Knitting the Web Together

Henry S. Thompson

HCRC Language Technology Group

Division of Informatics

University of Edinburgh

The old Web

Mark 1

Hand-authored HTML

Marking up human-generated prose

For humans to read

(Mark 1a)


Mark 2

Mechanically generated HTML

Marking up non-prose data

For machines to read

The new Web

Part 1

Machine-created XML

Marking up application data

For other applications to process

Part 2

Human-authored XML

Adding value to existing XML

For humans and machines to process

Part 3

. . . distributed in space and time

XML is ASCII for the 21st century

ASCII (ISO 646) solved a fundamental interchange problem for flat text documents

What bits encode what characters

(For a pretty parochial definition of 'character')

UNICODE/ISO 10646 extends that solution to the whole world

XML thought it was doing the same for simple tree-structured documents

The emphasis in the XML design was on simplifying SGML to move it to the Web

XML didn't touch SGML's architectural vision

flexible linearisation/transfer syntax

for tree-structured documents with internal links

The essence of XML

It's a markup language used for annotating text

It is concerned with logical structure

to identify sections, titles, section headers, chapters, paragraphs,…

It is not concerned with appearance

you say 'this is a subtitle'
not 'this is in bold, 14pt, centered'

you say 'this is an example'
not 'this is in verbatim, indented by 5pts, ragged right'

Why is XML a big deal?

It is an official W3C Recommendation

It is vendor-independent, platform independent, application independent,…

unlike Word documents, RTF documents, PDF documents, Postscript documents,…

It is human readable

ditto (for most values of 'human')

Unformatted text

Internet-based Application Architectures for the 21st Century:
The Role of XML
Let's skip straight to an example of XML syntax for a simple bit of structure:

<tip><emph>Never</emph> stand up in a canoe!</tip>

XML marked up text


  <title> Internet-based Application Architectures for the 21st Century: </title>
  <subtitle>The Role of XML</subtitle>

  <para> Let's skip <emph>straight</emph> to an example of XML syntax for a simple bit of structure:</para>
  <example> &lt;tip>&lt;emph>Never&lt;/emph> stand up in a canoe!&lt;/tip></example>

Who is in charge of XML?

XML is a W3C Recommendation

The W3C is The World Wide Web Consortium, a voluntary association of companies and non-profit organisations. Membership costs serious money, confers voting rights.  Complex procedures, with the Chairman (Tim Berners-Lee) having ultimate authority, guided by a committee of the whole called the Advisory Council.

The XML recommendation was written by the W3C's XML Working Group.

The essence of XML, try again

It's a markup language used for transferring data

It is concerned with data models

to convert between application-appropriate and transfer-appropriate forms

It is not concerned with human beings

It's produced and consumed by programs

What just happened!?

The whole transfer syntax story just went meta, that's what happened!

XML has been a runaway success, on a much greater scale than its designers anticipated

Not  for the reason they had hoped

Because separation of form from content is right

But for a reason they barely thought about

Data must travel the web

Tree structured documents are a useable transfer syntax for just about anything

So data-oriented web users think of XML as a transfer mechanism for their data

Components of the XML family


Transforming XML


Connecting XML documents

XML Schema

Defining XML document families

XML Protocols

XML-based communication

XSLT: Structure into form

There is a stylesheet language called XSLT

Rules for transforming from one vocabulary to another

Common case: output vocabulary is HTML

Coming soon: HQ print-orientated vocabulary

For example

  <template match='emph'>

   will do part of the Transformation job

XSLT Status:

W3C approved REC since November 1999

Three or four fully conformant implementations

All free

Including IE5

As of last week J

Most are offline

Written in Java

IE5 is online

Written in C++


What is XLink

Together with XPointer, a reconstruction and enrichment of the hyperlink concept at the heart of the web

Browsing is not the only application

"Follow Me" is not the only link semantics

Take HTML's <A HREF="…"/>, and do it right

Not tied to a particular element type

Not restricted to two endpoints

Not restricted to be inline

A careful separation between

The ontology and its notation (XLink)

The syntax of resource identification (Xpointe/XPathr)

XLink/XPointer status

In Candidate Recommendation phase

Several near-complete implementations recently announced

Retrospective integration with e.g. XHTML and SVG underway

XML Schema: some details

XML Schema is a language for defining the structure of XML documents

  Notated in XML itself

So there are elements defined for use in schemas to define. . .

Elements :-)



A type is a collection of constraints on element content and attribute values

A type may be either

Simple, for constraining string values

Complex, for constraining elements which contain other elements

A simple example

<!ELEMENT text (#PCDATA|emph|name)*>

<!ATTLIST text
        timestamp NMTOKEN #REQUIRED>

<xs:element name="text">

  <xs:complexType content="mixed">

  <xs:choice minOccurs='0'

   <xs:element ref="emph"/>

   <xs:element ref="name"/>


  <xs:attribute name="timestamp"
         type="date" minOccurs="1"/>

XML Schema Status

Last Call finished in May

Entering Candidate Recommendation very soon

Small number of weeks

At least five (partial) implementations

Three free

Big players strongly committed

IBM/Lotus, Oracle, Microsoft

W3C eating its own cooking

Subsequent RECs based on XML Schema

XML Protocols

Replace application-specific wire protocols with XML

Define an XML messaging story just above the transport layer

Use the modularity of XML Schema to allow application-specific specialisation of payload

Lack of consensus about exactly what the right level is

XML Protocol Status

W3C Working Group formation just announced

First meeting next month

Starting points



Microsoft just announced a major development effort

Linking vs. Messaging

People tend to think about distributed applications at too low a level



E-business and E-commerce are struggling to use XML versions of these technologies

With less success than originally expected

I think distributed, dynamic documents are a better fit


XML has a lot to offer e-Business and e-Commerce

Separating hype from reality is not easy

Careful requirements analysis is still the only sensible starting point

Old paradigms are not always the right model

Creative exploration/exploitation of new architectures is needed

Pilot first, before you bet the company

Look for help from established practitioners

Start now, if you haven't already!

XML and e-Business

Ed Feigenbaum once described Terry Winograd's work as "a breakthrough in enthusiasm"

I worry sometimes if XML and e-business is vulnerable to the same criticism

Negotiation between producers and consumers is the key

If you can't describe what you want, you can't have it

If you can't describe what you've got, no-one will use it

If you can't dicker, you'll always lose

So as far as I can see, for e-Business to be successful the Web badly needs a solution to the metadata problem

What is the metadata problem

There's been a lot of talk about metadata.

What is metadata?

It's just data.

But it's data about other data

Data intended for machine consumption

What could metadata do for us?

Give search engines something to work with that is designed for their needs.

Give us all a place to record what a document, or any other resource or service, is for or about.

Requirements for metadata

What would we need to make this work?

A standard syntax, so metadata can be recognised as such;

One or more standard vocabularies, so search engines, producers and consumers all speak the same language;

Lots of resources with metadata attached;

Attribution and trust

Is this resource really about Pamela Anderson?

Some choices for the GRID

Design our own languages/data structures for describing problem and resource components

Just define the ontologies, and use an existing data modelling meta-language




Topic Maps

XML Schema and RDF are the W3C-designed vehicles of choice

What is RDF?

RDF is actually two standardisation efforts, under the aegis of the W3C.

It stands for Resource Description Framework (in other words, metadata).

The two efforts are:

Standardising the syntax and abstract semantics;

RDF Model and Syntax

Providing a standard way of defining standard vocabularies (but not actually defining any).

RDF Schema

Distributed Dynamic Documents

Ted Nelson identified a powerful link semantics over twenty years ago

He called it transclusion

We're only just able to implement it

A document with transclusions in it is synthesised from the parts it points to

The separation of form from content is crucial here

First you pull it together

Then you render it

In the dynamic case, if what you're point at changes

You re-knit, and re-style

I've used document language, but the layered story works here too