How to think about Namespaces

Outline (How to think about Namespaces)

How to think about Namespaces [5]
Using Namespaces [5]
Defining Namespaces [4]
Processing Namespaces [3]
Conclusions [1]

How to think about Namespaces R. Larson: XML Namespaces

(4) Namespaces are Simple

XML Namespaces are often misunderstood
- the biggest problem is to get rid of some assumptions
- XML Namespaces are too simple and thus confusing
Instincts of Web users
1. URIs identify something that can be retrieved by a browser
2. URIs identify something that can be displayed by a browser
3. if I cannot get it and cannot look at it, what good can it be?
However, these assumptions are not always true
1. URIs identify resources which often, but not always, can be accessed over the Web
2. URIs identify resources which often, but not always, have a Web-accessible representation
3. sharing URIs means sharing an identity, which can mean sharing semantics (associated with this identity)

How to think about Namespaces R. Larson: XML Namespaces

(5) Simple Examples

<html xmlns="http://www.w3.org/1999/xhtml">
 <body>
  <p>Some MathML Formula:</p>
  <math xmlns="http://www.w3.org/1998/Math/MathML">
   <mfrac>

mathml1.xml (line 2-6)

<html xmlns="http://www.w3.org/1999/xhtml">
 <body>
  <p>Some MathML Formula:</p>
  <math:math xmlns:math="http://www.w3.org/1998/Math/MathML">
   <math:mfrac>

mathml2.xml (line 2-6)

<html xmlns="http://www.w3.org/1999/xhtml" xmlns:math="http://www.w3.org/1998/Math/MathML">
 <body>
  <p>Some MathML Formula:</p>
  <math:math>
   <math:mfrac>

mathml3.xml (line 2-6)

<html:html xmlns:html="http://www.w3.org/1999/xhtml" xmlns:math="http://www.w3.org/1998/Math/MathML">
 <html:body>
  <html:p>Some MathML Formula:</html:p>
  <math:math>
   <math:mfrac>

mathml4.xml (line 2-6)

How to think about Namespaces R. Larson: XML Namespaces

(6) Name Spaces

Names are one form of identification
Identification is essential for communications
Names in XML are not suitable for identification
- they are local to their context (where they are defined)
- if the context is uniquely identified, the names would be, too
Name Spaces: Put names into spaces
- how to identify the space? Web identifiers are URIs!

How to think about Namespaces R. Larson: XML Namespaces

(7) URI Philosophy

Uniform Resource Identifier (URI) [XML Basics; Uniform Resource Identifier (URI) (1)] uniquely identify resources
URIs often provide access information
- pretty clear in http://dret.net/lectures/xml-fall10/
- less clear in urn:ietf:rfc:2648 (RFC 2648 [http://tools.ietf.org/html/rfc2648])
- very (and purposely) unclear in tag:9327493874329 (RFC 4151 [http://tools.ietf.org/html/rfc4151])
URIs often return resource representations
- the resource itself is never returned (how to return a lecture?)
- some representation often is useful (HTML, PDF, maybe video/audio)
- the resource exists and is useful without a representation!
- geo:37.87133;-122.258463 [http://maps.google.com/maps?ll=37.87133,-122.258463&om=1] is not an information resource (but it exists)
URIs are much more than just addresses of HTML pages

How to think about Namespaces R. Larson: XML Namespaces

(8) The Namespace Problem

People assume that URIs point to Web pages
- a namespace name (a URI) may point to a Web page
- it may also have no Web page associated with it
- it may even use a URI scheme which cannot be retrieved
- but it is always possible to compare URIs (Do I know this namespace?)
People assume some standardized content format
- friendly namespaces provide HTML portals (XHTML [http://www.w3.org/1999/xhtml])
- some namespaces just give you the schema (SOAP [http://www.w3.org/2001/12/soap-envelope])
- less friendly namespaces provide minimal information (XSLT [http://www.w3.org/1999/XSL/Transform])
- but they all are correct, because no resource representation is required!
- very unfriendly namespaces may return a 404 or even use inaccessible schemes
Namespaces are used by comparing URIs
- anything else maybe useful, but is not required and unsafe on the Web
- when searching for a namespace definition, use Google (string search)

Using Namespaces

Outline (Using Namespaces)

How to think about Namespaces [5]
Using Namespaces [5]
Defining Namespaces [4]
Processing Namespaces [3]
Conclusions [1]

Using Namespaces R. Larson: XML Namespaces

(10) Declaring Namespaces

Using a namespace means referencing names from it
- unfortunately, there is no really standard way of writing these names
- (the Clark notation [http://www.jclark.com/xml/xmlns.htm] is useful: {http://www.w3.org/1999/xhtml}html)
- Namespaces are first declared and then used
xmlns-prefixed attributes are used for declaring namespaces
- Default namespace: html xmlns="http://www.w3.org/1999/xhtml"
- Prefix notation: xhtml:html xmlns:xhtml="http://www.w3.org/1999/xhtml"
Namespace declarations are inherited and can be overwritten
- the default namespace can be undeclared
- Namespace declarations can be used in a myriad of ways

Using Namespaces R. Larson: XML Namespaces

(11) Unhealthy Namespace Usages

Namespaces can be (and are) used in very weird ways
- these are syntax variations of identical structures
- without a good (i.e., conforming) parser, interpretation is very hard
- copy/paste can become hard or impossible
Namespaces can be neurotic, psychotic, borderline, or normal [http://lists.xml.org/archives/xml-dev/200204/msg00170.html]
Each of the insane cases complicates processing
None of these has any real technical inaccuracies
XML should be used with humans in mind

Using Namespaces R. Larson: XML Namespaces

(12) Unhealthy Namespace Usages in Practice

<neurotic>
 <prefix:elem xmlns:prefix="http://ns1.com/">
  <prefix:child>content</prefix:child>
 </prefix:elem>
 <prefix:elem xmlns:prefix="http://ns2.com/">
  <prefix:child>content</prefix:child>
 </prefix:elem>
</neurotic>

neurotic.xml (line 2-9)

<borderline>
 <prefix1:elem xmlns:prefix1="http://ns1.com/">
  <prefix1:child>content</prefix1:child>
 </prefix1:elem>
 <prefix2:elem xmlns:prefix2="http://ns1.com/">
  <prefix2:child>content</prefix2:child>
 </prefix2:elem>
</borderline>

borderline.xml (line 2-9)

<psychotic xmlns:prefix1="http://ns1.com/" xmlns:prefix2="http://ns1.com/">
 <prefix1:elem>
  <prefix2:child>content</prefix2:child>
 </prefix1:elem>
 <prefix2:elem>
  <prefix1:child>content</prefix1:child>
 </prefix2:elem>
</psychotic>

psychotic.xml (line 2-9)

Using Namespaces R. Larson: XML Namespaces

(13) Elements and Attributes

Namespaces often apply to elements and attributes
- if an element name has no prefix, it has no namespace or the default namespace is used
- if a name has a prefix, the prefix must be bound to a namespace name
- names like this are called Qualified Names (QNames)
Elements and Attributes are treated differently
- the default namespace only applies to unprefixed element names
- unprefixed attribute names are in no namespace
- XSD [XSD – Part I] deals with this by keeping attributes local [XSD – Part III; Names and Namespaces (1)]
Applications should interpret QNames
- naïve implementations will break when processing unhealthy instances
- implementing namespaces is not very hard (but not trivial either)

Using Namespaces R. Larson: XML Namespaces

(14) Other Usages

Increasingly, QNames are used in content
- XSLT [XML Transformations (XSLT) – Part I] was the first specification using this
- many other technologies have followed

<xsl:template match="section" xmlns:mathml="http://www.w3.org/1998/Math/MathML/">
<xsl:if test="exists(.//mathml:*)">

Technically, everything is well-defined
- for processing, the namespace bindings must be known
- copy/paste on a textual basis may not work or even work wrong

Defining Namespaces

Outline (Defining Namespaces)

How to think about Namespaces [5]
Using Namespaces [5]
Defining Namespaces [4]
Processing Namespaces [3]
Conclusions [1]

Defining Namespaces R. Larson: XML Namespaces

(16) Any URI is Possible

A namespace name is a URI, that's all!
- it may not be accessible (because of the URI scheme)
- when retrieving it, nothing may be returned
- when retrieving it, something may be returned
The only important thing is the name
- the name is mentioned in the documentation
- if you know the documentation, you known the name
- shared names mean shared knowledge

Defining Namespaces R. Larson: XML Namespaces

(17) Namespace Definitions

Namespaces can be defined by a DTD (XHTML [http://www.w3.org/TR/xhtml1/#strict])
Namespaces can be defined by XSD (SOAP [http://www.w3.org/TR/soap12-part1/#tabnsprefixes])
Namespaces can be defined by RELAX NG (XHTML 2.0 [http://www.w3.org/TR/xhtml2/conformance.html#strict])
Namespaces can be defined by prose (XSLT [http://www.w3.org/TR/xslt#xslt-namespace])
If schemas are provided, additional information is required
- it is unlikely that a namespace can be fully described by a schema
- additional constraints and semantics are specified in prose

Defining Namespaces R. Larson: XML Namespaces

(18) Structured Namespaces

Namespaces have no structure
- a collection of names grouped by their namespace name
- inside the namespace, names have local meaning
Namespace definitions can invent their own rules
- but then they must also make rules how to deal with conflicts

Defining Namespaces R. Larson: XML Namespaces

(19) Fixed or Extensible?

Can a namespace change over time?
- may the namespace description become outdated? extended? replaced?
- this should be clearly documented in the namespace description
The XML XML Namespace was widely believed to be defined by XML [http://www.w3.org/XML/1998/namespace]
- xml:lang and xml:space defined by XML
- xml:base was added by XML Base [http://www.w3.org/TR/xmlbase/]
- xml:id was added by xml:id [XQuery – Part IV; XML IDs with xml:id (1)]
When defining namespaces, plan ahead and publish everything
- dependencies, change management, and versioning issues are important
- there still is no established standard for namespace descriptions

Processing Namespaces

Outline (Processing Namespaces)

How to think about Namespaces [5]
Using Namespaces [5]
Defining Namespaces [4]
Processing Namespaces [3]
Conclusions [1]

Processing Namespaces R. Larson: XML Namespaces

(21) Namespaces and Validity

Namespaces define an additional layer on top of XML
- they define additional semantics (assignment to namespaces)
- they define additional constraints (declaration and usage of namespaces)
Namespace-awareness is a basic requirement for XML tools
- XML not compliant with XML Namespaces will break most tools
- processing namespaces should be done by tools
- a namespace-aware parser translates namespace declarations into nodes

Processing Namespaces R. Larson: XML Namespaces

(22) Namespaces in the Document

<?xml version="1.0" encoding="UTF-8"?>
<html:html xmlns:html="http://www.w3.org/1999/xhtml" xmlns:math="http://www.w3.org/1998/Math/MathML">
 <html:body>
  <html:p>Some MathML Formula:</html:p>
  <math:math>
   <math:mfrac>
    <math:mn>2</math:mn>
    <math:mi>x</math:mi>
   </math:mfrac>
   <math:mo>=</math:mo>
   <math:mfrac>
    <math:mn>3</math:mn>
    <math:mi>y</math:mi>
   </math:mfrac>
  </math:math>
 </html:body>
</html:html>

mathml4.xml

Processing Namespaces R. Larson: XML Namespaces

(23) Namespaces in the Tree

XML Namespaces

XML Foundations [./]
Fall 2011 — INFO 242 (CCN 42596)

Ray LarsonUC Berkeley School of Information, UC Berkeley School of Information
2011-09-20

Contents

(2) Abstract

How to think about Namespaces

Outline (How to think about Namespaces)

(4) Namespaces are Simple

(5) Simple Examples

(6) Name Spaces

(7) URI Philosophy

(8) The Namespace Problem

Using Namespaces

Outline (Using Namespaces)

(10) Declaring Namespaces

(11) Unhealthy Namespace Usages

(12) Unhealthy Namespace Usages in Practice

(13) Elements and Attributes

(14) Other Usages

Defining Namespaces

Outline (Defining Namespaces)

(16) Any URI is Possible

(17) Namespace Definitions

(18) Structured Namespaces

(19) Fixed or Extensible?

Processing Namespaces

Outline (Processing Namespaces)

(21) Namespaces and Validity

(22) Namespaces in the Document

(23) Namespaces in the Tree

Conclusions

Outline (Conclusions)

(25) Name Spaces

XML Namespaces

XML Foundations [./]Fall 2011 — INFO 242 (CCN 42596)

Ray LarsonUC Berkeley School of Information, UC Berkeley School of Information2011-09-20

Contents

(2) Abstract

How to think about Namespaces

Outline (How to think about Namespaces)

(4) Namespaces are Simple

(5) Simple Examples

(6) Name Spaces

(7) URI Philosophy

(8) The Namespace Problem

Using Namespaces

Outline (Using Namespaces)

(10) Declaring Namespaces

(11) Unhealthy Namespace Usages

(12) Unhealthy Namespace Usages in Practice

(13) Elements and Attributes

(14) Other Usages

Defining Namespaces

Outline (Defining Namespaces)

(16) Any URI is Possible

(17) Namespace Definitions

(18) Structured Namespaces

(19) Fixed or Extensible?

Processing Namespaces

Outline (Processing Namespaces)

(21) Namespaces and Validity

(22) Namespaces in the Document

(23) Namespaces in the Tree

Conclusions

Outline (Conclusions)

(25) Name Spaces

XML Foundations [./]
Fall 2011 — INFO 242 (CCN 42596)

Ray LarsonUC Berkeley School of Information, UC Berkeley School of Information
2011-09-20