XML

Question 1

What is XML?

Answer 1

XML is the Extensible Markup Language. It improves the functionality of the Web by letting you identify your information in a more accurate, flexible, and adaptable way.
It is extensible because it is not a fixed format like HTML (which is a single, predefined markup language). Instead, XML is actually a metalanguage—a language for describing other languages—which lets you design your own markup languages for limitless different types of documents. XML can do this because it’s written in SGML, the international standard metalanguage for text document markup (ISO 8879).

Question 2

What is a markup language?

Answer 2

A markup language is a set of words and symbols for describing the identity of pieces of a document (for example ‘this is a paragraph’, ‘this is a heading’, ‘this is a list’, ‘this is the caption of this figure’, etc). Programs can use this with a style sheet to create output for screen, print, audio, video, Braille, etc.
Some markup languages (e.g. those used in word processors) only describe appearances (‘this is italics’, ‘this is bold’), but this method can only be used for display, and is not normally re-usable for anything else

Question 3

Where should I use XML?

Answer 3

Its goal is to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with HTML. XML has been designed for ease of implementation and for interoperability with both SGML and HTML.
Despite early attempts, browsers never allowed other SGML, only HTML (although there were plugins), and they allowed it (even encouraged it) to be corrupted or broken, which held development back for over a decade by making it impossible to program for it reliably. XML fixes that by making it compulsory to stick to the rules, and by making the rules much simpler than SGML.
But XML is not just for Web pages: in fact it’s very rarely used for Web pages on its own because browsers still don’t provide reliable support for formatting and transforming it. Common uses for XML include:
Information identification
because you can define your own markup, you can define meaningful names for all your information items. Information storage
because XML is portable and non-proprietary, it can be used to store textual information across any platform. Because it is backed by an international standard, it will remain accessible and processable as a data format. Information structure
XML can therefore be used to store and identify any kind of (hierarchical) information structure, especially for long, deep, or complex document sets or data sources, making it ideal for an information-management back-end to serving the Web. This is its most common Web application, with a transformation system to serve it as HTML until such time as browsers are able to handle XML consistently. Publishing
The original goal of XML as defined in the quotation at the start of this section. Combining the three previous topics (identity, storage, structure) means it is possible to get all the benefits of robust document management and control (with XML) and publish to the Web (as HTML) as well as to paper (as PDF) and to other formats (e.g. Braille, Audio, etc) from a single source document by using the appropriate style sheets. Messaging and data transfer
XML is also very heavily used for enclosing or encapsulating information in order to pass it between different computing systems which would otherwise be unable to communicate. By providing a lingua franca for data identity and structure, it provides a common envelope for inter-process communication (messaging). Web services
Building on all of these, as well as its use in browsers, machine-processable data can be exchanged between consenting systems, where before it was only comprehensible by humans (HTML). Weather services, e-commerce sites, blog newsfeeds, AJAX sites, and thousands of other data-exchange services use XML for data management and transmission, and the web browser for display and interaction.

Question 4

Why is XML such an important development?

Answer 4

It removes two constraints which were holding back Web developments:

1. dependence on a single, inflexible document type (HTML) which was being much abused for tasks it was never designed for;
2. the complexity of full SGML, whose syntax allows many powerful but hard-to-program options.

XML allows the flexible development of user-defined document types. It provides a robust, non-proprietary, persistent, and verifiable file format for the storage and transmission of text and data both on and off the Web; and it removes the more complex options of SGML, making it easier to program for.

Question 5

Describe the role that XSL can play when dynamically generating HTML pages from a relational database.

Answer 5

Even if candidates have never participated in a project involving this type of architecture, they should recognize it as one of the common uses of XML. Querying a database and then formatting the result set so that it can be validated as an XML document allows developers to translate the data into an HTML table using XSLT rules. Consequently, the format of the resulting HTML table can be modified without changing the database query or application code since the document rendering logic is isolated to the XSLT rules

Question 6

What is SGML?

Answer 6

SGML is the Standard Generalized Markup Language (ISO 8879:1986), the international standard for defining descriptions of the structure of different types of electronic document. There is an SGML FAQ from David Megginson at http://math.albany.edu:8800/hm/sgml/cts-faq.htmlFAQ; and Robin Cover’s SGML Web pages are at http://www.oasis-open.org/cover/general.html. For a little light relief, try Joe English’s ‘Not the SGML FAQ’ at http://www.flightlab.com/~joe/sgml/faq-not.txtFAQ.
SGML is very large, powerful, and complex. It has been in heavy industrial and commercial use for nearly two decades, and there is a significant body of expertise and software to go with it.
XML is a lightweight cut-down version of SGML which keeps enough of its functionality to make it useful but removes all the optional features which made SGML too complex to program for in a Web environment.

Question 7

Aren’t XML, SGML, and HTML all the same thing?

Answer 7

Not quite; SGML is the mother tongue, and has been used for describing thousands of different document types in many fields of human activity, from transcriptions of ancient Irish manuscripts to the technical documentation for stealth bombers, and from patients’ clinical records to musical notation. SGML is very large and complex, however, and probably overkill for most common office desktop applications.
XML is an abbreviated version of SGML, to make it easier to use over the Web, easier for you to define your own document types, and easier for programmers to write programs to handle them. It omits all the complex and less-used options of SGML in return for the benefits of being easier to write applications for, easier to understand, and more suited to delivery and interoperability over the Web. But it is still SGML, and XML files may still be processed in the same way as any other SGML file (see the question on XML software).
HTML is just one of many SGML or XML applications—the one most frequently used on the Web.
Technical readers may find it more useful to think of XML as being SGML– rather than HTML++.

Question 8

Who is responsible for XML?

Answer 8

XML is a project of the World Wide Web Consortium (W3C), and the development of the specification is supervised by an XML Working Group. A Special Interest Group of co-opted contributors and experts from various fields contributed comments and reviews by email.
XML is a public format: it is not a proprietary development of any company, although the membership of the WG and the SIG represented companies as well as research and academic institutions. The v1.0 specification was accepted by the W3C as a Recommendation on Feb 10, 1998.

Question 9

Why is XML such an important development?

Answer 9

It removes two constraints which were holding back Web developments:

1. dependence on a single, inflexible document type (HTML) which was being much abused for tasks it was never designed for;
2. the complexity of full question A.4, SGML, whose syntax allows many powerful but hard-to-program options.
   XML allows the flexible development of user-defined document types. It provides a robust, non-proprietary, persistent, and verifiable file format for the storage and transmission of text and data both on and off the Web; and it removes the more complex options of SGML, making it easier to program for.

Question 10

Give a few examples of types of applications that can benefit from using XML.

Answer 10

There are literally thousands of applications that can benefit from XML technologies. The point of this question is not to have the candidate rattle off a laundry list of projects that they have worked on, but, rather, to allow the candidate to explain the rationale for choosing XML by citing a few real world examples. For instance, one appropriate answer is that XML allows content management systems to store documents independently of their format, which thereby reduces data redundancy. Another answer relates to B2B exchanges or supply chain management systems. In these instances, XML provides a mechanism for multiple companies to exchange data according to an agreed upon set of rules. A third common response involves wireless applications that require WML to render data on hand held devices.

Question 11

What is DOM and how does it relate to XML?

Answer 11

The Document Object Model (DOM) is an interface specification maintained by the W3C DOM Workgroup that defines an application independent mechanism to access, parse, or update XML data. In simple terms it is a hierarchical model that allows developers to manipulate XML documents easily Any developer that has worked extensively with XML should be able to discuss the concept and use of DOM objects freely. Additionally, it is not unreasonable to expect advanced candidates to thoroughly understand its internal workings and be able to explain how DOM differs from an event-based interface like SAX.

Question 12

What is SOAP and how does it relate to XML?

Answer 12

The Simple Object Access Protocol (SOAP) uses XML to define a protocol for the exchange of information in distributed computing environments. SOAP consists of three components: an envelope, a set of encoding rules, and a convention for representing remote procedure calls. Unless experience with SOAP is a direct requirement for the open position, knowing the specifics of the protocol, or how it can be used in conjunction with HTTP, is not as important as identifying it as a natural application of XML

Question 13

Why not just carry on extending HTML?

Answer 13

HTML was already overburdened with dozens of interesting but incompatible inventions from different manufacturers, because it provides only one way of describing your information.
XML allows groups of people or organizations to question C.13, create their own customized markup applications for exchanging information in their domain (music, chemistry, electronics, hill-walking, finance, surfing, petroleum geology, linguistics, cooking, knitting, stellar cartography, history, engineering, rabbit-keeping, question C.19, mathematics, genealogy, etc).

HTML is now well beyond the limit of its usefulness as a way of describing information, and while it will continue to play an important role for the content it currently represents, many new applications require a more robust and flexible infrastructure.

Question 14

Why should I use XML?

Answer 14

Here are a few reasons for using XML (in no particular order). Not all of these will apply to your own requirements, and you may have additional reasons not mentioned here (if so, please let the editor of the FAQ know!).

• XML can be used to describe and identify information accurately and unambiguously, in a way that computers can be programmed to ‘understand’ (well, at least manipulate as if they could understand).
• XML allows documents which are all the same type to be created consistently and without structural errors, because it provides a standardised way of describing, controlling, or allowing/disallowing particular types of document structure. [Note that this has absolutely nothing whatever to do with formatting, appearance, or the actual text content of your documents, only the structure of them.]
• XML provides a robust and durable format for information storage and transmission. Robust because it is based on a proven standard, and can thus be tested and verified; durable because it uses plain-text file formats which will outlast proprietary binary ones.
• XML provides a common syntax for messaging systems for the exchange of information between applications. Previously, each messaging system had its own format and all were different, which made inter-system messaging unnecessarily messy, complex, and expensive. If everyone uses the same syntax it makes writing these systems much faster and more reliable.
• XML is free. Not just free of charge (free as in beer) but free of legal encumbrances (free as in speech). It doesn’t belong to anyone, so it can’t be hijacked or pirated. And you don’t have to pay a fee to use it (you can of course choose to use commercial software to deal with it, for lots of good reasons, but you don’t pay for XML itself).
• XML information can be manipulated programmatically (under machine control), so XML documents can be pieced together from disparate sources, or taken apart and re-used in different ways. They can be converted into almost any other format with no loss of information.
• XML lets you separate form from content. Your XML file contains your document information (text, data) and identifies its structure: your formatting and other processing needs are identified separately in a stylesheet or processing system. The two are combined at output time to apply the required formatting to the text or data identified by its structure (location, position, rank, order, or whatever).

Question 15

Can you walk us through the steps necessary to parse XML documents?

Answer 15

Superficially, this is a fairly basic question. However, the point is not to determine whether candidates understand the concept of a parser but rather have them walk through the process of parsing XML documents step-by-step. Determining whether a non-validating or validating parser is needed, choosing the appropriate parser, and handling errors are all important aspects to this process that should be included in the candidate’s response.

Question 16

Give some examples of XML DTDs or schemas that you have worked with.

Answer 16

Although XML does not require data to be validated against a DTD, many of the benefits of using the technology are derived from being able to validate XML documents against business or technical architecture rules. Polling for the list of DTDs that developers have worked with provides insight to their general exposure to the technology. The ideal candidate will have knowledge of several of the commonly used DTDs such as FpML, DocBook, HRML, and RDF, as well as experience designing a custom DTD for a particular project where no standard existed.

Question 17

Using XSLT, how would you extract a specific attribute from an element in an XML document?

Answer 17

XML Interview Questions and Answers
Give a few examples of types of applications that can benefit from using XML.
There are literally thousands of applications that can benefit from XML technologies. The point of this question is not to have the candidate rattle off a laundry list of projects that they have worked on, but, rather, to allow the candidate to explain the rationale for choosing XML by citing a few real world examples. For instance, one appropriate answer is that XML allows content management systems to store documents independently of their format, which thereby reduces data redundancy. Another answer relates to B2B exchanges or supply chain management systems. In these instances, XML provides a mechanism for multiple companies to exchange data according to an agreed upon set of rules. A third common response involves wireless applications that require WML to render data on hand held devices.

What is DOM and how does it relate to XML?
The Document Object Model (DOM) is an interface specification maintained by the W3C DOM Workgroup that defines an application independent mechanism to access, parse, or update XML data. In simple terms it is a hierarchical model that allows developers to manipulate XML documents easily Any developer that has worked extensively with XML should be able to discuss the concept and use of DOM objects freely. Additionally, it is not unreasonable to expect advanced candidates to thoroughly understand its internal workings and be able to explain how DOM differs from an event-based interface like SAX.

What is SOAP and how does it relate to XML?
The Simple Object Access Protocol (SOAP) uses XML to define a protocol for the exchange of information in distributed computing environments. SOAP consists of three components: an envelope, a set of encoding rules, and a convention for representing remote procedure calls. Unless experience with SOAP is a direct requirement for the open position, knowing the specifics of the protocol, or how it can be used in conjunction with HTTP, is not as important as identifying it as a natural application of XML

Why not just carry on extending HTML?
HTML was already overburdened with dozens of interesting but incompatible inventions from different manufacturers, because it provides only one way of describing your information.
XML allows groups of people or organizations to question C.13, create their own customized markup applications for exchanging information in their domain (music, chemistry, electronics, hill-walking, finance, surfing, petroleum geology, linguistics, cooking, knitting, stellar cartography, history, engineering, rabbit-keeping, question C.19, mathematics, genealogy, etc).
HTML is now well beyond the limit of its usefulness as a way of describing information, and while it will continue to play an important role for the content it currently represents, many new applications require a more robust and flexible infrastructure.

Why should I use XML?
Here are a few reasons for using XML (in no particular order). Not all of these will apply to your own requirements, and you may have additional reasons not mentioned here (if so, please let the editor of the FAQ know!).
* XML can be used to describe and identify information accurately and unambiguously, in a way that computers can be programmed to ‘understand’ (well, at least manipulate as if they could understand).
* XML allows documents which are all the same type to be created consistently and without structural errors, because it provides a standardised way of describing, controlling, or allowing/disallowing particular types of document structure. [Note that this has absolutely nothing whatever to do with formatting, appearance, or the actual text content of your documents, only the structure of them.]
* XML provides a robust and durable format for information storage and transmission. Robust because it is based on a proven standard, and can thus be tested and verified; durable because it uses plain-text file formats which will outlast proprietary binary ones.
* XML provides a common syntax for messaging systems for the exchange of information between applications. Previously, each messaging system had its own format and all were different, which made inter-system messaging unnecessarily messy, complex, and expensive. If everyone uses the same syntax it makes writing these systems much faster and more reliable.
* XML is free. Not just free of charge (free as in beer) but free of legal encumbrances (free as in speech). It doesn’t belong to anyone, so it can’t be hijacked or pirated. And you don’t have to pay a fee to use it (you can of course choose to use commercial software to deal with it, for lots of good reasons, but you don’t pay for XML itself).
* XML information can be manipulated programmatically (under machine control), so XML documents can be pieced together from disparate sources, or taken apart and re-used in different ways. They can be converted into almost any other format with no loss of information.
* XML lets you separate form from content. Your XML file contains your document information (text, data) and identifies its structure: your formatting and other processing needs are identified separately in a stylesheet or processing system. The two are combined at output time to apply the required formatting to the text or data identified by its structure (location, position, rank, order, or whatever).

Can you walk us through the steps necessary to parse XML documents?

Superficially, this is a fairly basic question. However, the point is not to determine whether candidates understand the concept of a parser but rather have them walk through the process of parsing XML documents step-by-step. Determining whether a non-validating or validating parser is needed, choosing the appropriate parser, and handling errors are all important aspects to this process that should be included in the candidate’s response.

Give some examples of XML DTDs or schemas that you have worked with.
Although XML does not require data to be validated against a DTD, many of the benefits of using the technology are derived from being able to validate XML documents against business or technical architecture rules. Polling for the list of DTDs that developers have worked with provides insight to their general exposure to the technology. The ideal candidate will have knowledge of several of the commonly used DTDs such as FpML, DocBook, HRML, and RDF, as well as experience designing a custom DTD for a particular project where no standard existed.

Using XSLT, how would you extract a specific attribute from an element in an XML document?
Successful candidates should recognize this as one of the most basic applications of XSLT. If they are not able to construct a reply similar to the example below, they should at least be able to identify the components necessary for this operation: xsl:template to match the appropriate XML element, xsl:value-of to select the attribute value, and the optional xsl:apply-templates to continue processing the document.

Extract Attributes from XML Data
Example 1.

Attribute Value:

Question 18

When constructing an XML DTD, how do you create an external entity reference in an attribute value?

Answer 18

Every interview session should have at least one trick question. Although possible when using SGML, XML DTDs don’t support defining external entity references in attribute values. It’s more important for the candidate to respond to this question in a logical way than than the candidate know the somewhat obscure answer.

Question 19

How would you build a search engine for large volumes of XML data?

Answer 19

The way candidates answer this question may provide insight into their view of XML data. For those who view XML primarily as a way to denote structure for text files, a common answer is to build a full-text search and handle the data similarly to the way Internet portals handle HTML pages. Others consider XML as a standard way of transferring structured data between disparate systems. These candidates often describe some scheme of importing XML into a relational or object database and relying on the database’s engine for searching. Lastly, candidates that have worked with vendors specializing in this area often say that the best way the handle this situation is to use a third party software package optimized for XML data.

Question 20

What is the difference between XML and C or C++ or Java ?

Answer 20

C and C++ (and other languages like FORTRAN, or Pascal, or Visual Basic, or Java or hundreds more) are programming languages with which you specify calculations, actions, and decisions to be carried out in order:
mod curconfig[if left(date,6) = “01-Apr”,
t.put “April googlel!”,
f.put days(‘31102005’,’DDMMYYYY’) -
days(sdate,’DDMMYYYY’)
“ more shopping days to Samhain”];
XML is a markup specification language with which you can design ways of describing information (text or data), usually for storage, transmission, or processing by a program. It says nothing about what you should do with the data (although your choice of element names may hint at what they are for):

Camshaft end bearing retention circlip
Ringtown Fasteners Ltd
Angle-nosed insertion tool is required for the removal
and replacement of this part.

On its own, an SGML or XML file (including HTML) doesn’t do anything. It’s a data format which just sits there until you run a program which does something with it.

Question 21

Does XML replace HTML?

Answer 21

No. XML itself does not replace HTML. Instead, it provides an alternative which allows you to define your own set of markup elements. HTML is expected to remain in common use for some time to come, and the current version of HTML is in XML syntax. XML is designed to make the writing of DTDs much simpler than with full SGML. (See the question on DTDs for what one is and why you might want one.)

Question 22

Do I have to know HTML or SGML before I learn XML?

Answer 22

No, although it’s useful because a lot of XML terminology and practice derives from two decades’ experience of SGML.
Be aware that ‘knowing HTML’ is not the same as ‘understanding SGML’. Although HTML was written as an SGML application, browsers ignore most of it (which is why so many useful things don’t work), so just because something is done a certain way in HTML browsers does not mean it’s correct, least of all in XML.

Question 23

What does an XML document actually look like (inside)?

Answer 23

The basic structure of XML is similar to other applications of SGML, including HTML. The basic components can be seen in the following examples. An XML document starts with a Prolog:
1. The XML Declaration

which specifies that this is an XML document;
2. Optionally a Document Type Declaration

which identifies the type of document and says where the Document Type Description (DTD) is stored;
The Prolog is followed by the document instance:

1. A root element, which is the outermost (top level) element (start-tag plus end-tag) which encloses everything else: in the examples below the root elements are conversation and titlepage;
2. A structured mix of descriptive or prescriptive elements enclosing the character data content (text), and optionally any attributes (‘name=value’ pairs) inside some start-tags.
   XML documents can be very simple, with straightforward nested markup of your own design:

<br></br>
Hello, world!
Stop the planet, I want to get
off!

Or

they can be more complicated, with a Schema or question C.11, Document Type Description (DTD) or internal subset (local DTD changes in [square brackets]), and an arbitrarily complex nested structure:

<!DOCTYPE titlepage
SYSTEM “http://www.google.bar/dtds/typo.dtd”
[<!ENTITY % active.links “INCLUDE”>]>

Hello, world!

<!-- In some copies the following
decoration is hand-colored, presumably
by the author -->

Vitam capias

Or

they can be anywhere between: a lot will depend on how you want to define your document type (or whose you use) and what it will be used for. Database-generated or program-generated XML documents used in e-commerce is usually unformatted (not for human reading) and may use very long names or values, with multiple redundancy and sometimes no character data content at all, just values in attributes:

Question 24

How does XML handle white-space in my documents?

Answer 24

All white-space, including linebreaks, TAB characters, and normal spaces, even between ‘structural’ elements where no text can ever appear, is passed by the parser unchanged to the application (browser, formatter, viewer, converter, etc), identifying the context in which the white-space was found (element content, data content, or mixed content, if this information is available to the parser, eg from a DTD or Schema). This means it is the application’s responsibility to decide what to do with such space, not the parser’s:
* insignificant white-space between structural elements (space which occurs where only element content is allowed, ie between other elements, where text data never occurs) will get passed to the application (in SGML this white-space gets suppressed, which is why you can put all that extra space in HTML documents and not worry about it)
* significant white-space (space which occurs within elements which can contain text and markup mixed together, usually mixed content or PCDATA) will still get passed to the application exactly as under SGML. It is the application’s responsibility to handle it correctly.
The parser must inform the application that white-space has occurred in element content, if it can detect it. (Users of SGML will recognize that this information is not in the ESIS, but it is in the Grove.)

My title for
Chapter 1.

text

In the example above, the application will receive all the pretty-printing linebreaks, TABs, and spaces between the elements as well as those embedded in the chapter title. It is the function of the application, not the parser, to decide which type of white-space to discard and which to retain. Many XML applications have configurable options to allow programmers or users to control how such white-space is handled.

Question 25

Which parts of an XML document are case-sensitive?

Answer 25

All of it, both markup and text. This is significantly different from HTML and most other SGML applications. It was done to allow markup in non-Latin-alphabet languages, and to obviate problems with case-folding in writing systems which are caseless.

• Element type names are case-sensitive: you must follow whatever combination of upper- or lower-case you use to define them (either by first usage or in a DTD or Schema). So you can’t say …: upper- and lower-case must match; thus <img></img>, <img></img>, and <img></img> are three different element types;
• For well-formed XML documents with no DTD, the first occurrence of an element type name defines the casing;
• Attribute names are also case-sensitive, for example the two width attributes in and (if they occurred in the same file) are separate attributes, because of the different case of width and WIDTH;
• Attribute values are also case-sensitive. CDATA values (eg Url=”MyFile.SGML”) always have been, but NAME types (ID and IDREF attributes, and token list attributes) are now case-sensitive as well;
• All general and parameter entity names (eg Á), and your data content (text), are case-sensitive as always.

Question 26

How can I make my existing HTML files work in XML?

Answer 26

Either convert them to conform to some new document type (with or without a DTD or Schema) and write a stylesheet to go with them; or edit them to conform to XHTML.
It is necessary to convert existing HTML files because XML does not permit end-tag minimisation (missing
, etc), unquoted attribute values, and a number of other SGML shortcuts which have been normal in most HTML DTDs. However, many HTML authoring tools already produce almost (but not quite) well-formed XML.
You may be able to convert HTML to XHTML using the Dave Raggett’s HTML Tidy program, which can clean up some of the formatting mess left behind by inadequate HTML editors, and even separate out some of the formatting to a stylesheet, but there is usually still some hand-editing to do.

Question 27

Is there an XML version of HTML?

Answer 27

Yes, the W3C recommends using XHTML which is ‘a reformulation of HTML 4 in XML 1.0’. This specification defines HTML as an XML application, and provides three DTDs corresponding to the ones defined by HTML 4.* (Strict, Transitional, and Frameset).
The semantics of the elements and their attributes are as defined in the W3C Recommendation for HTML 4. These semantics provide the foundation for future extensibility of XHTML. Compatibility with existing HTML browsers is possible by following a small set of guidelines (see the W3C site).

Question 28

If XML is just a subset of SGML, can I use XML files directly with existing SGML tools?

Answer 28

Yes, provided you use up-to-date SGML software which knows about the WebSGML Adaptations TC to ISO 8879 (the features needed to support XML, such as the variant form for EMPTY elements; some aspects of the SGML Declaration such as NAMECASE GENERAL NO; multiple attribute token list declarations, etc).
An alternative is to use an SGML DTD to let you create a fully-normalised SGML file, but one which does not use empty elements; and then remove the DocType Declaration so it becomes a well-formed DTDless XML file. Most SGML tools now handle XML files well, and provide an option switch between the two standards.

Question 29

Can XML use non-Latin characters?

Answer 29

Yes, the XML Specification explicitly says XML uses ISO 10646, the international standard character repertoire which covers most known languages. Unicode is an identical repertoire, and the two standards track each other. The spec says (2.2): ‘All XML processors must accept the UTF-8 and UTF-16 encodings of ISO 10646…’. There is a Unicode FAQ at http://www.unicode.org/faq/FAQ.
UTF-8 is an encoding of Unicode into 8-bit characters: the first 128 are the same as ASCII, and higher-order characters are used to encode anything else from Unicode into sequences of between 2 and 6 bytes. UTF-8 in its single-octet form is therefore the same as ISO 646 IRV (ASCII), so you can continue to use ASCII for English or other languages using the Latin alphabet without diacritics. Note that UTF-8 is incompatible with ISO 8859-1 (ISO Latin-1) after code point 127 decimal (the end of ASCII).
UTF-16 is an encoding of Unicode into 16-bit characters, which lets it represent 16 planes. UTF-16 is incompatible with ASCII because it uses two 8-bit bytes per character (four bytes above U+FFFF).

Question 30

What’s a Document Type Definition (DTD) and where do I get one?

Answer 30

A DTD is a description in XML Declaration Syntax of a particular type or class of document. It sets out what names are to be used for the different types of element, where they may occur, and how they all fit together. (A question C.16, Schema does the same thing in XML Document Syntax, and allows more extensive data-checking.)
For example, if you want a document type to be able to describe Lists which contain Items, the relevant part of your DTD might contain something like this:
<!ELEMENT List (Item)+>
<!ELEMENT Item (#PCDATA)>

This defines a list as an element type containing one or more items (that’s the plus sign); and it defines items as element types containing just plain text (Parsed Character Data or PCDATA). Validators read the DTD before they read your document so that they can identify where every element type ought to come and how each relates to the other, so that applications which need to know this in advance (most editors, search engines, navigators, and databases) can set themselves up correctly. The example above lets you create lists like:

Chocolate
Music
Surfingv

(The indentation in the example is just for legibility while editing: it is not required by XML.)
A DTD provides applications with advance notice of what names and structures can be used in a particular document type. Using a DTD and a validating editor means you can be certain that all documents of that particular type will be constructed and named in a consistent and conformant manner.
DTDs are not required for processing the tip in question Bwell-formed documents, but they are needed if you want to take advantage of XML’s special attribute types like the built-in ID/IDREF cross-reference mechanism; or the use of default attribute values; or references to external non-XML files (‘Notations’); or if you simply want a check on document validity before processing.
There are thousands of DTDs already in existence in all kinds of areas (see the SGML/XML Web pages for pointers). Many of them can be downloaded and used freely; or you can write your own (see the question on creating your own DTD. Old SGML DTDs need to be converted to XML for use with XML systems: read the question on converting SGML DTDs to XML, but most popular SGML DTDs are already available in XML form.
The alternatives to a DTD are various forms of question C.16, Schema. These provide more extensive validation features than DTDs, including character data content validation.

Question 31

Does XML let me make up my own tags?

Answer 31

No, it lets you make up names for your own element types. If you think tags and elements are the same thing you are already in considerable trouble: read the rest of this question carefully.

Question 32

How do I create my own document type?

Answer 32

Document types usually need a formal description, either a DTD or a Schema. Whilst it is possible to process well-formed XML documents without any such description, trying to create them without one is asking for trouble. A DTD or Schema is used with an XML editor or API interface to guide and control the construction of the document, making sure the right elements go in the right places. 
Creating your own document type therefore begins with an analysis of the class of documents you want to describe: reports, invoices, letters, configuration files, credit-card verification requests, or whatever. Once you have the structure correct, you write code to express this formally, using DTD or Schema syntax.

Question 33

How do I write my own DTD?

Answer 33

You need to use the XML Declaration Syntax (very simple: declaration keywords begin with
<!ELEMENT Shopping-List (Item)+>
<!ELEMENT Item (#PCDATA)>

It says that there shall be an element called Shopping-List and that it shall contain elements called Item: there must be at least one Item (that’s the plus sign) but there may be more than one. It also says that the Item element may contain only parsed character data (PCDATA, ie text: no further markup).
Because there is no other element which contains Shopping-List, that element is assumed to be the ‘root’ element, which encloses everything else in the document. You can now use it to create an XML file: give your editor the declarations:

<!DOCTYPE Shopping-List SYSTEM “shoplist.dtd”>

(assuming you put the DTD in that file). Now your editor will let you create files according to the pattern:

Chocolate
Sugar
Butter

It is possible to develop complex and powerful DTDs of great subtlety, but for any significant use you should learn more about document systems analysis and document type design. See for example Developing SGML DTDs: From Text to Model to Markup (Maler and el Andaloussi, 1995): this was written for SGML but perhaps 95% of it applies to XML as well, as XML is much simpler than full SGML—see the list of restrictions which shows what has been cut out.

Warning
Incidentally, a DTD file never has a DOCTYPE Declaration in it: that only occurs in an XML document instance (it’s what references the DTD). And a DTD file also never has an XML Declaration at the top either. Unfortunately there is still software around which inserts one or both of these.

Question 34

Can a root element type be explicitly declared in the DTD?

Answer 34

No. This is done in the document’s Document Type Declaration, not in the DTD.

Question 35

I keep hearing about alternatives to DTDs. What’s a Schema?

Answer 35

The W3C XML Schema recommendation provides a means of specifying formal data typing and validation of element content in terms of data types, so that document type designers can provide criteria for checking the data content of elements as well as the markup itself. Schemas are written in XML Document Syntax, like XML documents are, avoiding the need for processing software to be able to read XML Declaration Syntax (used for DTDs).
There is a separate Schema FAQ at http://www.schemavalid.comFAQ. The term ‘vocabulary’ is sometimes used to refer to DTDs and Schemas together. Schemas are aimed at e-commerce, data control, and database-style applications where character data content requires validation and where stricter data control is needed than is possible with DTDs; or where strong data typing is required. They are usually unnecessary for traditional text document publishing applications.
Unlike DTDs, Schemas cannot be specified in an XML Document Type Declaration. They can be specified in a Namespace, where Schema-aware software should pick it up, but this is optional:

…

More commonly, you specify the Schema in your processing software, which should record separately which Schema is used by which XML document instance.
In contrast to the complexity of the W3C Schema model, Relax NG is a lightweight, easy-to-use XML schema language devised by James Clark (see http://relaxng.org/) with development hosted by OASIS. It allows similar richness of expression and the use of XML as its syntax, but it provides an additional, simplified, syntax which is easier to use for those accustomed to DTDs.

Question 36

How do I get XML into or out of a database?

Answer 36

Ask your database manufacturer: they all provide XML import and export modules to connect XML applications with databases. In some trivial cases there will be a 1:1 match between field names in the database table and element type names in the XML Schema or DTD, but in most cases some programming will be required to establish the desired match. This can usually be stored as a procedure so that subsequent uses are simply commands or calls with the relevant parameters.

In less trivial, but still simple, cases, you could export by writing a report routine that formats the output as an XML document, and you could import by writing an XSLT transformation that formatted the XML data as a load file.

Question 37

Can I encode mathematics using XML ?

Answer 37

Yes, if the document type you use provides for math, and your users’ browsers are capable of rendering it. The mathematics-using community has developed the MathML Recommendation at the W3C, which is a native XML application suitable for embedding in other DTDs and Schemas.
It is also possible to make XML fragments from other DTDs, such as ISO 12083 Math, or OpenMath, or one of your own making. Browsers which display math embedded in SGML existed for many years (eg DynaText, Panorama, Multidoc Pro), and mainstream browsers are now rendering MathML. David Carlisle has produced a set of stylesheets for rendering MathML in browsers. It is also possible to use XSLT to convert XML math markup to LATEX for print (PDF) rendering, or to use XSL:FO.

Please note that XML is not itself a programming language, so concepts such as arithmetic and if-statements (if-then-else logic) are not meaningful in XML documents.

Question 38

How will XML affect my document links?

Answer 38

The linking abilities of XML systems are potentially much more powerful than those of HTML, so you’ll be able to do much more with them. Existing href-style links will remain usable, but the new linking technology is based on the lessons learned in the development of other standards involving hypertext, such as TEI and HyTime, which let you manage bidirectional and multi-way links, as well as links to a whole element or span of text (within your own or other documents) rather than to a single point. These features have been available to SGML users for many years, so there is considerable experience and expertise available in using them. Currently only Mozilla Firefox implements XLink.
The XML Linking Specification (XLink) and the XML Extended Pointer Specification (XPointer) documents contain the details. An XLink can be either a URI or a TEI-style Extended Pointer (XPointer), or both. A URI on its own is assumed to be a resource; if an XPointer follows it, it is assumed to be a sub-resource of that URI; an XPointer on its own is assumed to apply to the current document (all exactly as with HTML).

An XLink may use one of #, ?, or |. The # and ? mean the same as in HTML applications; the | means the sub-resource can be found by applying the link to the resource, but the method of doing this is left to the application. An XPointer can only follow a #.
The TEI Extended Pointer Notation (EPN) is much more powerful than the fragment address on the end of some URIs, as it allows you to specify the location of a link end using the structure of the document as well as (or in addition to) known, fixed points like IDs. For example, the linked second occurrence of the word ‘XPointer’ two paragraphs back could be referred to with the URI (shown here with linebreaks and spaces for clarity: in practice it would of course be all one long string):

http://xml.silmaril.ie/faq.xml#ID(hypertext)
.child(1,#element,’answer’)
.child(2,#element,’para’)
.child(1,#element,’link’)

This means the first link element within the second paragraph within the answer in the element whose ID is hypertext (this question). Count the objects from the start of this question (which has the ID hypertext) in the XML source:

1. the first child object is the element containing the question ();
2. the second child object is the answer (the element);
3. within this element go to the second paragraph;
4. find the first link element.

Eve Maler explained the relationship of XLink and XPointer as follows:
XLink governs how you insert links into your XML document, where the link might point to anything (eg a GIF file); XPointer governs the fragment identifier that can go on a URL when you’re linking to an XML document, from anywhere (eg from an HTML file).

[Or indeed from an XML file, a URI in a mail message, etc…Ed.]

David Megginson has produced an xpointer function for Emacs/psgml which will deduce an XPointer for any location in an XML document. XML Spy has a similar function.

Question 39

How do I use graphics in XML?

Answer 39

Graphics have traditionally just been links which happen to have a picture file at the end rather than another piece of text. They can therefore be implemented in any way supported by the XLink and XPointer specifications (see question C.18, ‘How will XML affect my document links?’), including using similar syntax to existing HTML images. They can also be referenced using XML’s built-in NOTATION and ENTITY mechanism in a similar way to standard SGML, as external unparsed entities.
However, the SVG specification (see the tip below, by Peter Murray-Rust) lets you use XML markup to draw vector graphics objects directly in your XML file. This provides enormous power for the inclusion of portable graphics, especially interactive or animated sequences, and it is now slowly becoming supported in browsers.
The XML linking specifications for external images give you much better control over the traversal and activation of links, so an author can specify, for example, whether or not to have an image appear when the page is loaded, or on a click from the user, or in a separate window, without having to resort to scripting.
XML itself doesn’t predicate or restrict graphic file formats: GIF, JPG, TIFF, PNG, CGM, EPS, and SVG at a minimum would seem to make sense; however, vector formats (EPS, SVG) are normally essential for non-photographic images (diagrams).
You cannot embed a raw binary graphics file (or any other binary [non-text] data) directly into an XML file because any bytes happening to resemble markup would get misinterpreted: you must refer to it by linking (see below). It is, however, possible to include a text-encoded transformation of a binary file as a CDATA Marked Section, using something like UUencode with the markup characters ], & and > removed from the map so that they could not occur as an erroneous CDATA termination sequence and be misinterpreted. You could even use simple hexadecimal encoding as used in PostScript. For vector graphics, however, the solution is to use SVG (see the tip below, by Peter Murray-Rust).
Sound files are binary objects in the same way that external graphics are, so they can only be referenced externally (using the same techniques as for graphics). Music files written in MusiXML or an XML variant of SMDL could however be embedded in the same way as for SVG.
The point about using entities to manage your graphics is that you can keep the list of entity declarations separate from the rest of the document, so you can re-use the names if an image is needed more than once, but only store the physical file specification in a single place. This is available only when using a DTD, not a Schema.

Question 40

How do I include one XML file in another?

Answer 40

This works exactly the same as for SGML. First you declare the entity you want to include, and then you reference it by name:

<!DOCTYPE novel SYSTEM “/dtd/novel.dtd” [
<!ENTITY chap1 SYSTEM “mydocs/chapter1.xml”>
<!ENTITY chap2 SYSTEM “mydocs/chapter2.xml”>
<!ENTITY chap3 SYSTEM “mydocs/chapter3.xml”>
<!ENTITY chap4 SYSTEM “mydocs/chapter4.xml”>
<!ENTITY chap5 SYSTEM “mydocs/chapter5.xml”>
]>

…blah blah…

&chap1; 
&chap2;
&chap3;
&chap4; 
&chap5; 

The difference between this method and the one used for including a DTD fragment (see question D.15, ‘How do I include one DTD (or fragment) in another?’) is that this uses an external general (file) entity which is referenced in the same way as for a character entity (with an ampersand).
The one thing to make sure of is that the included file must not have an XML or DOCTYPE Declaration on it. If you’ve been using one for editing the fragment, remove it before using the file in this way. Yes, this is a pain in the butt, but if you have lots of inclusions like this, write a script to strip off the declaration (and paste it back on again for editing).

Question 41

What is parsing and how do I do it in XML

Answer 41

Parsing is the act of splitting up information into its component parts (schools used to teach this in language classes until the teaching profession collectively caught the anti-grammar disease).
‘Mary feeds Spot’ parses as
1. Subject = Mary, proper noun, nominative case
2. Verb = feeds, transitive, third person singular, present tense
3. Object = Spot, proper noun, accusative case
In computing, a parser is a program (or a piece of code or API that you can reference inside your own programs) which analyses files to identify the component parts. All applications that read input have a parser of some kind, otherwise they’d never be able to figure out what the information means. Microsoft Word contains a parser which runs when you open a .doc file and checks that it can identify all the hidden codes. Give it a corrupted file and you’ll get an error message.
XML applications are just the same: they contain a parser which reads XML and identifies the function of each the pieces of the document, and it then makes that information available in memory to the rest of the program.
While reading an XML file, a parser checks the syntax (pointy brackets, matching quotes, etc) for well-formedness, and reports any violations (reportable errors). The XML Specification lists what these are.
Validation is another stage beyond parsing. As the component parts of the program are identified, a validating parser can compare them with the pattern laid down by a DTD or a Schema, to check that they conform. In the process, default values and datatypes (if specified) can be added to the in-memory result of the validation that the validating parser gives to the application.

Judy O’Grady
The example above parses as: 1. Element person identified with Attribute corpid containing abc123 and Attribute birth containing 1960-02-31 and Attribute gender containing female containing …
2. Element name containing …
3. Element forename containing text ‘Judy’ followed by …
4. Element surname containing text ‘O’Grady’
(and lots of other stuff too).
As well as built-in parsers, there are also stand-alone parser-validators, which read an XML file and tell you if they find an error (like missing angle-brackets or quotes, or misplaced markup). This is essential for testing files in isolation before doing something else with them, especially if they have been created by hand without an XML editor, or by an API which may be too deeply embedded elsewhere to allow easy testing.

Question 42

When should I use a CDATA Marked Section?

Answer 42

You should almost never need to use CDATA Sections. The CDATA mechanism was designed to let an author quote fragments of text containing markup characters (the open-angle-bracket and the ampersand), for example when documenting XML (this FAQ uses CDATA Sections quite a lot, for obvious reasons). A CDATA Section turns off markup recognition for the duration of the section (it gets turned on again only by the closing sequence of double end-square-brackets and a close-angle-bracket).
Consequently, nothing in a CDATA section can ever be recognised as anything to do with markup: it’s just a string of opaque characters, and if you use an XML transformation language like XSLT, any markup characters in it will get turned into their character entity equivalent.
If you try, for example, to use:
some text with <![CDATA[markup]]> in it.
in the expectation that the embedded markup would remain untouched, it won’t: it will just output
some text with <em>markup</em> in it.
In other words, CDATA Sections cannot preserve the embedded markup as markup. Normally this is exactly what you want because this technique was designed to let people do things like write documentation about markup. It was not designed to allow the passing of little chunks of (possibly invalid) unparsed HTML embedded inside your own XML through to a subsequent process—because that would risk invalidating the output.
As a result you cannot expect to keep markup untouched simply because it looked as if it was safely ‘hidden’ inside a CDATA section: it can’t be used as a magic shield to preserve HTML markup for future use as markup, only as characters.

Question 43

How can I handle embedded HTML in my XML

Answer 43

Apart from using CDATA Sections, there are two common occasions when people want to handle embedded HTML inside an XML element:
1. when they have received (possibly poorly-designed) XML from somewhere else which they must find a way to handle;
2. when they have an application which has been explicitly designed to store a string of characters containing < and & character entity references with the objective of turning them back into markup in a later process (eg FreeMind, Atom).
Generally, you want to avoid this kind of trick, as it usually indicates that the document structure and design has been insufficiently thought out. However, there are occasions when it becomes unavoidable, so if you really need or want to use embedded HTML markup inside XML, and have it processable later as markup, there are a couple of techniques you may be able to use:
* Provide templates for the handling of that markup in your XSLT transformation or whatever software you use which simply replicates what was there, eg

<b></b>

</b>

• Use XSLT’s ‘deep copy’ instruction, which outputs nested well-formed markup verbatim, eg
• As a last resort, use the disable-output-escaping attribute on the xsl:text element of XSL[T] which is available in some processors, eg
  <![CDATA[<b>Now!</b>]]>
• Some processors (eg JX) are now providing their own equivalents for disabling output escaping. Their proponents claim it is ‘highly desirable’ or ‘what most people want’, but it still needs to be treated with care to prevent unwanted (possibly dangerous) arbitrary code from being passed untouched through your system. It also adds another dependency to your software.
  For more details of using these techniques in XSL[T], see the relevant question in the XSL FAQ.

Question 44

What are the special characters in XML ?

Answer 44

For normal text (not markup), there are no special characters: just make sure your document refers to the correct encoding scheme for the language and/or writing system you want to use, and that your computer correctly stores the file using that encoding scheme. See the question on non-Latin characters for a longer explanation.
If your keyboard will not allow you to type the characters you want, or if you want to use characters outside the limits of the encoding scheme you have chosen, you can use a symbolic notation called ‘entity referencing’. Entity references can either be numeric, using the decimal or hexadecimal Unicode code point for the character (eg if your keyboard has no Euro symbol (€) you can type €); or they can be character, using an established name which you declare in your DTD (eg ) and then use as € in your document. If you are using a Schema, you must use the numeric form for all except the five below because Schemas have no way to make character entity declarations.
If you use XML with no DTD, then these five character entities are assumed to be predeclared, and you can use them without declaring them:
<
The less-than character () starts entity markup (the first character of a character entity reference).
>
The greater-than character (>) ends a start-tag or an end-tag.
"
The double-quote character (“) can be symbolised with this character entity reference when you need to embed a double-quote inside a string which is already double-quoted.
‘
The apostrophe or single-quote character (‘) can be symbolised with this character entity reference when you need to embed a single-quote or apostrophe inside a string which is already single-quoted.
If you are using a DTD then you must declare all the character entities you need to use (if any), including any of the five above that you plan on using (they cease to be predeclared if you use a DTD). If you are using a Schema, you must use the numeric form for all except the five above because Schemas have no way to make character entity declarations.

Question 45

Do I have to change any of my server software to work with XML?

Answer 45

The only changes needed are to make sure your server serves up .xml, .css, .dtd, .xsl, and whatever other file types you will use as the correct MIME content (media) types.
The details of the settings are specified in RFC 3023. Most new versions of Web server software come preset.
If not, all that is needed is to edit the mime-types file (or its equivalent: as a server operator you already know where to do this, right?) and add or edit the relevant lines for the right media types. In some servers (eg Apache), individual content providers or directory owners may also be able to change the MIME types for specific file types from within their own directories by using directives in a .htaccess file. The media types required are:
* text/xml for XML documents which are ‘readable by casual users’;
* application/xml for XML documents which are ‘unreadable by casual users’;
* text/xml-external-parsed-entity for external parsed entities such as document fragments (eg separate chapters which make up a book) subject to the readability distinction of text/xml;
* application/xml-external-parsed-entity for external parsed entities subject to the readability distinction of application/xml;
* application/xml-dtd for DTD files and modules, including character entity sets.
The RFC has further suggestions for the use of the +xml media type suffix for identifying ancillary files such as XSLT (application/xslt+xml).
If you run scripts generating XHTML which you wish to be treated as XML rather than HTML, they may need to be modified to produce the relevant Document Type Declaration as well as the right media type if your application requires them to be validated.

Question 46

I’m trying to understand the XML Spec: why does it have such difficult terminology?

Answer 46

For implementation to succeed, the terminology needs to be precise. Design goal eight of the specification tells us that ‘the design of XML shall be formal and concise’. To describe XML, the specification therefore uses formal language drawn from several fields, specifically those of text engineering, international standards and computer science. This is often confusing to people who are unused to these disciplines because they use well-known English words in a specialised sense which can be very different from their common meanings—for example: grammar, production, token, or terminal.
The specification does not explain these terms because of the other part of the design goal: the specification should be concise. It doesn’t repeat explanations that are available elsewhere: it is assumed you know this and either know the definitions or are capable of finding them. In essence this means that to grok the fullness of the spec, you do need a knowledge of some SGML and computer science, and have some exposure to the language of formal standards.
Sloppy terminology in specifications causes misunderstandings and makes it hard to implement consistently, so formal standards have to be phrased in formal terminology. This FAQ is not a formal document, and the astute reader will already have noticed it refers to ‘element names’ where ‘element type names’ is more correct; but the former is more widely understood.

Question 47

Can I still use server-side inclusions?

Answer 47

Yes, so long as what they generate ends up as part of an XML-conformant file (ie either valid or just well-formed).
Server-side tag-replacers like shtml, PHP, JSP, ASP, Zope, etc store almost-valid files using comments, Processing Instructions, or non-XML markup, which gets replaced at the point of service by text or XML markup (it is unclear why some of these systems use non-HTML/XML markup). There are also some XML-based preprocessors for formats like XVRL (eXtensible Value Resolution Language) which resolve specialised references to external data and output a normalised XML file.

Question 48

Can I (and my authors) still use client-side inclusions?

Answer 48

The same rule applies as for server-side inclusions, so you need to ensure that any embedded code which gets passed to a third-party engine (eg calls to SQL, VB, Java, etc) does not contain any characters which might be misinterpreted as XML markup (ie no angle brackets or ampersands). Either use a CDATA marked section to avoid your XML application parsing the embedded code, or use the standard <, and & character entity references instead.

Question 49

How can I include a conditional statement in my XML?

Answer 49

You can’t: XML isn’t a programming language, so you can’t say things like
bar
If you need to make an element optional, based on some internal or external criteria, you can do so in a Schema. DTDs have no internal referential mechanism, so it isn’t possible to express this kind of conditionality in a DTD at the individual element level.
It is possible to express presence-or-absence conditionality in a DTD for the whole document, by using parameter entities as switches to include or ignore certain sections of the DTD based on settings either hardwired in the DTD or supplied in the internal subset. Both the TEI and Docbook DTDs use this mechanism to implement modularity.
Alternatively you can make the element entirely optional in the DTD or Schema, and provide code in your processing software that checks for its presence or absence. This defers the checking until the processing stage: one of the reasons for Schemas is to provide this kind of checking at the time of document creation or editing.

Question 50

I have to do an overview of XML for my manager/client/investor/advisor. What should I mention?

Answer 50

You can’t: XML isn’t a programming language, so you can’t say things like
bar
If you need to make an element optional, based on some internal or external criteria, you can do so in a Schema. DTDs have no internal referential mechanism, so it isn’t possible to express this kind of conditionality in a DTD at the individual element level.
It is possible to express presence-or-absence conditionality in a DTD for the whole document, by using parameter entities as switches to include or ignore certain sections of the DTD based on settings either hardwired in the DTD or supplied in the internal subset. Both the TEI and Docbook DTDs use this mechanism to implement modularity.
Alternatively you can make the element entirely optional in the DTD or Schema, and provide code in your processing software that checks for its presence or absence. This defers the checking until the processing stage: one of the reasons for Schemas is to provide this kind of checking at the time of document creation or editing.

I have to do an overview of XML for my manager/client/investor/advisor. What should I mention?
* XML is not a markup language. XML is a ‘metalanguage’, that is, it’s a language that lets you define your own markup languages (see definition).
* XML is a markup language [two (seemingly) contradictory statements one after another is an attention-getting device that I’m fond of], not a programming language. XML is data: is does not ‘do’ anything, it has things done to it.
* XML is non-proprietary: your data cannot be held hostage by someone else.
* XML allows multi-purposing of your data.
* Well-designed XML applications most often separate ‘content’ from ‘presentation’. You should describe what something is rather what something looks like (the exception being data content which never gets presented to humans).
Saying ‘the data is in XML’ is a relatively useless statement, similar to saying ‘the book is in a natural language’. To be useful, the former needs to specify ‘we have used XML to define our own markup language’ (and say what it is), similar to specifying ‘the book is in French’.
A classic example of multipurposing and separation that I often use is a pharmaceutical company. They have a large base of data on a particular drug that they need to publish as:
* reports to the FDA;
* drug information for publishers of drug directories/catalogs;
* ‘prescribe me!’ brochures to send to doctors;
* little pieces of paper to tuck into the boxes;
* labels on the bottles;
* two pages of fine print to follow their ad in Reader’s Digest;
* instructions to the patient that the local pharmacist prints out;
* etc.
Without separation of content and presentation, they need to maintain essentially identical information in 20 places. If they miss a place, people die, lawyers get rich, and the drug company gets poor. With XML (or SGML), they maintain one set of carefully validated information, and write 20 programs to extract and format it for each application. The same 20 programs can now be applied to all the hundreds of drugs that they sell.
In the Web development area, the biggest thing that XML offers is fixing what is wrong with HTML:

• browsers allow non-compliant HTML to be presented;
• HTML is restricted to a single set of markup (‘tagset’).

If you let broken HTML work (be presented), then there is no motivation to fix it. Web pages are therefore tag soup that are useless for further processing. XML specifies that processing must not continue if the XML is non-compliant, so you keep working at it until it complies. This is more work up front, but the result is not a dead-end.
If you wanted to mark up the names of things: people, places, companies, etc in HTML, you don’t have many choices that allow you to distinguish among them. XML allows you to name things as what they are:
Charles Goldfarb worked
at IBM
gives you a flexibility that you don’t have with HTML:
<b>Charles Goldfarb</b> worked at<b>IBM«/b>
With XML you don’t have to shoe-horn your data into markup that restricts your options.</b>

Question 51

What is the purpose of XML namespaces?

Answer 51

XML namespaces are designed to provide universally unique names for elements and attributes. This allows people to do a number of things, such as:
* Combine fragments from different documents without any naming conflicts. (See example below.)
* Write reusable code modules that can be invoked for specific elements and attributes. Universally unique names guarantee that such modules are invoked only for the correct elements and attributes.
* Define elements and attributes that can be reused in other schemas or instance documents without fear of name collisions. For example, you might use XHTML elements in a parts catalog to provide part descriptions. Or you might use the nil attribute defined in XML Schemas to indicate a missing value.
As an example of how XML namespaces are used to resolve naming conflicts in XML documents that contain element types and attributes from multiple XML languages, consider the following two XML documents:

<address>
Apple 7
Color
State
Country
H98d69
</address>
and:

OurWebServer

<address>888.90.67.8</address>

Each document uses a different XML language and each language defines an Address element type. Each of these Address element types is different – that is, each has a different content model, a different meaning, and is interpreted by an application in a different way. This is not a problem as long as these element types exist only in separate documents. But what if they are combined in the same document, such as a list of departments, their addresses, and their Web servers? How does an application know which Address element type it is processing?
One solution is to simply rename one of the Address element types – for example, we could rename the second element type IPAddress. However, this is not a useful long term solution. One of the hopes of XML is that people will standardize XML languages for various subject areas and write modular code to process those languages. By reusing existing languages and code, people can quickly define new languages and write applications that process them. If we rename the second Address element type to IPAddress, we will break any code that expects the old name.
A better answer is to assign each language (including its Address element type) to a different namespace. This allows us to continue using the Address name in each language, but to distinguish between the two different element types. The mechanism by which we do this is XML namespaces.
(Note that by assigning each Address name to an XML namespace, we actually change the name to a two-part name consisting of the name of the XML namespace plus the name Address. This means that any code that recognizes just the name Address will need to be changed to recognize the new two-part name. However, this only needs to be done once, as the two-part name is universally unique.

Question 52

What is an XML namespace?

Answer 52

An XML namespace is a collection of element type and attribute names. The collection itself is unimportant – in fact, a reasonable argument can be made that XML namespaces don’t actually exist as physical or conceptual entities . What is important is the name of the XML namespace, which is a URI. This allows XML namespaces to provide a two-part naming system for element types and attributes. The first part of the name is the URI used to identify the XML namespace – the namespace name. The second part is the element type or attribute name itself – the local part, also known as the local name. Together, they form the universal name.
This two-part naming system is the only thing defined by the XML namespaces recommendation.

Question 53

Does the XML namespaces recommendation define anything except a two-part naming system for element types and attributes?

Answer 53

No.
This is a very important point and a source of much confusion, so we will repeat it:
THE XML NAMESPACES RECOMMENDATION DOES NOT DEFINE ANYTHING EXCEPT A TWO-PART NAMING SYSTEM FOR ELEMENT TYPES AND ATTRIBUTES.
In particular, they do not provide or define any of the following:
* A way to merge two documents that use different DTDs.
* A way to associate XML namespaces and schema information.
* A way to validate documents that use XML namespaces.
* A way to associate element type or attribute declarations in a DTD with an XML namespace.

Question 54

What do XML namespaces actually contain?

Answer 54

XML namespaces are collections of names, nothing more. That is, they contain the names of element types and attributes, not the elements or attributes themselves. For example, consider the following document.

<b></b>

The element type name A and the attribute name C are in the http://www.google.org/ namespace because they are mapped there by the google prefix. The element type name B and the attribute name D are not in any XML namespace because no prefix maps them there. On the other hand, the elements A and B and the attributes C and D are not in any XML namespace, even though they are physically within the scope of the http://www.google.org/ namespace declaration. This is because XML namespaces contain names, not elements or attributes.
XML namespaces also do not contain the definitions of the element types or attributes. This is an important difference, as many people are tempted to think of an XML namespace as a schema, which it is not.

Question 55

Are the names of all element types and attributes in some XML namespace?

Answer 55

No.
If an element type or attribute name is not specifically declared to be in an XML namespace – that is, it is unprefixed and (in the case of element type names) there is no default XML namespace – then that name is not in any XML namespace. If you want, you can think of it as having a null URI as its name, although no “null” XML namespace actually exists. For example, in the following, the element type name B and the attribute names C and E are not in any XML namespace:

<b></b>

Question 56

Do XML namespaces apply to entity names, notation names, or processing instruction targets?

Answer 56

No.
XML namespaces apply only to element type and attribute names. Furthermore, in an XML document that conforms to the XML namespaces recommendation, entity names, notation names, and processing instruction targets must not contain colons.

Question 57

Who can create an XML namespace?

Answer 57

Anybody can create an XML namespace – all you need to do is assign a URI as its name and decide what element type and attribute names are in it. The URI must be under your control and should not be being used to identify a different XML namespace, such as by a coworker.
(In practice, most people that create XML namespaces also describe the element types and attributes whose names are in it – their content models and types, their semantics, and so on. However, this is not part of the process of creating an XML namespace, nor does the XML namespace include or provide a way to discover such information.)

Question 58

Do I need to use XML namespaces?

Answer 58

Maybe, maybe not.
If you don’t have any naming conflicts in the XML documents you are using today, as is often the case with documents used inside a single organization, then you probably don’t need to use XML namespaces. However, if you do have conflicts today, or if you expect conflicts in the future due to distributing your documents outside your organization or bringing outside documents into your organization, then you should probably use XML namespaces.
Regardless of whether you use XML namespaces in your own documents, it is likely that you will use them in conjunction with some other XML technology, such as XSL, XHTML, or XML Schemas. For example, the following XSLT (XSL Transformations) stylesheet uses XML namespaces to distinguish between element types defined in XSLT and those defined elsewhere:

<!-- The Addresses element type is not part of the XSLT namespace. -->

Question 59

What is the relationship between XML namespaces and the XML 1.0 recommendation?

Answer 59

Although the XML 1.0 recommendation anticipated the need for XML namespaces by noting that element type and attribute names should not include colons, it did not actually support XML namespaces. Thus, XML namespaces are layered on top of XML 1.0. In particular, any XML document that uses XML namespaces is a legal XML 1.0 document and can be interpreted as such in the absence of XML namespaces. For example, consider the following document:

If this document is processed by a namespace-unaware processor, that processor will see two elements whose names are google:A and google:B. The google:A element has an attribute named xmlns:google and the google:B element has an attribute named google:C. On the other hand, a namespace-aware processor will see two elements with universal names {http://www.google.org}A and {http://www.google.org}B. The {http://www.google.org}A does not have any attributes; instead, it has a namespace declaration that maps the google prefix to the URI http://www.google.org. The {http://www.google.org}B element has an attribute named {http://www.google.org}C.
Needless to say, this has led to a certain amount of confusion. One area of confusion is the relationship between XML namespaces and validating XML documents against DTDs. This occurs because the XML namespaces recommendation did not describe how to use XML namespaces with DTDs. Fortunately, a similar situation does not occur with XML schema languages, as all of these support XML namespaces.
The other main area of confusion is in recommendations and specifications such as DOM and SAX whose first version predates the XML namespaces recommendation. Although these have since been updated to include XML namespace support, the solutions have not always been pretty due to backwards compatibility requirements. All recommendations in the XML family now support XML namespaces.

Question 60

What is the difference between versions 1.0 and 1.1 of the XML namespaces recommendation?

Answer 60

There are only two differences between XML namespaces 1.0 and XML namespaces 1.1:

• Version 1.1 adds a way to undeclare prefixes. For more information, see question 4.7.
• Version 1.1 uses IRIs (Internationalized Resource Identifiers) instead of URIs. Basically, URIs are restricted to a subset of ASCII characters, while IRIs allow much broader use of Unicode characters. For complete details, see section 9 of Namespaces in XML 1.1.
  NOTE: As of this writing (February, 2003), Namespaces in XML 1.1 is still a candidate recommendation and not widely used. PART II: DECLARING AND USING XML NAMESPACES

Question 61

How do I declare an XML namespace in an XML document?

Answer 61

To declare an XML namespace, you use an attribute whose name has the form:
xmlns:prefix

–OR–

xmlns
These attributes are often called xmlns attributes and their value is the name of the XML namespace being declared; this is a URI. The first form of the attribute (xmlns:prefix) declares a prefix to be associated with the XML namespace. The second form (xmlns) declares that the specified namespace is the default XML namespace.

For example, the following declares two XML namespaces, named http://www.google.com/ito/addresses and http://www.google.com/ito/servers. The first declaration associates the addr prefix with the http://www.google.com/ito/addresses namespace and the second declaration states that the http://www.google.com/ito/servers namespace is the default XML namespace.

NOTE: Technically, xmlns attributes are not attributes at all – they are XML namespace declarations that just happen to look like attributes. Unfortunately, they are not treated consistently by the various XML recommendations, which means that you must be careful when writing an XML application.

For example, in the XML Information Set (http://www.w3.org/TR/xml-infoset), xmlns “attributes” do not appear as attribute information items. Instead, they appear as namespace declaration information items.

On the other hand, both DOM level 2 and SAX 2.0 treat namespace attributes somewhat ambiguously. In SAX 2.0, an application can instruct the parser to return xmlns “attributes” along with other attributes, or omit them from the list of attributes.

Similarly, while DOM level 2 sets namespace information based on xmlns “attributes”, it also forces applications to manually add namespace declarations using the same mechanism the application would use to set any other attributes.