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XSD elementFormDefault="unqualified" destroys the benefits ofnamespace modularity
- From: "Costello, Roger L." <costello@mitre.org>
- To: "'xml-dev@lists.xml.org'" <xml-dev@lists.xml.org>
- Date: Wed, 28 Jan 2009 08:34:16 -0500
Hi Folks,
Recall that with XML Schemas you can specify that all elements in conforming instance documents must be namespace-qualified using:
elementFormDefault="qualified"
Or, you can specify that only top-level, global elements must be namespace-qualified using:
elementFormDefault="unqualified"
These can yield remarkably different instance documents.
INSTANCE #1
Here is an example of an instance where the schema specifies "qualified":
<?xml version="1.0"?>
<c:camera xmlns:c="http://www.camera.org";
xmlns:nikon="http://www.nikon.com";
xmlns:olympus="http://www.olympus.com";
xmlns:pentax="http://www.pentax.com";>
<c:body>
<nikon:description>Ergonomically designed casing for easy
handling</nikon:description>
</c:body>
<c:lens>
<olympus:zoom>300mm</olympus:zoom>
<olympus:f-stop>1.2</olympus:f-stop>
</c:lens>
<c:manual_adapter>
<pentax:speed>1/10,000 sec to 100 sec</pentax:speed>
</c:manual_adapter>
</c:camera>
INSTANCE #2
Using the same schema, but specifying "unqualified," here is the instance:
<?xml version="1.0"?>
<c:camera xmlns:c="http://www.camera.org";>
<body>
<description>Ergonomically designed
casing for easy handling
</description>
</body>
<lens>
<zoom>300mm</zoom>
<f-stop>1.2</f-stop>
</lens>
<manual_adapter>
<speed>1/10,000 sec to 100 sec</speed>
</manual_adapter>
</c:camera>
BENEFITS OF NAMESPACES
Namespaces modularize instance documents. They enable applications to perform module-specific processing.
MODULES IN INSTANCE #1
In instance #1 the namespaces modularize the document. The document can be readily partitioned into these five modules:
-------------------------------------------
<c:camera xmlns:c="http://www.camera.org";>
<c:body>
</c:body>
<c:lens>
</c:lens>
<c:manual_adapter>
</c:manual_adapter>
</c:camera>
-------------------------------------------
<nikon:description xmlns:nikon="http://www.nikon.com";>
Ergonomically designed casing for easy handling
</nikon:description>
-------------------------------------------
<olympus:zoom xmlns:olympus="http://www.olympus.com";>
300mm
</olympus:zoom>
-------------------------------------------
<olympus:f-stop xmlns:olympus="http://www.olympus.com";>
1.2
</olympus:f-stop>
-------------------------------------------
<pentax:speed xmlns:pentax="http://www.pentax.com";>
1/10,000 sec to 100 sec
</pentax:speed>
-------------------------------------------
MODULES IN INSTANCE #2
In instance #2 there is one namespace module and three no-namespace modules:
-------------------------------------------
<c:camera xmlns:c="http://www.camera.org";>
</c:camera>
-------------------------------------------
<body>
<description>Ergonomically designed
casing for easy handling
</description>
</body>
-------------------------------------------
<lens>
<zoom>300mm</zoom>
<f-stop>1.2</f-stop>
</lens>
-------------------------------------------
<manual_adapter>
<speed>1/10,000 sec to 100 sec</speed>
</manual_adapter>
-------------------------------------------
PROBLEMS WITH INSTANCE #2
The no-namespace modules are comprised of multiple XML vocabularies. For example, in the lens module the <lens> element comes from the camera schema, whereas the <zoom> and <f-stop> elements come from the olympus schema.
An application written for the olympus schema would not be able to process the lens module for these two reasons:
a. The elements are not associated with the olympus namespace. Consider how the olympus application would behave upon encountering the lens module: "Is that <zoom> element really the element I know about, or is it somebody else's zoom element?
b. The olympus application knows nothing about the <lens> element.
RECOMMENDATION
Don't use elementFormDefault="unqualified" in your XML Schemas. It destroys the modularity benefits provided by namespaces.
Do you agree with this recommendation?
/Roger
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