I would like to understand the purpose of targetNamespace as used both in XML Schema and in WSDL. In fact, to keep things simple, let's limit this question to XML Schema.
I feel like I fully understand the notion of (simple) XML namespaces. By convention we use URI/URLs, but we could use any string, which we then assign to a prefix for reuse by XML nodes and attributes, or use simply as the default namespace for the scope at hand. So far, so good ?
Now enters XML Schema. For some reason the inventors of XML Schema felt the notion of simple namespaces wasn't enough and they had to introduce the targetNamespace. My question is : what significant benefit does a targetNamespace introduce that couldn't be provided by a normal XML namespace ? If an XML document references a xsd document, either by schemaLocation or with an import statement, in either case I give the path to the actual xsd document being referenced. This is what uniquely defines the Schema I want to refer to. If in addition I want to bind this Schema to a particular namespace in my referencing document, why should I be obliged to replicate the precise targetNamespace already defined in the XML Schema I am referencing? Why couldn't I simply redefine this namespace however I want within the XML document in which this namespace will be used to refer to that particular XML Schema document I want to reference ?
Update:
To give an example, if I have the following in an XML instance document:
<p:Person xmlns:p="http://contoso.com/People" xmlns:v="http://contoso.com/Vehicles" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation= "http://contoso.com/schemas/Vehicles http://contoso.com/schemas/vehicles.xsd http://contoso.com/schemas/People http://contoso.com/schemas/people.xsd"> <name>John</name> <age>28</age> <height>59</height> <v:Vehicle> <color>Red</color> <wheels>4</wheels> <seats>2</seats> </v:Vehicle> </p:Person>
Why does e.g. the people.xsd Schema need to define a targetNamespace which is "http://contoso.com/schemas/People"? Why do we need the targetNamespace definition in the xsd document at all? It seems to me all you have to gain from the namespace part of the schemaLocation is already contained in the XML instance document. What is the benefit of enforcing the existence of a targetNamespace with equal value over in the xsd document ?
Follow-up question to Paul's answer:
Can you give me a concrete example where such "clashes" between xsd element names becomes apparent and that would explain the need for targetNamespace ?
Ok, here's an attempt to answer my own question. Let me know if it seems coherent to you. Looking at the examples on the page linked by Paul helped me.
If we take the XML instance example in the original question above, we have two references to the definition of the vehicle element. One is explicit and visible in the XML instance document itself, but we must also imagine that the person.xsd XML Schema references the same vehicle definition again as an allowed child element of person. If we were to use normal namespaces where each document were allowed to define its own namespace for vehicle, how would we know that the XML instance is referencing the same XML Schema definition for vehicle as is the person.xsd ? The only way is by enforcing a concept of namespace which is stricter than the original simple one and which must be written exactly the same way across multiple documents.
If I wasn't writing this on a tablet I would provide a code example, but here I will just attempt to describe the example I have in mind.
Imagine that we have two different XML Schema definitions for a vehicle element. location1/vehicles.xsd would contain the definition that validates the example from the question of this post (containing color, wheels, and seats child elements), whereas location2/vehicles.xsd would contain an entirely different definition for a vehicle element, (say, with child elements year, model, and volume). Now, if the XML instance document refers to the location1 Schema, as is the case in the example above, but person.xsd says that the person element can contain a vehicle child element of the type defined in the location2 Schema, then without the notion of a targetNamespace, the XML instance would validate, even though it clearly doesn't have the right kind of vehicle as a child element of its person element.
Target namespaces then help us make sure that if two different documents are referencing the same third XML Schema, that they are both in deed referencing the same Schema and not just a Schema that contains elements that are similar, but not identical to one another...
Does that make any sense ?
Figure 1: Elements and attributes in XML Schema namespace are used to write an XML Schema document, which generates elements and attributes as defined by user and puts them in {target namespace}. This {target namespace} is then used to validate the XML instance.
targetNamespace is the logical namespace for information about this service. WSDL documents can import other WSDL documents, and setting targetNamespace to a unique value ensures that the namespaces do not clash. xmlns is the default namespace of the WSDL document, and it is set to http://schemas.xmlsoap.org/wsdl/ .
An XML namespace is a collection of names that can be used as element or attribute names in an XML document. The namespace qualifies element names uniquely on the Web in order to avoid conflicts between elements with the same name.
How xmlns attribute is different from targetNamespace attribute? The target namespace is used for the internal referencing in the xml document to make sure it is valid as declared from the schema. The xmlns attribute actually declares the namespace of a particular element or group of elements.
You seem to be on the right track. I'll make a few points here that might help.
I think what you were assuming is that the instance document could specify the namespace of the elements and attributes declared in some schema document, using xsi:schemaLocation. That doesn't work. For one thing, the parser may locate other schema documents than those listed, and it needs to know what namespace they are for. For another, it would make reasoning about schemas difficult or impossible: you wouldn't be able to look at a schema and know the namespaces that everything belonged in because that decision would be postponed until an instance was written.
Q: "By convention we use URI/URLs, but we could use any string, which we then assign to a prefix for reuse by XML nodes and attributes, or use simply as the default namespace for the scope at hand."
A: Yes, exactly.
Q: "For some reason the inventors of XML Schema felt the notion of simple namespaces wasn't enough and they had to introduce the targetNamespace."
A: http://www.liquid-technologies.com/Tutorials/XmlSchemas/XsdTutorial_04.aspx
Breaking schemas into multiple files can have several advantages. You can create re-usable definitions that can be used across several projects. They make definitions easier to read and version as they break down the schema into smaller units that are simpler to manage.
...
This all works fine without namespaces, but if different teams start working on different files, then you have the possibility of name clashes, and it would not always be obvious where a definition had come from. The solution is to place the definitions for each schema file within a distinct namespace.
Clarification:
The primary purpose of XML Schemas is to declare "vocabularies".
These vocabularies can be identified by a namespace that is specified in the targetNamespace attribute.
The Schema (an XML document) can have a "namespace". The "vocabulary" the document describes can have a "targetNamespace".
Just as XML Schemas provide a higher level of abstraction than SGML DTD's (the original architects of XML thought DTD's were sufficient), XML Schema "targetNamespaces" provide a level of abstraction over "simple namespaces".
'Hope that helps
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