[NeXus-tech] NeXus data file schema

[Akeroyd, FA (Freddie)]

> 
> Freddie:
> 
> Seems we now have a slightly different approach to use in preparing
the
> NeXus
> data file schema, based on yesterday's tech committee meeting.
> Changes to what we have now will be necessary.
> 
> First, to validate a NeXus data file, we need to
>  * if HDF, convert NeXus HDF  to NeXus XML but don't include the big
> blocks of data
>  * transform the NeXus XML data file to an internal XML file format
for
> validation
>     (mostly, this is needed to handle the NXentry/definition that
> identifies if a NXDL
>

Pete,

nxvalidate would look at the "definition" field in the NXentry and then
has two options:
(1) It can either write <NXentry xsi:type="NXmonopdType"> ... </NXentry>
where NXmonopdType is inherited from NXentryType
or (2) instead of the NXentry it can write <NXmonopd> ... contents of
NXentry ... </NXmonodp> 

The advantage of (1) is that the use of inheritance means NXroot does
not need to know about NXmonopd. We may be able to do (2) by setting up
"substitution groups", but I think they are more restrictive than
inheritance. Another advantage of (1) is that the transformed XML file
would still be a valid NeXus XML data file if we left in the data - all
we have added is one extra attribute. Thus (1) leaves open the option of
a single schema file being able to validate an XML document against all
definitions - to make this automatic we just need to make an API change
such that an "xsi:type" attribute is added to the NXentry whenever a
"definition" field is written to the entry. 

Regards,

Freddie

>
>     application is to override the NXentry)
>  * validate the internal XML file against the master XML Schema (one
> single file)
> 
> We can generate one master XML Schema to validate XML files that
> contain
> the structure of NeXus data files.  Here is a brief description:
>     transform
> {base_classes,applications,contributed_definitions}/*.nxdl.xml
>     into XML Schema rules
>     and combine,
>     with content from nxdlTypes.xsd,
>     into one master XML Schema file
>     for use in validating transformed NeXus data files.
> 
> The base_class NXDL files will be transformed according to the
existing
> rules
> _EXCEPT_ that, since they will be combined together into a master
> XML Schema, it is not necessary to use to "import" other groups.
> 
> An XML Comment can be used to describe the version of each NXDL
> that contributes to the master schema.
> 
> The NXDL for applications will be transformed so that a
> NXmonopd.nxdl.xml
> will still result in a NXmonopd group being created in the master
> schema.
> The NeXus data file will be transformed so that
> NXentry/definition=NXmonopd
> elements will become NXmonopd elements.  This means the part of the
> master
> schema that validates NXroot elements needs to accept NXmonopd as an
> alternative to NXentry.  (And other application NXDLs similarly to
> NXmonopd.)
> 
> Transformation rules for NXDL of base_classes and applications is
> nearly
> identical
> except that fields and groups in base_classes are always optional
while
> fields and groups in applications are always required unless
designated
> optional.
> (This set of rules is not in the existing transformations or XML
Schema
> yet.)
> 
> A couple disadvantages to the approach of a master schema are
apparent:
>  *   the master schema file is large (relative term, maybe 100-200
kB?)
>  *   master schema file contains more than necessary for any one data
> file
>  *   cannot be used directly to validate NeXus data files, the data
> files
>         must always be transformed, even if they are XML
>  *   any changes in the NXDL requires creation of a new master schema
> 
> Despite these disadvantages, the advantages of apparent simplicity to
> the user
> and the data file validation process seem superior.
> 
> Pete
> 
> --
> ----------------------------------------------------------
>   Pete R. Jemian, Ph.D.                <jemian at anl.gov>
>   Beam line Controls and Data Acquisition, Group Leader
>   Advanced Photon Source,   Argonne National Laboratory
>   Argonne, IL  60439                   630 - 252 - 3189
> -----------------------------------------------------------
>      Education is the one thing for which people
>         are willing to pay yet not receive.
> -----------------------------------------------------------
> 

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