Native XML Databases

Native XML Databases

eXQuisite or eXcruciating?

Elliotte Rusty Harold

Software Development 2008 West

Wednesday, March 5, 2008

elharo@metalab.unc.edu

http://www.cafeconleche.org/

What is a Database?

That is, what services does it provide?

These services are independent of database models

Relational (i.e. SQL) databases handle about 20% of the world's information

But it's a very important 20%:

Relational models work very well for problems that can be decomposed into predictable sets of small atomic pieces of information that can be managed independently.

Characteristics of normalizable data

But what about the other 80%?

Of course you can always fit anything into anything

But should you?

Index Engines: The Other Extreme

XML Databases

Why Did Hierarchical Databases Fail?

But what if

30 Years Makes a Big Difference

What are XML Databases Good For?

What XML Databases cannot do:

The Players

Open Source Native

Closed source Native

Hybrid Relational-XML

Questions to Ask

i.e. characteristics that distinguish products

An XML Database should include

Berkeley DB XML

Installing

Creating a Database

$ dbxml

dbxml> createContainer website.dbxml
Creating node storage container with nodes indexed

Put Documents Into the Database

dbxml> putDocument pages '/home/elharo/initialdata.xml' f
Document added, name = pages

Run A Query

dbxml> query 'collection("website.dbxml")//*'
220 objects returned for eager expression ' collection("website.dbxml")//*'

dbxml> print
<feed xmlns="http://www.w3.org/2005/Atom" 
      xmlns:thr="http://purl.org/syndication/thread/1.0" 
      xml:lang="en">
  <id>http://www.elharo.com/blog/feed/atom/</id>
  <updated>2007-10-07T15:43:26Z</updated>
  <title type="text">Mokka mit Schlag</title>
...

Queries with Namespaces

dbxml> setNamespace atom http://www.w3.org/2005/Atom
Binding atom -> http://www.w3.org/2005/Atom

dbxml> query 'collection("website.dbxml")//atom:id'
11 objects returned for eager expression 'collection("website.dbxml")//atom:id'

dbxml> print
<id xmlns="http://www.w3.org/2005/Atom">http://www.elharo.com/blog/feed/atom/</id>
<id xmlns="http://www.w3.org/2005/Atom">http://www.elharo.com/blog/birding/2007/10/07/brown-lipped-snail/</id>
<id xmlns="http://www.w3.org/2005/Atom">http://www.elharo.com/blog/birding/2007/10/06/common-house-spider/</id>
<id xmlns="http://www.w3.org/2005/Atom">http://www.elharo.com/blog/networks/2007/10/05/were-back/</id>
<id xmlns="http://www.w3.org/2005/Atom">http://www.elharo.com/blog/networks/2007/10/01/shopping-for-isps/</id>
<id xmlns="http://www.w3.org/2005/Atom">http://www.elharo.com/blog/birding/2007/09/29/autumn-meadowhawks/</id>
<id xmlns="http://www.w3.org/2005/Atom">http://www.elharo.com/blog/new-york/2007/09/28/new-york-skyline/</id>
<id xmlns="http://www.w3.org/2005/Atom">http://www.elharo.com/blog/birding/2007/09/28/connecticut-warbler/</id>
<id xmlns="http://www.w3.org/2005/Atom">http://www.elharo.com/blog/science/astronomy/2007/09/28/adaptive-optics-on-the-cheap/</id>
<id xmlns="http://www.w3.org/2005/Atom">http://www.elharo.com/blog/birding/2007/09/28/390-connecticut-warbler-at-metrotech/</id>
<id xmlns="http://www.w3.org/2005/Atom">http://www.elharo.com/blog/software-development/web-development/2007/09/26/amazon-breaks-their-site/</id>

Run A Query Against a Named Document in the database

dbxml> query 'doc("website.dbxml/pages")//*'
220 objects returned for eager expression ' collection("website.dbxml")//*'

dbxml> print
<feed xmlns="http://www.w3.org/2005/Atom" 
    xmlns:thr="http://purl.org/syndication/thread/1.0" 
    xml:lang="en">
  <id>http://www.elharo.com/blog/feed/atom/</id>
  <updated>2007-10-07T15:43:26Z</updated>
  <title type="text">Mokka mit Schlag</title>
...

XQuery

Three parts:

XQuery Language

Documents to Query

Physical Representations to Query

Where is XQuery used?

The XML Model vs. the Relational Model

A relational database contains tables An XML database contains collections
A relational table contains records with the same schema A collection contains XML documents with the same DTD
A relational record is an unordered list of named values An XML document is a tree of nodes
A SQL query returns an unordered set of records An XQuery returns an ordered sequence of nodes

Query Data Types

XPath 2.0 Data Model Type Hierarchy

Picture taken from XQuery 1.0 and XPath 2.0 Functions and Operators W3C Working Draft 3 November 2005

An example document to query

Most of the examples in this talk query this bibliography document at the (relative) URL bib.xml: 

<?xml version="1.0"?>
<bib>
  <book year="1994">
    <title>TCP/IP Illustrated</title>
    <author><last>Stevens</last><first>W.</first></author>
    <publisher>Addison-Wesley</publisher>
    <price>65.95</price>
  </book>

  <book year="1992">
    <title>Advanced Programming in the Unix Environment</title>
    <author><last>Stevens</last><first>W.</first></author>
    <publisher>Addison-Wesley</publisher>
    <price>65.95</price>
  </book>

  <book year="2000">
    <title>Data on the Web</title>
    <author><last>Abiteboul</last><first>Serge</first></author>
    <author><last>Buneman</last><first>Peter</first></author>
    <author><last>Suciu</last><first>Dan</first></author>
    <publisher>Morgan Kaufmann Publishers</publisher>
    <price>39.95</price>
  </book>

  <book year="1999">
    <title>The Economics of Technology and Content for Digital TV</title>
    <editor>
      <last>Gerbarg</last><first>Darcy</first>
      <affiliation>CITI</affiliation>
    </editor>
    <publisher>Kluwer Academic Publishers</publisher>
    <price>129.95</price>
  </book>

</bib>

Adapted from Mary Fernandez, Jerome Simeon, and Phil Wadler: XML Query Languages: Experiences and Exemplars, 1999, as adapted in XML Query Use Cases

The XQuery FLWOR

photo of flower

Query: List titles of all books in one document

   for $t in doc("bib.xml")/bib/book/title
   return
      $t

Adapted from XML Query Use Cases

Query Result: Book Titles

% java -classpath saxon8.jar net.sf.saxon.Query query1
<?xml version="1.0" encoding="UTF-8"?>
<title>TCP/IP Illustrated</title>
<title>Advanced Programming in the Unix Environment</title>
<title>Data on the Web</title>
<title>The Economics of Technology and Content for Digital TV</title>

Serialization Format

XPath 1.0 Data Model

(Adapted from Jeni Tennison)

XPath 2.0 Data Model

(Adapted from Jeni Tennison)

Constructing sequences

Sequence Math

Sequence example

for $a in (1 to 10)
return $a

Output:

1
2
3
4
5
6
7
8
9
10

Data types and the PSVI

Element Constructors

List titles of all books in a bib element. Put each title in a book element. 

<bib>
  {
   for $title in doc("bib.xml")/bib/book/title
   return
    <book>
     { $title }
    </book>
  }
</bib>

Adapted from XML Query Use Cases

Query Result: Book Titles

<?xml version="1.0" encoding="UTF-8"?>
<bib>
   <book>
      <title>TCP/IP Illustrated</title>
   </book>
   <book>
      <title>Advanced Programming in the Unix Environment</title>
   </book>
   <book>
      <title>Data on the Web</title>
   </book>
   <book>
      <title>The Economics of Technology and Content for Digital TV</title>
   </book>
</bib>

Attribute Constructors

Adapted from XML Query Use Cases

Query Result

<?xml version="1.0" encoding="UTF-8"?>
<bib>
   <book year="1994">
      <title>TCP/IP Illustrated</title>
   </book>
   <book year="1992">
      <title>Advanced Programming in the Unix Environment</title>
   </book>
   <book year="2000">
      <title>Data on the Web</title>
   </book>
   <book year="1999">
      <title>The Economics of Technology and Content for Digital TV</title>
   </book>
</bib>

Text Constructors

Query Result

<?xml version="1.0" encoding="UTF-8"?>
<bib>
   <h1>Bibliography</h1>
   <title>TCP/IP Illustrated</title>
   <title>Advanced Programming in the Unix Environment</title>
   <title>Data on the Web</title>
   <title>The Economics of Technology and Content for Digital TV</title>
</bib>

Other Constructors

Expected Query Result

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="application/xml" href="bibliography.css"?>
<bib>
   <h1>Bibliography</h1>
   <title>TCP/IP Illustrated</title>
   <title>Advanced Programming in the Unix Environment</title>
   <title>Data on the Web</title>
   <title>The Economics of Technology and Content for Digital TV</title>
</bib>
<!-- An example from Elliotte Rusty Harold's 
  Native XML Databases presentation -->

Query with where

Adapted from XML Query Use Cases

Query Result: Titles of books published by Addison-Wesley

<?xml version="1.0" encoding="UTF-8"?>
<bib>
   <title>TCP/IP Illustrated</title>
   <title>Advanced Programming in the Unix Environment</title>
</bib>

Adapted from XML Query Use Cases

Query with Booleans

Adapted from XML Query Use Cases

Query Result: books published by Addison-Wesley before 1993

<?xml version="1.0" encoding="UTF-8"?>
<bib>
   <title>Advanced Programming in the Unix Environment</title>
</bib>

Adapted from XML Query Use Cases

Query with multiple variables

Create a list of all the title-author pairs, with each pair enclosed in a result element.

<results>
 {
   for $book in doc("bib.xml")/bib/book,
     $title in $book/title,
     $author in $book/author
   return
    <result>
    { $title }
    { $author }
    </result>
  }
</results>

Adapted from XML Query Use Cases

Query Result: A list of all the title-author pairs

<?xml version="1.0" encoding="UTF-8"?>
<results>
   <result>
      <title>TCP/IP Illustrated</title>
      <author>
         <last>Stevens</last>
         <first>W.</first>
      </author>
   </result>
   <result>
      <title>Advanced Programming in the Unix Environment</title>
      <author>
         <last>Stevens</last>
         <first>W.</first>
      </author>
   </result>
   <result>
      <title>Data on the Web</title>
      <author>
         <last>Abiteboul</last>
         <first>Serge</first>
      </author>
   </result>
   <result>
      <title>Data on the Web</title>
      <author>
         <last>Buneman</last>
         <first>Peter</first>
      </author>
   </result>
   <result>
      <title>Data on the Web</title>
      <author>
         <last>Suciu</last>
         <first>Dan</first>
      </author>
   </result>
</results>

Adapted from XML Query Use Cases

Nested FLWOR Queries

For each book in the bibliography, list the title and authors, grouped inside a result element.

<results>
 {
   for $b in doc("bib.xml")/bib/book
     return
      <result>
       { $b/title }
       {  
         for $a in $b/author
         return $a
       }
      </result>
 }
</results>

Adapted from XML Query Use Cases

Query Result: A list of the title and authors of each book in the bibliography

<?xml version="1.0" encoding="UTF-8"?>
<results>
   <result>
      <title>TCP/IP Illustrated</title>
      <author>
         <last>Stevens</last>
         <first>W.</first>
      </author>
   </result>
   <result>
      <title>Advanced Programming in the Unix Environment</title>
      <author>
         <last>Stevens</last>
         <first>W.</first>
      </author>
   </result>
   <result>
      <title>Data on the Web</title>
      <author>
         <last>Abiteboul</last>
         <first>Serge</first>
      </author>
      <author>
         <last>Buneman</last>
         <first>Peter</first>
      </author>
      <author>
         <last>Suciu</last>
         <first>Dan</first>
      </author>
   </result>
   <result>
      <title>The Economics of Technology and Content for Digital TV</title>
   </result>
</results>

Adapted from XML Query Use Cases

Query with let

Query Result: price differences

<?xml version="1.0" encoding="UTF-8"?>
<results>
   <data>
      <title>TCP/IP Illustrated</title> is -9.5 more expensive than the average. </data>
   <data>
      <title>Advanced Programming in the Unix Environment</title> is -9.5 more expensive than the average. </data>
   <data>
      <title>Data on the Web</title> is -35.5 more expensive than the average. </data>
   <data>
      <title>The Economics of Technology and Content for Digital TV</title> is 54.499999999999986 more expensive than the average. </data>
</results>

if then else

For each book in the bibliography, list the difference between the book's price and the average price, but this time indicate whether the book is more or less expensive than the average

<results> 
  {
   let $doc := doc("bib.xml")
   let $average := avg($doc//price)
   for $b in $doc/bib/book
     return
       if ($b/price > $average) then
         <data>
           { $b/title } is ${$b/price - $average} 
           more expensive than the average.
         </data>
       else  
         <data>
           { $b/title } is ${$average - $b/price} 
           less expensive than the average.
         </data>
  }    
</results>

Query Result: Price differences

<?xml version="1.0" encoding="UTF-8"?>
<results>
   <data>
      <title>TCP/IP Illustrated</title> is $9.5 less expensive than the average.</data>
   <data>
      <title>Advanced Programming in the Unix Environment</title> is $9.5 less expensive than the average.</data>
   <data>
      <title>Data on the Web</title> is $35.5 less expensive than the average.</data>
   <data>
      <title>The Economics of Technology and Content for Digital TV</title> is $54.499999999999986 more expensive than the average.</data>
</results>

Query with sorting

List the titles and years of all books published by Addison-Wesley after 1991, in alphabetic order.

<bib>
 {
   for $b in doc("bib.xml")//book[publisher = "Addison-Wesley"]
   order by ($b/title)
   return
    <book>
     { $b/@year } { $b/title }
    </book> 
 }
</bib>

Adapted from XML Query Use Cases

Query Result

<?xml version="1.0" encoding="UTF-8"?>
<bib>
   <book year="1992">
      <title>Advanced Programming in the Unix Environment</title>
   </book>
   <book year="1994">
      <title>TCP/IP Illustrated</title>
   </book>
</bib>

Adapted from XML Query Use Cases

ORDER BY modifiers

<bib>
 {
   for $b in doc("bib.xml")//book[publisher = "Addison-Wesley"]
   order by ($b/title) descending
   return
    <book>
     { $b/@year } { $b/title }
    </book> 
 }
</bib>

Adapted from XML Query Use Cases

Query Result

<?xml version="1.0" encoding="UTF-8"?>
<bib>
   <book year="1994">
      <title>TCP/IP Illustrated</title>
   </book>
   <book year="1992">
      <title>Advanced Programming in the Unix Environment</title>
   </book>
</bib>

Adapted from XML Query Use Cases

A different document about books

Sample data at "reviews.xml":

<?xml version="1.0"?>
<reviews>
  <entry>
    <title>Data on the Web</title>
    <price>34.95</price>
    <review>
       A very good discussion of semi-structured database
       systems and XML.
    </review>
  </entry>
  <entry>
    <title>Advanced Programming in the Unix Environment</title>
    <price>65.95</price>
    <review>
      A clear and detailed discussion of UNIX programming.
    </review>
  </entry>
  <entry>
    <title>TCP/IP Illustrated</title>
    <price>65.95</price>
    <review>
      One of the best books on TCP/IP.
    </review>
  </entry>
</reviews>

Adapted from XML Query Use Cases

This document uses a different DTD

<!ELEMENT reviews (entry*)>
<!ELEMENT entry   (title, price, review)>
<!ELEMENT title   (#PCDATA)>
<!ELEMENT price   (#PCDATA)>
<!ELEMENT review  (#PCDATA)>

Query that joins two documents

For each book found in both bib.xml and reviews.xml, list the title of the book and its price from each source.

<books-with-prices>
 {
   for $b in doc("bib.xml")//book,
     $a in doc("reviews.xml")//entry
   where $b/title = $a/title
   return
    <book-with-prices>
     { $b/title },
       <price-amazon> { $a/price/text() } </price-amazon>
       <price-bn> { $b/price/text() } </price-bn>
    </book-with-prices>
 }
</books-with-prices>

Adapted from XML Query Use Cases

Result 

<?xml version="1.0" encoding="UTF-8"?>
<books-with-prices>
   <book-with-prices>
      <title>TCP/IP Illustrated</title>,
       <price-amazon>65.95</price-amazon>
      <price-bn>65.95</price-bn>
   </book-with-prices>
   <book-with-prices>
      <title>Advanced Programming in the Unix Environment</title>,
       <price-amazon>65.95</price-amazon>
      <price-bn>65.95</price-bn>
   </book-with-prices>
   <book-with-prices>
      <title>Data on the Web</title>,
       <price-amazon>34.95</price-amazon>
      <price-bn>39.95</price-bn>
   </book-with-prices>
</books-with-prices>

Adapted from XML Query Use Cases

prices.xml Query Sample Data

The next query also uses an input document named "prices.xml": 

<?xml version="1.0"?>
<prices>
  <book>
    <title>Advanced Programming in the Unix Environment</title>
    <source>www.amazon.com</source>
    <price>65.95</price>
  </book>
  <book>
    <title>Advanced Programming in the Unix Environment</title>
    <source>www.bn.com</source>
    <price>65.95</price>
  </book>
  <book>
    <title>TCP/IP Illustrated</title>
    <source>www.amazon.com</source>
    <price>65.95</price>
  </book>
  <book>
    <title>TCP/IP Illustrated</title>
    <source>www.bn.com</source>
    <price>65.95</price>
  </book>
  <book>
    <title>Data on the Web</title>
    <source>www.amazon.com</source>
    <price>34.95</price>
  </book>
  <book>
    <title>Data on the Web</title>
    <source>www.bn.com</source>
    <price>39.95</price>
  </book>
</prices>

Adapted from XML Query Use Cases

Query with reused variables

<results>
 {
   let $doc := doc("prices.xml")
   for $t in distinct-values($doc/prices/book/title)
     let $p := $doc/prices/book[title = $t]/price
     return
       <minprice title="{$t}">
         { min($p) }
       </minprice>
 }
</results>

Adapted from XML Query Use Cases

Query Result

<?xml version="1.0" encoding="UTF-8"?>
<results>
   <minprice title="Advanced Programming in the Unix Environment">65.95</minprice>
   <minprice title="TCP/IP Illustrated">65.95</minprice>
   <minprice title="Data on the Web">34.95</minprice>
</results>

Adapted from XML Query Use Cases

Multiple FLWOR Queries

<bib>
 {
   for $b in doc("bib.xml")//book[author]
   return
    <book>
     { $b/title }
     { $b/author }
    </book>,
   for $b in doc("bib.xml")//book[editor]
   return
    <reference>
     { $b/title }
     <org> { $b/editor/affiliation/text() } </org>
    </reference>
 }
</bib>

Adapted from XML Query Use Cases

Query Result

<?xml version="1.0" encoding="UTF-8"?>
<bib>
   <book>
      <title>TCP/IP Illustrated</title>
      <author>
         <last>Stevens</last>
         <first>W.</first>
      </author>
   </book>
   <book>
      <title>Advanced Programming in the Unix Environment</title>
      <author>
         <last>Stevens</last>
         <first>W.</first>
      </author>
   </book>
   <book>
      <title>Data on the Web</title>
      <author>
         <last>Abiteboul</last>
         <first>Serge</first>
      </author>
      <author>
         <last>Buneman</last>
         <first>Peter</first>
      </author>
      <author>
         <last>Suciu</last>
         <first>Dan</first>
      </author>
   </book>
   <reference>
      <title>The Economics of Technology and Content for Digital TV</title>
      <org>CITI</org>
   </reference>
</bib>

Adapted from XML Query Use Cases

Querying documents that use namespaces

Query Software

What's the difference between XQuery and XSLT?

XPath 2.0

XPath 2.0 Goals

Held over from XPath 1.0

Accessor Functions

fn:node-name(Node)
returns zero or one QName
fn:string(Object)
returns the string value of anything
fn:data(Node)
returns a sequence of zero or more typed simple values
fn:base-uri(node)
returns the base URI of an Element or Document node
fn:document-uri(node)
returns the document URI of an Element or Document node

Constructor Functions

Casting 

if ($x castable as xs:gYear) then 
  $x cast as xs:gYear
else if ($x castable as xs:integer) then 
  $x cast as xs:integer
else if ($x castable as xs:decimal) then 
  $x cast as xs:decimal
else 
  $x cast as string

Four kinds of comparison operators

Value comparison operators

General comparisons

Node comparisons

Order comparisons

Functions and operators

Arithmetic operators

Numeric Functions

String functions

Regular expressions

Boolean Functions and Operators

Date and time functions

Qualified Name Functions

Node Functions

Sequence Functions

Sequence size Functions

fn:zero-or-one($arg as item()*) => item()?
Returns $arg if it contains zero or one items. Otherwise, raises an error
fn:one-or-more($arg as item()*) => item()?
Returns $arg if it containsone or more items. Otherwise, raises an error
fn:exactly-one($arg as item()*) => item()?
Returns $arg if it contains exactly one item. Otherwise, raises an error

Context Functions

Other New features in XPath 2.0

XPath Comments 

<xsl:apply-templates 
 select="(: The difference between the context node and the 
             current node is crucial here :)
 ../composition[@composer=current()/@id]"/>

Namespace wildcards

<xsl:template match="*:set">
  This matches MathML set elements, SVG set elements, set
  elements in no namespace at all, etc. 
</xsl:template>

Can use functions as location steps

Can use parenthesized expressions as location steps

Dereference steps

For Expressions

for Example

Consider the list of weblogs at http://static.userland.com/weblogMonitor/logs.xml 

<?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE foo SYSTEM "http://msdn.microsoft.com/xml/general/htmlentities.dtd">
<weblogs>
    <log>
        <name>MozillaZine</name>
        <url>http://www.mozillazine.org</url>
        <changesUrl>http://www.mozillazine.org/contents.rdf</changesUrl>
        <ownerName>Jason Kersey</ownerName>
        <ownerEmail>kerz@en.com</ownerEmail>
        <description>THE source for news on the Mozilla Organization.  DevChats, Reviews, Chats, Builds, Demos, Screenshots, and more.</description>
        <imageUrl></imageUrl>
        <adImageUrl>http://static.userland.com/weblogMonitor/ads/kerz@en.com.gif</adImageUrl>
    </log>
    <log>
        <name>SalonHerringWiredFool</name>
        <url>http://www.salonherringwiredfool.com/</url>
        <ownerName>Some Random Herring</ownerName>
        <ownerEmail>salonfool@wiredherring.com</ownerEmail>
        <description></description>
    </log>
    <log>
        <name>SlashDot.Org</name>
        <url>http://www.slashdot.org/</url>
        <ownerName>Simply a friend</ownerName>
        <ownerEmail>afriendofweblogs@weblogs.com</ownerEmail>
        <description>News for Nerds, Stuff that Matters.</description>
    </log>
</weblogs>

The changesUrl element points to a document like this: 

<?xml version="1.0"?>
<!DOCTYPE rss PUBLIC "-//Netscape Communications//DTD RSS 0.91//EN" 
                     "http://my.netscape.com/publish/formats/rss-0.91.dtd">
<rss version="0.91">
  <channel>
    <title>MozillaZine</title>
    <link>http://www.mozillazine.org/</link>
    <language>en-us</language>
    <description>Your source for Mozilla news, advocacy, interviews, builds, and more!</description>
    <copyright>Copyright 1998-2002, The MozillaZine Organization</copyright>
    <managingEditor>jason@mozillazine.org</managingEditor>
    <webMaster>jason@mozillazine.org</webMaster>
    <image>
      <title>MozillaZine</title>
      <url>http://www.mozillazine.org/image/mynetscape88.gif</url>
      <description>Your source for Mozilla news, advocacy, interviews, builds, and more!</description>
      <link>http://www.mozillazine.org/</link>
    </image>

    <item>
      <title>BugDays Are Back!</title>
      <link>http://www.mozillazine.org/talkback.html?article=2151</link>
    </item>

    <item>
      <title>Independent Status Reports</title>
      <link>http://www.mozillazine.org/talkback.html?article=2150</link>
    </item>

  </channel>

</rss>

We want to process all the item elements from each weblog.

for Example


<xsl:template match="weblogs">
  <xsl:apply-templates select="
    for $url in log/changesUrl
    return doc($url)//item
  "/>
</xsl:template>

Conditional Expressions

Not all weblogs have a changesUrl

<xsl:template match="log">
  <xsl:apply-templates select="
    if (changesUrl)
     then document(changesUrl)
     else document(url)"/>
</xsl:template>

Quantified Expressions

<xsl:template match="weblogs">
  <xsl:if test="some $log in log satisfies changesURL">
     At least one log has a changesURL
  </xsl:if>
</xsl:template>

<xsl:template match="weblogs">
  <xsl:if test="every $log in log satisfies url">
    Every log has a url
  </xsl:if>
</xsl:template>

A Closing Thought

Traditionally we have distinguished between documents, which are static, linear files that live in a file system and data, which are small chunks of information that are stored in a database. Much, perhaps most, PHP is little more than a glue language for inserting data into templates to form documents. Ditto for many report generation tools.

What if we break down the walls? Suppose everything goes into the database, not just the "data"? A single controller turns every request to the web server into a database query. Everything is stored in one place, and gets the benefits of the database: ACID, backups, distribution, professional management, etc. Everything can be queried, and the architecture is simpler: one piece (a database) instead of two (database+file system). Furthermore the system is more flexible. Modern web servers are still crippled by the assumption that what they're doing is serving files, and that the URL structure maps to a file system. That's too limiting, especially for fully RESTful applications. To enable the next generation of web applications, we need to pull the file system out from under the web servers and teach developers to think in terms of URL design and database queries. SQL is too limited to handle data this broad. XQuery can.

To Learn More

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Copyright 2002-2008 Elliotte Rusty Harold
elharo@metalab.unc.edu
Last Modified January 11, 2008