[Fiware-ngsi] interpretation of attributeExpressions

[Tobias Jacobs]

Dear all,

I would like to open the discussion on which syntax to use for attributeExpressions used as restrictions. The OMA NGSI specs say about this

"The XPath expression will be evaluated against ContextEntity structures."

This is rather ambiguous in my opinion. How NEC suggests to define the semantics of attributeExpressions is:

-          AttributeExpressions are correct if they return a list of "Id" strings from EntityIds when evaluated against the xml given by the response message of the operation (QueryContextResponse or NotifyContextRequest).

-          The attributeExpression is applied to a response message by removing all information about any Entity whose Id does not appear in that list.

Secondly we propose to make the "AttributeExpression" field in the Restriction structure (5.5.6 in the NGSI specs) optional, because we believe that there will be many situations where only a scope and no AttributeExpression is needed. This would be another change in the NGSI specs.

What do you think? Have you already implemented the attributeExpression restriction, and do you use a different interpretation?

Best regards
Tobias
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