XML interview questions and answers

Describe the differences between XML and HTML.
It's amazing how many developers claim to be proficient programming with
XML, yet do not understand the basic differences between XML and HTML. Anyone
with a fundamental grasp of XML should be able describe some of the main
differences outlined in the table below.

XML	HTML
User definable tags	Defined set of tags designed for web display
Content driven	Format driven
End tags required for well formed documents	End tags not required
Quotes required around attributes values	Quotes not required
Slash required in empty tags	Slash not required

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

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.

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.
Example 1:
<xsl:template match="element-name">
Attribute Value:
<xsl:value-of select="@attribute"/>
<xsl:apply-templates/>
</xsl:template>

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

How would you build a search engine for large volumes of XML data?
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.
Obviously, some important areas of XML technologies were not included in this list --
namespaces, XPointer, XLink, and so on -- and should be added to the interviewer's
set of questions if applicable to the particular position that the candidate is applying
for. However, these questions in conjunction with others to assess soft skills
(communication skills, ability to work on teams, leadership ability, etc.) will help
determine how well candidates understand the fundamental principles of XML.

Why is XML such an important development?
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 hardto-
program options.
3. 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.