XML Interview Questions with Answers Page I

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1. What is XML?

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).

2. What is a markup language?

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

stylesheet to create output for screen, print, audio, video, Braille, etc.

Some markup languages (eg those used in wordprocessors) 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.

3. Where should I use XML?

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 (eg Braille, Audio, etc) from a single source document by

using the appropriate stylesheets. 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.

4. 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 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.

5. 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.

6. Aren't XML, SGML, and HTML all the same thing?

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).

7. Who is responsible for XML?

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.

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++.

8. 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 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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).

15. 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.