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MRML Extensions
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In order to demonstrate how easily MRML can be extended to other query
paradigms, we give as an example QBE for images with user annotation. We
assume that the user is invited to associate textual comments with images
he or she marks as relevant or irrelevant. Since a tag for this purpose
does not yet exist in MRML, we add an
attribute cui-user-annotation="..." to the element. The
prefix cui- is added to avoid name clashes with extensions
from other groups which use MRML.
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<user-relevance-list
>
<user-relevance-element
image-location = "file:/images/1.jpg"
user-relevance = "1"
cui-user-annotation = "tropical fish"
/>
</user-relevance-list>
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It is important to note here that servers which do not recognize the
cui-user-annotation attribute still can make use of the
remaining information contained in the user-relevance-element element.
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As an example of how not to extend MRML, we give an extension with
the same semantics but which does not respect the principle of graceful
degradation:
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<user-relevance-list
>
<cui-user-relevance-element
image-location = "file:/images/1.jpg"
user-relevance = "1"
user-annotation = "tropical fish"
/>
</user-relevance-list>
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Instead of adding an attribute to an existing MRML element (user-relevance-element),
a new element was defined that
contained the same kind of extension, namely cui-user-relevance-element.
Consequently, servers which do not
recognize this element will not be able to exploit any relevance
information.
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