[graap-wg] Re: Alternative Approach to Templates and Constraints

[Heiko Ludwig]

Karl,

thank you for your specific proposal. This enables some concrete 
discussion. Overall, I see a trade-off between your proposal (let's call 
it offer pointer proposal (OPP) ) and the one we came up yesterday 
(template pointer proposal (TPP) ) are quite similar in expressiveness.The 
OPP requires a little less specification lines while the TPP is a little 
easier to implement in a verifier and provide a good aditor. Also, The TPP 
seems to be a bit more convenient for a mostly static template with fields 
of moderate complexity while the OFP appears to be easier when a template 
is very open, with relatively "big" fields in a rudimentary main agreement 
prototype but insertable content using prototype content in and for the 
fill-in items. I will detail this in a second.

First, i want to raise my only real conceptual concern with the OPP: There 
is a dependency between items which is a little bit hidden in the XPATHs, 
which we don't require as location instrument. An item 1 at /a/b/c can 
only be inserted if /a/b exist. If /a/b is optional, determined by 
choosing to fill in another item 2, item 1 depends on item 2. This 
dependency is only understandable by analysing the XPATH expression, which 
is no tpart of the standard. This is a serioud concern. It could be dealt 
with by explicitly listing dependencies, but this is additional hastle.

My other concerns mostly relate to the pragmatics and my lack of 
understanding of how to implement this well:

- The pointers in the OFP may point to not yet existing locations in the 
editing process of the templates. XPATHs, if this is the chosen location 
mechanism (and I guess it will), are not nice to write by hand reliably, 
really. Ideally, you want to have an editor where you can point to an 
element and the XPath is returned. Xerces, for example supports this. With 
OPP pointers the author is probably on its own, as far as I understand the 
current tooling environment.

- For the same reason, the pointers to potential future offer locations, 
the consistency check of authored templates is more difficult, or at least 
appears to me so as of now.

- The verification of an offer with its constraints appears to become 
harder. In the TPP approach, all item constraint follow the exact 
interpretation semantics of the schema, no assumptions. One can verify an 
items value by taking the XML to which an XPath points, take the XML 
schema constraint associated with item and match it, e.g., using standard 
Xerces funtionality. In the OPP case, when verifying an item, one has to 
look through all items to analyze which contraints or type definitions 
apply and then merge them with the local constraint to execute the full 
verification using an XML parser - and hope that they are more consistent.

Hence, for evaluating our alternatives, we must take into account the 
dependency issue as well as the pragmatic concerns that I was raising. An 
additional decision -criterion is where we see the most probable uses 
cases: relatively simple fill-in items or items with a lot of 
compostitional complexity that must be guided.

Heiko

----
Heiko Ludwig, Dr. rer. pol.
IBM TJ Watson Research Center, PO Box 704, Yorktown, NY, 10598
hludwig at us.ibm.com, tel. +1 914 784 7160,  mob. +1 646 675 8469
http://www.research.ibm.com/people/h/hludwig/



Karl Czajkowski <karlcz at univa.com> wrote on 07/20/2005 02:13:50 PM:

> 
> Heiko and GRAAP:
> 
> I think I have narrowed down my discomfort with the current template
> proposal... basically, I think we are on the right track with the
> creation constraints but the prototype aspect is still too biased
> towards a particular flavor of domain-specific term language that is
> "flat" and has no variable-cardinality content.  Once you start
> wanting to admit a domain language with optional fields or repeatable
> fields WITHOUT TOTALLY CONSTRAINING ITS USE, this approach gets in the
> way.
> 
> 
> EVALUATION CRITERIA
> 
> We of course need to keep focused on delivering a specification, so I
> am writing this email with the intent that people can review my
> suggestions and decide if they make sense and are easy enough to write
> into the specification without any non-trivial change to the schedule,
> i.e. Heiko should feel that it isn't more complex or open-ended than
> his proposal in terms of specification content, and that it also seems
> beneficial, and others should agree as well. :-)
> 
> My concept of an "ideal" template is one that does the following in
> a very concise manner (in rough order of importance):
> 
>   1. identify the domain-specific language(s) that are allowed
>      in terms
> 
>   2. specify the prototype values for terms, e.g. fixed or default**
>      values which are specific to one WS-Agreement service rather
>      than schema-level fixed/default values
> 
>   3. specify other more general content restrictions, e.g. identify
>      a place where the domain-language has open content and specify
>      a smaller list of recognized extensions that are acceptable
> 
>   4. related to (1), specify the compositional rules/restrictions
>      for terms in an offer, e.g. the whole "template composition"
>      discussion.  this is basically a matter of asserting restrictions
>      to the generic wsag:compositor syntax of the offer.
> 
>   5. assumes schema-level understanding of the content model in
>      the absence of prototypes and restrictions, i.e. do not require
>      the template to re-state the structural requirements of the
>      domain language if the WS-Agreement service is not trying to
>      restrict it further
> 
> In other words, the offer MUST (SHOULD?) conform to the WS-Agreement
> envelope and domain-specific term schemas, and it also MUST (SHOULD?)
> conform to the additional prototypes and restrictions of the template!
> 
> The current proposal doesn't allow concise mixing of the prototype and
> restriction mechanisms.  You end up having to either generate a large
> set of creation constraints that express the different parts of the
> domain schema (redundantly) against a prototype, or you cannot use the
> prototype feature and you provide one more compact creation
> constraint.
> 
> My view is that the current prototype mechanism is not valuable enough
> to keep, when a slight modification to the constraint model can cover
> the case where it is concise (a simple fixed list of values).
> 
> 
> ALTERNATE PROPOSAL
> 
> Make the entire template a set of constraint rules which have a new
> "triggering" and "reference" model: they refer to the eventual offer
> document by XPath instead of the (now non-existent) prototype
> document.  Furthermore, they are softened by default to only apply IF
> their XPath reference matches content in the prototype. (If they
> select an empty "nodeset" from the offer, they are ignored by
> default.) An extra attribute is permitted to override this soft
> triggering model and REQUIRE that the nodeset references by the Xpath
> (even if empty) match the constraint's content model.
> 
>    <Item>
>       <Location required="boolean"?>XPath</Location>
>       ...XSD content model...
>    </Item>
> 
> Whatever content in the offer is addressed by /Item/Location must
> conform to the content model in the rule. The Location is evaluated
> with the wsag:offer element as the root. This content model can be any
> of the following to cover our use cases:
> 
>    1. a literal value or subtree to cover the fixed/prototype case
> 
>    2. a simple restriction for the old enumeration use case
> 
>    3. a complex restriction/group for the new structural restriction
> 
>    4. an attribute group to cover the bases for reasonable XPath
>       references?
> 
> There is an implicit rule in every template which could be made
> explicit without harm:
> 
>    <Item>
>       <Location required=true>/wsag:AgreementOffer</Location>
>       <xsd:sequence>
>          <xsd:element ref="wsag:AgreementOffer"/>
>       </xsd:sequence>
>    </Item>
> 
> Which by itself means that the offer must conform to the WS-Agreement
> normative schema.
> 
> A minimal open-ended template might just want to identify the domain
> language and a simple compositor structure without making additional
> restrictions:
> 
>    <Item>
>       <Location required=true>
>          /wsag:AgreementOffer/wsag:Terms/wsag:All/*
>       </Location>
>       <xsd:sequence>
>          <xsd:element ref="wsag:ServiceDescriptionTerm"/>
>       </xsd:sequence>
>    </Item>
>    <Item>
>       <Location>//wsag:ServiceDescriptionTerm/*</Location>
>       <xsd:sequence>
>          <xsd:element ref="tns:docroot"/>
>       </xsd:sequence>
>    </Item>
> 
> The first rule says that the offer must have the very simple form of
> an All compositor with one child element ServiceDescriptionTerm... the
> number of children could be opened up with cardinality attributes in
> the sequence definition of the rule.  Alternatively, someone could
> write a rule with Location /wsag:AgreementOffer and then provide more
> inline typing to give a restriction of it?
> 
> The second rule says that the (any) service description term must have
> the tns:docroot domain-specific element in it.  A benefit of this
> approach for complex domain languages is that the variable structure
> of the domain language is preserved with all its flexibility UNLESS
> the template lists constraints against the structure.
> 
> Someone more familiar with XPath and/or XSD may be able to show a more
> compact way to write both of these conditions into one rule.
> 
> To do the fixed value fieldA from Heiko's email:
> 
>    <Item>
>       <Location>//tns:docroot/fieldA/text()</Location>
>       <wsag:literal>fixed value</wsag:literal>
>    </Item>
> 
> i.e. we define one "literal" content model to handle the prototype
> cases concisely.  (Is there an XSD feature that works as well? The
> most concise I can think of is a one-element enumeration
> restriction...)
> 
> These rules can be overlapping at different depths in the tree, so
> that the "top-level" rules fix the superstructure with brief
> ref=elementname descriptions of structure, implying that the general
> purpose elementname schema is acceptable, while later rules further
> restrict the use of that schema by referencing substructures in the
> descendents of elementname.
> 
> Thus, I hope I have demonstrated that this approach is just as
> expressive in terms of conveying information about offer requirements
> to the human.  I do not think we have use-cases in mind yet where we
> can evaluate the automatic consumption of templates by schema-aware
> software. :-( I think this is more general and consistent w/ the
> nodeset/infoset model of XPath selection, e.g. rules select parts of
> the offer and define schematic requirements on those selected parts.
> 
> 
> EXAMPLE
> 
> Here is the concrete example w/ the same implied offer content as
> Heiko's email example, e.g. I've left out the same contextual bits as
> were left out in his!
> 
> <template>
>    <Item>
>       <Location required=true>//wsag:ServiceDescriptionTerm/*</Location>
>       <xsd:sequence>
>          <xsd:element ref="tns:docroot"/>
>       </xsd:sequence>
>    </Item>
> 
>    <Item>
>       <Location>//tns:docroot/fieldA/text()</Location>
>       <wsag:literal>fixed value</wsag:literal>
>    </Item>
> 
>    <Item>
>       <Location>//tns:docroot/fieldB/text()</Location>
>       <xsd:restriction base="xsd:string">
>          <xsd:enumeration value="foo"/>
>          <xsd:enumeration value="bar"/>
>       </xsd:restriction>
>    </Item>
> 
>    <Item>
>       <Location>//tns:docroot/fieldC</Location>
>       <xsd:sequence>
>          <xsd:element ref="tns:fieldC" maxOccurs="128"/>
>       </xsd:sequence>
>    </Item>
> 
>    <Item>
> <Location>//tns:docroot/[fn:namespace-uri()!=http://tns/path]</Location>
>       <xsd:sequence>
>          <xsd:element ref="wsgram:FileStageIn" minOccurs=0 
> maxOccurs=unbounded/>
>       </xsd:sequence>
>    </Item>
> </template>
> 
> Note that the only reason I have to assert a constraint for fieldC is
> because I am restricting its cardinality. Otherwise, I could leave it
> off and the consumer of the template would assume the schema
> definition for tns:docroot is the only constraint, e.g. fieldC would
> be unbounded or whatever cardinality is given in the schema.
> 
> 
> karl
> 
> 
> ** I want to raise the idea that a default value semantics (B) that
> differs from the domain-specific schema is a confusing topic.
> Shouldn't a term document have the same meaning everywhere? (Shouldn't
> the domain schema remain normative for interpreting the absence of
> content?)  If we want to advertise a "default", maybe it should really
> be a weaker form of constraint which has the meaning "we recommend
> this value if you don't have any other preference", but it is still
> the initiators responsibility to fill in some value?
> 
> 
> 
> -- 
> Karl Czajkowski
> karlcz at univa.com
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