support for dynamic containment

[bert.robben@agfa.com]

Support for dynamic containment
-------------------------------

In SDO, the boundaries of a datagraph are defined by the containment 
relation. Only objects which can be reached from the root object by 
following properties that are contained are part of the datagraph. 
Containment is defined at the type level.
In cases where applications need to dynamically select what information 
they want, this fixed containment relationship is an issue. For instance, 
suppose in a medical context you have defined a number of types defines to 
represent patients together with their clinical (e.g. procedures they have 
taken) and administrative data (for instance their address). The type 
definition needs to decide on the containment of the clinical and 
administrative data. However it is hard to decide whether or not the 
administrative and clinical data should be contained because some 
applications might only need clinical or administrative data and others 
might need both. In cases where the type system is large or where there 
are large volumes of data involved (for instance in the example, 
procedures could have an associated pdf-report property) this becomes a 
real issue.
Current solutions within the SDO framework could be (for the interested, 
there has been a mail thread about this a while ago in the user mailing 
list)
- Each app shoud define its own type with an appropriate containment 
relation. The downside of this is a proliferation of types.
- The main types should not have any containment relations. Containment is 
specified using a synthetic type. Think of this as a special list type 
that contains its elements. The root of the datagraph then would be an 
instance of such a list type. All instances that are needed should be put 
in this flat list.
I would like to propose an alternative solution. In this solution, 
containment would not be specified at the type level. Whenever the 
boundary of a datagraph is needed (for instance when an xml document it be 
generated or a datagraph is to be exchanged between for instance a client 
and a server), the application should provide appropriate information that 
specifies exactly what is part of the graph and what not. This can be seen 
as a select clause for sql, or even better as a set of fetch joins in 
Hibernate. This would give the application control over exactly what it 
wants. In the example for instance, the application can easily decide at 
each point whether or not it would want the address information together 
with the patient data.
This proposal would have a number of interesting implications.
- What is the implication of this for cases where datagraphs are 
represented as xml documents that should be according to an xml schema?
- How to deal with links to objects that don't belong to the datagraph? 
One strategy could be just to drop them. Another one to provide some kind 
of proxy.

Frank Budinsky let me know that this is closely related to charter item 
(10.  Relaxing Containment Requirements).


I posted this also as a feature request on the Tuscany JIRA (
https://issues.apache.org/jira/browse/TUSCANY-1918).

Interested parties can have a look at our SDO implementation (which we 
donated to Tuscany) (see also Tuscany JIRA 1527 and 1493) where we try to 
support this.






Bert Robben | Agfa HealthCare
Platform Architect | HE/Technology Platform Team
T  +32 3444 8407 | F  +32 3444 8401

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