Update Constraint narrative

[riannella]

update the Constraint narrative to include differences in evaluating the expression outcome for Perms/Prohibs/Duties

[riannella]

Comments from Simon:

http://w3c.github.io/poe/model/#constraint states:

"[...] If multiple Constraint entities are linked to the same Permission, Prohibition, or Duty entity, then all of the Constraint entities MUST be satisfied. That is, all the Constraint entities are (boolean) anded. [...]"

Without actually explaining the concept of "satisfying a Constraint entity".

=> How can someone "satisfy a Constraint entity"?
=> What does it mean for permissions/prohibitions/duties if their constraints are unsatisfied?
=> Is a permission having an unsatisfied constraint equivalent to a prohibition? (and vice versa)
=> Should a permission/prohibition having an unsatisfied constraint simply be ignored? If so, is it possible for "ignored" perm/proh to become "active" again?

[riannella]

There are some interesting statements about handling stateful constraints in the OMA spec:

http://technical.openmobilealliance.org/Technical/Release_Program/docs/DRM/V2_2-20110419-C/OMA-TS-DRM_DRM-V2_2-20110419-C.pdf

Even though it was for ODRL V1.1, the concepts still apply
We should review this too

[riannella]

See Michaels' comments here: https://www.w3.org/2016/poe/wiki/Constraints_in_the_Information_Model

[iherman]

I think that core statement is:

The name identifies the left operand of the logic operation for the Constraint, it SHOULD include the entity it constrains and how its value for a comparison has to be retrieved/generated

Based on the discussions we had on Tuesday, I wonder whether the SHOULD shouldn't be a MUST. It would also help to see some specific examples of how this would work in practice for some of the specific constraints we define.

On 17 Nov 2016, at 03:21, Renato Iannella notifications@github.com wrote:

See Michaels' comments here: https://www.w3.org/2016/poe/wiki/Constraints_in_the_Information_Model https://www.w3.org/2016/poe/wiki/Constraints_in_the_Information_Model

[riannella]

@iherman @nitmws I think it is a "should" as the default entity that is constrained is the Action.

[iherman]

Hm. I apologize if I did not look it up (I am on a meeting) but isn't it so that the constraints are, usually, on an 'aspect' of an Action (e.g., time) rather than the Action itself, and that was exactly the issue of the discussion. What I am saying is that this 'aspect' MUST be exactly specified for the left operand of the logic operation, it should be more than a SHOULD.

I may miss something.

[nitmws]

In my proposed changes to constraint definitions on https://www.w3.org/2016/poe/wiki/Constraints the Action as constraint target is always explicitly included. By my experience with users of definitions I prefer no to state default values outside a definition, this is skipped in too many cases. And @iherman is partially right: a couple of constraints do no target the Action only, second ranking is the Asset.

[riannella]

We can make it a MUST

[riannella]

See #81