top 31-40 Giant list of XML interview questions and answers

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

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:

<List>
<Item>Chocolate</Item>
<Item>Music</Item>
<Item>Surfingv</Item>

</List>

(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.

32. Does XML let me make up my own tags?

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.

33. How do I create my own document type?

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.

34. How do I write my own DTD?

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:
<?xml version=”1.0″?>

<!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:
<Shopping-List>

<Item>Chocolate</Item>
<Item>Sugar</Item>

<Item>Butter</Item>
</Shopping-List>

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 SGMLÃ¢â‚¬â€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.

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

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

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

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.com.
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.

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

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.

38. Can I encode mathematics using XML?Updated

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.

39. How will XML affect my document links?

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, etcÃ¢â‚¬Â¦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.

40. How does XML handle metadata?

Because XML lets you define your own markup languages, you can
make full use of the extended hypertext features of XML (see the
question on Links) to store or link to metadata in any format (eg
using ISO 11179, as a Topic Maps Published Subject, with Dublin
Core, Warwick Framework, or with Resource Description Framework
(RDF), or even Platform for Internet Content Selection (PICS)).
There are no predefined elements in XML, because it is an architecture,
not an application, so it is not part of XML’s job to specify how
or if authors should or should not implement metadata. You are therefore
free to use any suitable method. Browser makers may also have their
own architectural recommendations or methods to propose.