Compound Attributes - Was: [xacml] Minutes for 27 April 2017 TC Meeting

[Steven Legg <steven.legg@viewds.com>]

On 28/04/2017 6:56 AM, rich levinson wrote:
>    Documentation of HL7 data types: ref from David wrt "compound attributes":
>      https://lists.oasis-open.org/archives/xacml/201704/msg00003.html
>
>         compound attrs: variation of xml
>      martin: datatype was sort of a code list: needed or covered?
>       look up official country code:
>       need datatype for "enumerated lists"
>
>      rich: maybe have a key:value pair as a datatype?

Isn't a key:value pair an XACML attribute? :-)

I would have described the HL7 data types as multi-part identifiers, but as the
IHE-XACML binding is only interested in at most two parts, and one mandatory part
is an OID, it would have been a viable approach to treat each OID as an XACML
attribute id and the other part as the XACML attribute value, thus avoiding the
need for new data-types and functions.

That aside, the entities profile provides two ways to represent
multi-part identifiers (or key:value pairs).

A value of the CV data-type would appear something like this according to HL7:

  <xacml:AttributeValue dataType="urn:hl7-org:v3#CV">
    <CodedValue code="1" codeSystem="1.0.14265.1"/>
  </xacml:AttributeValue>

Using the entities profile it could still be represented as XML without defining
a new data-type:

  <xacml:AttributeValue DataType="urn:oasis:names:tc:xacml:3.0:data-type:entity">
    <xacml:Content>
      <CodedValue code="1" codeSystem="1.0.14265.1" />
    </xacml:Content>
  </xacml:AttributeValue>

The parts of the value would have to be extracted using XPath and the
attribute-selector function.

Alternatively, using the entities profile, the parts could be represented as
nested attributes:

  <xacml:AttributeValue DataType="urn:oasis:names:tc:xacml:3.0:data-type:entity">
    <xacml:Attribute IncludeInResult="false" AttributeId="urn:example:hl7:attribute:code">
      <xacml:AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string";
         >1</xacml:AttributeValue>
    </xacml:Attribute>
    <xacml:Attribute IncludeInResult="false" AttributeId="urn:example:hl7:attribute:codeSystem">
      <xacml:AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string";
         >1.0.14265.1</xacml:AttributeValue>
    </xacml:Attribute>
  </xacml:AttributeValue>

The parts of the value would be extracted using the attribute-designator function.

The tricky part is writing XACML expressions that have the same effect as the
new HL7 functions. The CV-equal and II-equal functions are straightforward, if a
little verbose. The II-to-string and II-match functions are doable, but contorted.
It would be much easier if there were an XACML function that behaved like the
Java/C ternary operator "?:", i.e., if the first argument is true then return the
second argument, otherwise return the third argument. The remaining functions
involve transformations that could perhaps be done in the PEP and PIP rather than
the PDP.

It would be handy for policy writers using compound attributes if XACML supported
user-defined functions (think of a variable definition with parameters). To better
support use cases with compound attributes I would add user-defined functions and
a ternary operator. Representing compound attributes and processing them are
already covered by the entities profile.

Regards,
Steven