Action 218 Subject Attachment Extensibility in guidelines

The following text completes our action item 218 for Subject Attachment
Extensibility. It is proposed as a new subsection in Section 5. 

Umit and Maryann

[Action 218] http://www.w3.org/2005/06/tracker/wspolicy/actions/218
Note: The text will use the reference that is already existing in the
guidelines document to WS-RM Policy. 


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5.3 Subject Attachment Extensibility: 
Overtime, a policy expression may evolve to be applicable to policy
subjects different than the policy subjects that it was originally
designed for. When this type of evolution occurs, the author of the
assertion should carefully consider the applicability of this usage with
respect to the compatibility of the assertion's semantics. When an
assertion needs to evolve to indicate a behavior that is no longer
compatible with its originally intended semantics or policy subjects, a
new namespace should be designed to designate the new behavior that is
signified with the new namespace. Section 5.2 further clarifies the
usage of multiple behaviors. 
In general, it is NOT recommended to deprecate policy subjects by
reducing the policy subjects that an assertion was originally designed
for. 
When the assertion's semantics does not change to invalidate any of the
original policy subjects but new policy subjects need to be added, it
may be possible to use the same assertion to designate the additional
policy subjects without a namespace change.  The authors must retain the
compatible behavior of the policy assertion. For example, a policy
assertion for a protocol that is originally designed for endpoint policy
subject may add message policy subject to indicate finer granular
attachment provided that endpoint policy subject is also retained in its
design. This approach has been used by WS-RM Policy. 

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Dr. Umit Yalcinalp
Research Scientist
SAP Labs, LLC
Email: umit.yalcinalp@sap.com Tel: (650) 320-3095 
SDN: https://www.sdn.sap.com/irj/sdn/weblogs?blog=/pub/u/36238
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"Nearly all men can stand adversity, but if you want to test a man's
character, give him power." Abraham Lincoln.