XML Interview Questions and Answers

What is the purpose of XML namespaces?
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:
<?xml version="1.0" ?>
<Address>
<Street>Apple 7</Street>
<City>Color</City>
<State>State</State>
<Country>Country</Country>
<PostalCode>H98d69</PostalCode>
</Address>
and:
<?xml version="1.0" ?>
<Server>
<Name>OurWebServer</Name>
<Address>888.90.67.8</Address>
</Server>

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.

What is an XML namespace?
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.

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

What do XML namespaces actually contain?
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.
<google:A xmlns:google="http://www.google.org/">
<B google:C="google" D="bar"/>
</google:A>
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.

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