cases.

The only real advantage of XML is that the data is portable, and this

is less of an advantage than it seems due to the widespread

availability of tools for serializing databases as XML. . 3.0 Why Use a Database? The

first question you need to ask yourself when you start thinking about

XML and databases is why you want to use a database in the first place.

Do you have legacy data you want to expose? Are you looking for a place

to store your Web pages? Is the database used by an e-commerce

application in which XML is used as a data transport? The answers to

these questions will strongly influence your choice of database and

middleware (if any), as well as how you use that database. . For

example, suppose you have an e-commerce application that uses XML as a

data transport. It is a good bet that your data has a highly regular

structure and is used by non-XML applications. Furthermore, things like

entities and the encodings used by XML documents probably aren't

important to you -- after all, you are interested in the data, not how

it is stored in an XML document. In this case, you'll probably need a

relational database and software to transfer the data between XML

documents and the database. If your applications are object-oriented,

you might even want a system that can store those objects in the

database or serialize them as XML. . On the other hand, suppose

you have a Web site built from a number of prose-oriented XML

documents. Not only do you want to manage the site, you would like to

provide a way for users to search its contents. Your documents are

likely to have a less regular structure and things such as entity usage

are probably important to you because they are a fundamental part of

how your documents are structured. In this case, you might want a

product like a native XML database or a content management system. This

will allow you to preserve physical document structure, support

document-level transactions, and execute queries in an XML query

language. . .

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