[glue-wg] Decision on Flat XML Schema

[Shiraz Memon]

Dear Glue’s,

After continued requirements analysis within the WG and at OGF 35/36, we would like to propose that the flat XSD style is adopted as the official GLUE2 XML rendering style over the nested/hierarchical style (or a combined approach). The main reasons for this style choice are summarised below. Further details, including a pro/con comparison are available from the WG’s wiki (see http://redmine.ogf.org/dmsf_files/176?download=).

At the time of decision on the flat schema, the group is aware that there are early implementations of GLUE2 XML rendering that use a formerly proposed hierarchical XSD schema.

For the sake of inter-operability and compatibility between implementations, when considering the flat schema as the official one, the OGF GLUE group will guide (on best effort basis) on how to create a mapping between the previous hierarchical XSD schema and the agreed flat XSD schema.

If you have any remaining concerns regarding this style choice, please respond before 25th Nov. If we hear no major concerns before this date, we will progress with an update to the GFD and XSD for subsequent submission to the OGF.

Summary of the main reasons for a flat over nested/combined approach:

1.       The flat style enables a single document root element (<Entities>). This acts as an element bag that lists Glue entity elements in a defined order.

2.       The flat style more efficiently models many-to-many associations and a child with multiple parents (style mixing is unnecessary, no combinations of nesting and ID refs).

3.       The flat style is better suited for rendering the results of ‘projection’ queries, i.e. queries that slice the data and return selected entities. The nested style is less efficient as you need to cascade render all children/grandchildren/so on for completeness (whereas the flat style can simply show that an association exists using an element ID reference, which means that the whole entity graph does not need to be rendered).

4.       Abstract elements with corresponding xsd:substitutionGroups and concrete entity implementations will be included within the base schema. Only those concrete entity implementations defined within this XSD will be used for interop/compliance. However, this extension mechanism facilitates profiling of new sub-type specialisations in the future without having to modify the base schema.

5.       We recognise that the flat approach requires extra steps when validating as it is necessary to check the correctness of associations by traversing/cross referencing the element IDs. However, we believe the extra benefits provided by the flat approach outweigh the drawback of this additional validation step.

Best Regards,
--
Ahmed Shiraz Memon
Federated Systems and Data
Jülich Supercomputing Centre (JSC)

Phone: +49 2461 61 6899<tel:%2B49%202461%2061%206899>
Fax:     +49 2461 61 6656<tel:%2B49%202461%2061%206656>



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