% VODML - Status update 
% Omar Laurino
% June, 18 2015

# Specification

## Goals
>- provide formal rules for data modeling in IVOA
- provide standard, machine-readable Data Model descriptions
- define Portable Data Model References

. . .

>- define primitives types (IVOA model)

## Some benefits
From the Data Model WG charter:

>[The WG] provide[s] a **framework
for the description of metadata** attached to observed or simulated data.

. . .

>[...] focuses
on **logical relationships between these metadata**,
examines how an astronomer wants to retrieve, process and interpret astronomical data,
and **provides an architecture** to handle them.

. . .

>[...] What is defined in this WG can then be **re-used**
in the protocols defined **by the DAL WG or in VO aware applications**.

## Formal Data Modeling Rules
>- Based on UML Class Diagram meta-model
- Relational Model, server-friendly
- Object Model, client-friendly

## Data Model descriptions

```xml
<dataType>
 <vodml-id>source.SkyCoordinate</vodml-id>
 <name>SkyCoordinate</name>
 <description>...</description>
 <attribute>
   <vodml-id>source.SkyCoordinate.longitude</vodml-id>
   <name>longitude</name>
   <description>...</description>
   <datatype>
     <vodml-ref>ivoa:quantity.RealQuantity</vodml-ref>
   </datatype>
   <multiplicity>1</multiplicity>
 </attribute>
...
```

## Portable Data Model references
`src:source.SkyCoordinate.longitude`

*points to*

```
 <attribute>
   <vodml-id>source.SkyCoordinate.longitude</vodml-id>
   <name>longitude</name>
   <description>...</description>
   <datatype>
     <vodml-ref>ivoa:quantity.RealQuantity</vodml-ref>
   </datatype>
   <multiplicity>1</multiplicity>
 </attribute>
```

. . .

## Portable Data Model references
Can be used in VOTable (pending VOTable discussion)

```
<VODML>
  <ROLE>src:source.SkyCoordinate.longitude</ROLE>
</VODML>
```

# Updates

##
>- Normative part is basically unchanged since Heidelberg:
    - `utype` -> `vodml-ref`

. . .

- Informative sections improved before Banff
- Nothing major changed since Banff
    - Few minor *To Be Discussed* addressed.

. . .

- Implementation feedback is good: only one issue identified.

. . .

- Working on more XMI translation scripts.

. . .

Document seems stable. Should be ready for Proposed Recommendation
pending few TBDs.

## To Be Discussed
>- Few TBDs left in the document:
	- \<\<subsets\>>\ issues from implementation feedback
    - prefix: globally unique or defined in serialization?
    - SKOSConcept/RDF/Enumeration
- One TODO:
    - reformat a figure 

## Subsets property
Implements UML concepts of `redefines` and `subsets`

- `subsets`: WG chairs are a subset of the IVOA members
    - Common pattern in Relational Models
- `redefines`: the SphericalError on a SphericalCoordinate is a subtype of a generic Error on a Coordinate
    - Common pattern in Object Models

## Subsets property
STC2 uncovered issue with `subsets` property

. . .

The subsetted property would redefine its VODML-ID, breaking inheritance, e.g.:

```
  <ROLE>stc:SphericalCoordinate.error</ROLE>
```

vs

```
  <ROLE>stc:Coordinate.error</ROLE>
```

. . .

Fixed by making `subsets` an attribute of the super-type.

# Implementations

## Models
>- STC2
- Spectral DM 2.0
- Cube DM

. . .

Formal meta-modeling rules make models more robust and interoperable. We need more tools
for easily translate models to VODML/XML.

## Software
>- volib/Python: Django-like Models in MVC
    - Demonstrates that VODML can be adapted to Object Model

. . .

>- VO Mapper/Web: map models to TAP services
    - Demonstrates that compliant annotations can be made without knowledge of the specs

. . .

>- Validator/Java
    - Demonstrates that models can be validated by model-agnostic software

. . .

## Software
- For serializations we need the Mapping Document to settle, however:
    - prototypes of the current Mapping Document have been successful
    - plenty of pre-Banff prototypes


# Discussion items

## Model Prefix
>- Have them globally unique (e.g. `stc2`, `char`, `spec2`)

or

>- Define them dynamically (a la XML namespace)

Consensus seems to be that they are globally unique.

Can we confirm this?

. . .

Should models (and prefixes) be registered?

## Enumerations vs Vocabularies

>- Enumerations:
    - integral part of the model
    - a change requires an update of the model

. . .

>- Vocabularies:
    - can be extended without updating the model
    - VODML supports SKOS concepts, which are more flexible
        - e.g. relationships among concepts