[NeXus-committee] Validation: Against which application definition? Ignoring base classes?

[Joachim Wuttke]

Dear colleagues:

NeXus validation tools are currently broken.
Repairing them won't be just a coding task.
Rather, a number fundamental problems will come up.
Here a few of them. Your comments are welcome.

For simplicity of language, I suppose that
   https://github.com/nexusformat/definitions/issues/298
will be accepted: application definitions include
contributed ones.

(1) Validation of a NeXus data file means validation
against one application definition - right?

(2) It is desirable to have a tool that automatically
checks whether a data file is NeXus compliant.
Possible applications:
- Facilities could run the validator over all newly
acquired data, to make sure nobody is breaking NeXus.
- One could think of a web service where people can upload
a data file to have the format validated.

(3) If (1) and (2) are agreed, then the problem arises
how to know from a given file which application definition
it pretends to follow. I think there is only one clean solution:
the name of the application definition must be stored in each
data file. E.g. in form of an NXroot attribute @application_definition.

(4) A few weeks ago we clarified that base classes member
lists can be extended (by application definitions, and by
any single NeXus application, if I understood correctly).
With member lists being neither exclusive nor obligatory,
base classes turn out to be no more than recommendations
to application definition writers. Data file validation
tools will have to ignore the base class specs altogether.

Best regards - Joachim

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