[step-manufacturing] Re: Specification of the AP Binding
Tom Rando
trando at ebmail.gdeb.com
Fri Jul 16 18:22:57 EDT 2004
I have a couple issues with this proposal for the "simple" binding.
It does not appear to be any simpler than the default binding
in the current document. It contradicts many of its own purported
requirements.
It fails to address the major US comment regarding external mapping.
+++++++++++++++++++++++++++++++++++++++
>AP Binding Requirements
>1. The AP binding needs to follow the mainstream conventions for XML data
> (no surprises to the end user).
- The proposed naming convention (1) is one that I have not seen used
in any XML community. XML-ers typically prefer CamelCase for
elements and
lowerCamelCase for attributes. I believe this naming convention
would come as a surprise
to an XML-literate end user.
- The examples in (3) , though only examples, contain STEPisms for
floating point
numbers that would also come as a surprise to XML users. They have
adopted the
IEEE standard for representing floating point values.
- (7) introduces a STEP version of NULL. The XML community has already
adopted
xsi:nil as the out of band designator for null elements.
- (8) Handling of simple aggregates will also come as a surprise
to XML users who are used to seeing concise white-space separated
lists for aggregates of simple values. It may also come as a surprise
to AP209 users
or others whose instance documents consist of voluminous aggregates
of numerical values -- to find their instance files quadrupled in size.
-- (10) proposes the use of fragment identifiers for local references.
This
runs counter to the expectations of any XML users who rely on XSLT,
XPath or
XML schema. Fragment identifiers do not really work in those
environments.
>2. The AP binding must have sufficient legibility/understandability to
> allow the AP developers to add definitions to the schema that may come
from
> the AAM, ARM, mapping tables and AIM rules.
- One of the reasons that this summary appears simple is that it omits
the description of the accompanying XML Schema. I'm quite confident
that
the XML schema that is devised for this encoding will not be any
'simpler' than the default binding in the current draft (after the
key/keyrefs
are removed).
>3. The AP binding specification must be clear, concise and precise to
support
> easy conformance checking by vendors and end users.
-- As Gerry points out, the proposal omitted the header. Each such
omission
makes the proposal more complicated and diminishes the difference in
complexity
between this and the current default binding.
-- One of most vocal complaints has been with respect to the perceived
complexity
of the configurability capability. (2) proposes a
configurability option for the "Handling of entities". This should not
be allowed
in any "simple" binding. It gives the impression that we are saying
that configurability
is bad in all cases except the configurability option that we like.
-- (3) describes the notion of a path for SELECTs. I do not believe
that
traditional XML developers, such as those that Dave Price deals with,
will
find this concept clear and precise.
>4. The AP binding must be implementable by those who have developed
> P21 parsers without requiring them to purchase or otherwise obtain
> additional tooling for any reason including pre-processing or
> post-processing data described by a configuration.
Why? What is the world-wide population of those who have developed
P21 parsers? Is it over five? The paramount reason for using XML
is to take advantage of the universally-available XML parsing tools.
>5. The AP binding must allow AP implementations to inter-operate when
> those AP's contain harmonized AIM definitions.
Nothing in the proposal addresses interoperability across schemas. This
is accomplished in the XML world by means of XML namespaces. The CD P28
document addresses namespaces. The inclusion
of namespaces in the "simple" binding introduces yet another layer of
complexity.
OTHER RAMIFICATIONS
1 ) The proposal for external mapping (6) violates the major US comment
to support a late binding approach. This proposal is no different
than the approach in the current Part 28. The problem in both cases is
the
impracticality of generating XML schema for EXPRESS schemas that
implicitly
define a huge number of complex entity types. It adds nothing to say,
"There is no
requirement to generate the external mapping names in advance." If
explicit
names are used they must be generated in advance in order to create an
XML schema.
The only alternatives are to generate the XML Schema at runtime after
the
instance population is known OR to loosen the XML Schema so that any
entity elements
are allowed. Of course, the latter means that the XML schema no longer
validates even the
structural constraints on the data and, in fact, validates nothing at
all.
2) The proposal for order independent attributes (8) means that the
resulting
XML schema must be an inheritance-free schema. The Part 28 team has
spent a lot of time
dealing with the fact that if XML schema inheritance is used, then it
is not
possible to support order-independent attributes. The upshot of this is
that
the simple mapping fails to satisfy the requirements of those who want
an inheritance mapping. Most notably, the simple XML schema will not be
useful
to those that plan to use Microsoft tools to generate data objects (eg
AP210 team,
PLCS users, et al.).
3) I keep hearing that the simple binding should be like Part 21 but
once a specific
proposal is made, it is clear that this statement is misleading. Part
21 relies on
the order of attribute values (and their explicit association to an
accompanying EXPRESS
schema) in order to marry values with their attributes. XML is
fundamentally different;
it uses tags in the instance data and does not require ordering.
As Gerry points out
the identifiers in P21 are integers; XML identifiers must start with a
letter.
Unlike Part 21, well-formed XML requires a root element.
The current proposal does not address how the root element is
designated (that would make
the proposal more complicated).
============================================================================================
The following is a proposed:
+++++++++++++++++++++++++++++++++++++++
Specification of the AP Binding
1. Case conventions
Entity names converted to upper case. Attribute
names converted to lower case. Type names
converted to upper case.
2. Handling of entities
Entity is tagged with its name and an id.
Each attribute is tagged with its name.
Each attribute describe by an entity value
has either an href attribute to the entity
or the value is described in place using nesting.
AP developers can choose to define only one
form (href or nesting) in their XML Schema for
a given attribute and then state that the other
form may also be used.
Nested instances may be referenced by other instances.
<PRODUCT_DEFINITION id="pd45" >
<id>PR-3456-Manufacturing</id>
<formation href="#pdf78" />
</PRODUCT_DEFINITION>
<PRODUCT_DEFINITION_FORMATION id="pdf78" >
<id>PR23-67-PDF</id>
<of_product>
<PRODUCT id="p078">
<id>PR2345-76A</id>
<name>machining project</name>
</PRODUCT>
</of_product>
</PRODUCT_DEFINITION_FORMATION>
3. Handling of selects
Primitive types are always tagged with the most
specific type name.
Entity types are tagged with the name of the type
If a path of nested selects is necessary then the
XML tag contains an attribute called path which contains
a white space delimited list of type names as per the
Part 21 specification.
<CIRCLE id="c56" >
<position href="#a2p-102" /> <!-- this is a select -->
</CIRCLE>
<AXIS2_PLACEMENT_3D id="a2p-102">
...
</AXIS2_PLACEMENT_3D>
In the following EXPRESS (from P21)
TYPE Mass = SELECT(Mass_Spec, Mass_Substitute);
END_TYPE;
TYPE Mass_Spec = SELECT(Measured_Mass, Computed_Mass, Estimated_Mass);
END_TYPE;
TYPE Measured_Mass = REAL;
END_TYPE;
TYPE Computed_Mass = Extended_Real;
END_TYPE;
TYPE Estimated_Mass = REAL;
END_TYPE;
TYPE Mass_Substitute = SELECT=(Weight, Estimated_Mass);
END_TYPE;
TYPE Weight = REAL;
END_TYPE;
TYPE Extended_Real = SELECT(FloatingNumber, NotaNumber);
END_TYPE;
TYPE FloatingNUmber = REAL(7);
END_TYPE;
TYPE NotaNumber = ENUMERATION OF (plus_infinity, minus_infinity,
indeterminate, invalid);
END_TYPE;
ENTITY Steel_bar;
bar_length: Extended_Real;
bar_mass: Mass;
END_ENTITY;
#1=SEEL_BAR(FLOATINGNUMBER(77.0), MEASURED_MASS(13.25));
<STEEL_BAR id="id-1" >
<bar_length><FLOATINGNUMBER>77.0</FLOATINGNUMBER></bar_length>
<bar_mass><MEASURED_MASS>13.25</MEASURED_MASS></bar_mass>
</STEEL_BAR>
#2=SEEL_BAR(NOTANUMBER(.INDETERMINATE.)), ESTIMATED_MASS(10.0));
<STEEL_BAR id="id-2" >
<bar_length><NOTANUMBER>indeterminate</NOTANUMBER></bar_length>
<bar_mass><ESTIMATED_MASS>13.25</ESTIMATED_MASS></bar_mass>
</STEEL_BAR>
#3=STEEL_BAR(FLOATINGNUMBER(77.0),
COMPUTED_MASS(FLOATINGNUMBER(14.77719)));
<STEEL_BAR id="id-3" >
<bar_length><FLOATINGNUMBER>77.0</FLOATINGNUMBER></bar_length>
<bar_mass>
<FLOATINGNUMBER path
="COMPUTED_MASS">13.25</FLOATINGNUMBER>
</bar_mass>
</STEEL_BAR>
4. Handling of simple aggregates
The EXPRESS attribute serves as the container for the aggregate. Every
element in the aggregate is represented by an XML element. For those
data types that are not described by entities, the name of the most
specific
type is used as the element name.
<CARTESIAN_POINT id="cp45" >
<coordinates>
<LENGTH_MEASURE>6.78</LENGTH_MEASURE>
<LENGTH_MEASURE>3.2</LENGTH_MEASURE>
<LENGTH_MEASURE>10.32</LENGTH_MEASURE>
</coordinates>
</CARTESIAN_POINT>
<APPLIED_DATE_AND_TIME_ASSIGNMENT id="adta23">
<items>
<DATED_EFFECTIVITY href="#ef-45" />
<APPROVAL_STATUS>
<name>approved</name>
</APPROVAL_STATUS>
<PERSON_AND_ORGANIZATION href="#pno-24" />
</items>
</APPLIED_DATE_AND_TIME_ASSIGNMENT>
5. Handling of nested aggregates
New keyword <AGGREGATE> is used to tag each nest except the
outermost which is tagged by the attribute. Elements in
the aggregate are tagged using their most specific type
name.
<RATIONAL_B_SPLINE_SURFACE id="rbss89" >
<weights_data>
<AGGREGATE>
<REAL>5.6</REAL>
<REAL>6.7</REAL>
</AGGREGATE>
<AGGREGATE>
<REAL>3.2</REAL>
<REAL>4.3</REAL>
</AGGREGATE>
</weights_data>
</RATIONAL_B_SPLINE_SURFACE >
6. External mapping
Tagged using the Part 23 name generation algorithm but
with a "-" replacing the "_and_".
For example, an AND/OR of foo and bar, is foo_and_bar
in Part 23 and is foo-bar in the AP Binding
There is no requirement to generate the external mapping
names in advance.
AP developers can use the generated names to make assertions
about the AND/OR instance in XML Schema at their discretion.
Early bound implementations can choose to generate the names
in advance, or process the type using late bound External mapping
software by recognizing the hyphen.
#28=(LENGTH_UNIT()
NAMED_UNIT(*)
SI_UNIT(.MILLI.,.METRE.)
);
<NAMED_UNIT-SI_UNIT id="id-28" >
<prefix>milli</prefix>
<name>metre</name>
</NAMED_UNIT-SI_UNIT >
7. Null ($ sign equivalent).
The keyword NVL shall be used to indicate a NULL element.
<position><NVL/></position>
<items><REAL>1.5</REAL><NVL/><REAL>2.6</REAL></items>
When an AP developer wants to allow NULL values, the <NVL>
element should be included in the XMLSchema definition.
8. Attribute order.
The attribute order does not matter. If an entity inherits two
attributes with the same name then qualify each name with the
name of the entity that defined that attribute.
Attributes with unrecognized names shall be ignored.
Missing attributes shall be unset.
9. Entity instance order.
The order is unspecified. Unrecognized instances shall be
ignored.
10. href
The form of an href is "document#id"
Document shall be the URL of a file. Id shall be the id of
an entity instance in that file. Document may be omitted
if the instance is in the current file.
The id must be a valid XML NMTOKEN (must begin with a
a letter)
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