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Subject: Re: [sdo] Re: Comments on Blaises Proposal
Hi Guys,
I like the proposed mapping to XSD for models that don't have containment
references. However, that said, I really don't understand what change is
being proposed for containment here. I think I need a better answer to
Ron's question from below:
[SAP1] How does this differ from the UML concept of aggregation.
[Blaise] I guess this depends on your definition of aggregation in UML.
The following is the important concept:
Containment: employeeDO1.get("residence") ==
employeeDO2.get("residence"); // This can never be true
Non-Containment: employeeDO1.get("residence") ==
employeeDO2.get("residence"); // This can be true
[Frank] My ("the") definition of UML aggregation is "a has-a (whole/part)
relationship". Aggregations can either be shared (i.e., a part can be used
in more than one aggregations) or exclusive (also known as by-value
aggregation or composition). SDO containment is the latter (exclusive
aggregation); a part can be used in only one aggregation/container - which
is returned by the getContainer() method. Blaise's definition, above,
seems to be the same thing. If so, what exactly is the proposed
containment change? The only thing that seems different (and very strange,
in fact) is the example of an association where both ends have
containment=true. Allowing this seems to be fundamentally at odds with the
concept - i.e., a part cannot contain its whole. How would we know which
is the whole and which is the part, given both can be either according to
the metadata? Sounds like you want some new kind of bidirectional
relationship that says one end or the other is containment, but the
metadata won't say which end. Is that it? If so, how would the
DataObject.getContainer() method work?
Thanks, Frank.
"Blaise Doughan" <blaise.doughan@oracle.com>
04/04/2008 11:14 AM
To
<sdo@lists.oasis-open.org>
cc
Subject
[sdo] Re: Comments on Blaises Proposal
Hello All,
I think this is going to turn out to be a very good discussion on
containment :),
There are some points in the proposal I would like to draw attention to:
1. Full Backwards Compatibility
A number of TC members have made strong cases for backwards compatibility
and my proposal supports this. A user defining SDO 2.1 compatible
metadata will not see any change in behaviour wrt my proposed changes.
2. Relaxed Containment Requirements
The proposal allows for a data graph to have no containment relationships.
This reflects the case where SDO metadata is derived from unannotated
Java classes/interfaces. Since XML handling is a core part of the spec
(XMLHelper, XSDHelper, DataObject serialization), the proposal provides an
algorithm to keep these concepts valid. Ron has pointed out that the
algorithm can fail, but it fails in the same way as all other XML binding
tools. The alternative is to not allow the XML serialization of
DataObjects without all the necessary containment which is difficult to
determine and is unnecessarily restrictive.
3. DAS
The proposal contains an example of deriving SDO metadata from a
relational database. XML schemas are then derived wrt different SDO
types. I am excited about the idea that the data source metadata could be
defined once, and then per type XML messages (one for findAddress, another
for findEmployee) could be used each of which reflects the core metadata.
Below are my responses to the points raised by Ron's email:
[Ron] Much as I like aspects of this proposal, there are several
fundamental problems with it. In particular, Blaise's claim that "makes
use of containment properties when they are present and handles things
when they are not" is false. The fundemtental problem is that what Blaise
calls "orphans" can not be determined by analysing the metadata, you have
to look at the actual instances. Using the proposed algorithm, it is
still possible to generate documents with unresolved references.
[Blaise] I do not believe the claim is false. Solving the object-to-XML
impedance mismatch requires that each object be reachable by containment,
the onus is always on the user to ensure enough containment relationships
are in place (this is true in both SDO 2.1 and JAXB 2.0). The algorithm
could prevent all broken references by allowing remaining orphan instances
to be adopted by the complex type corresponding to the root type. The
algorithm borrows from the "technical root" originally proposed by Xcalia,
but instead of requiring an additional object, the algorithm gives a place
for non-contained data to go.
[Ron] I want to point out that this, or any other proposal regarding
containment within a context is somewhat orthogonal to the issue 66. Issue
66 solves the more general problem of moving data between contexts in
which the definition of the types varies slightly. Both containment and
instance class are our first targets, but certainly we believe this to be
a a way in which other problems, such as versioning, can be addressed.
[Ron] Nonetheless, let's compare, at a very high level, the approach to
containment of the two approaches.
[Ron] One of the problems we've got when dealing with different
application using different sources of metadata is that the definition
have to sync-up. In standard SDO 2.1, metadata comes basically from XSD.
The standard defines how to convert this to Java, and it's expected that
the users of the java interfaces take what they get.
[Ron]Blaise's proposal at least allows a second route. It allows the data
to be defined from Java, and any XML clients have to take what they get.
[Blaise] My proposal involves a reinterpretation of what containment
means. In SDO 2.1 containment represented a nested relationship between
DataObjects. A DataObject of Type A can be nested within a DataObject of
Type B in the resulting XML if an instance of Type A cannot be shared
between instances of Type B. Containment in SDO 3.0 should not simply
represent nesting, but the underlying concept that enables nesting. This
does not simply allow a "second route", but makes it easier to derive an
SDO metadata from multiple sources. It is more useful for containment to
model the data sharing rules from a relational database or JPA entities,
then to be an artificial mapping of these sources to an XML schema.
[Ron]That we've tried to achieve with Issue 66 is to "meet-in-the-middle".
That is, for instance, some aspects of the model can be defined in Java,
the things that effect XML can be defined in XSD. Both sides can be kept
a little happy.
[Blaise] The issue 66 proposal works well when the SDO model being
projected has no containment relationships and is projected into a context
with containment relationships. This is a well proven approach
demonstrated by technologies such as JAXB and JPA. It becomes more
difficult when projecting a model with containment to a model with
containment.
[Ron]In other words, the project method allows the XML structure to be
added in after the Properties are already defined, without touching the
application that is defining the properties. None the less, the XML
client has a large degree of flexibility in defining the document he
wants.
[Blaise] A common TopLink/EclipseLink use case is the following. A
programmer is creating an application to access relational data and expose
it as a web service (XML). They create an object model and apply JPA
metadata in the form of annotations to map the objects to the database.
Now in order to map these same objects to XML the user needs to apply JAXB
annotations. When I do this I always look at the JPA annotations to find
out the data sharing rules so I can apply containment/nesting
relationships. There are N ways to map an object model to an XML schema,
when I take into account the data sharing rules from the JPA annotations
there are always <N ways to map the annotated object model to an XML
schema. If SDO metadata evolves to represent real relationships between
data it can ease the transition from one form to another.
[Ron]Please find my detailed comments to Blaise's proposal in the attached
document.
Best Regards,
Ron
Responses to document comments:
[SAP1] How does this differ from the UML concept of aggregation.
[Blaise] I guess this depends on your definition of aggregation in UML.
The following is the important concept:
Containment: employeeDO1.get("residence") ==
employeeDO2.get("residence"); // This can never be true
Non-Containment: employeeDO1.get("residence") ==
employeeDO2.get("residence"); // This can be true
[SAP2] Address has ?eId?, is that because there is a 1:1 relationship
between employee and address. But if that?s the case, I wouldn?t expect
an SDO representation to give them both keys, the reference to the ?main?
object should be enough?
[Blaise] The domain model used in this example was derived from a
relational database so I brought in all the primary keys. If Address is
never used as a root type, and is only referenced through containment
relationships then it would not need an ID (to satisfy SDO).
[SAP3[You have a bi-directional relationship, that is containment in both
directions. In 2.1 this is not allowed (since it would result in an
endless loop when serializing). How would we ever find the ?root? of a
datagraph? Would we have to check for cycles?
[Blaise] When generating an XML schema if the value of containment
property is being converted then the property representing the back
pointer to the parent must not generate an XML element. Instead the XML
schema should be annotated to indicate that this relationship exists.
[SAP4]Wouldn?t it be possible to take the document structure you define
later and apply it here? What would be the effect? Would this be
equivalent to the transitive closure? I?m asking because when we?re
coming from unannotated classes, there is default no containment, and
therefore no meaningful change summary. It would be good if we could
apply your algorithm to solve this problem.
[Blaise]If the scope of ChangeSummary was changed from containment to the
datagraph itself then the document structure in the document would apply.
[SAP5]I like this. But it?s rather besides the point for the containment
proposal, isn?t it. Unless you require that all non-containment
properties have keys, which you?re not doing, right?
[Blaise] It is not the non-containment properties that keys but their
values. This is particulary useful for SDO Types with composite keys.
[SAP6]It?s interesting that you have non-containment as the default case,
exactly the opposite of what JAXB does. How does this proposal relate to
the topic of JAXB alignment. Are we defining new annotations?
[Blaise] For the purpose of this document I kept the annotations strongly
linked to the Property metadata. Since Property had a property called
"containment" I introduced an annotation called "Containment". The
specifics of the annotations can be worked out as part of a seperate
discussion.
[SAP7]In your definition of containment, Address is exclusively owned by
employee, right?
[Blaise] Yes see my response to [SAP2].
[SAP8]In your example the traversal path is always clear, but I don?t
believe this is generally true. Especially if we are coming from Java
interfaces, where the order of the properties is undefined.
[Blaise] The order of properties is unrelated to the traversal path.
[SAP9]I believe there is a conceptual problem here. It is not Types that
are ?orphaned?, but objects. A type could still have some containment
relationship to another type, but instances of that type used someplace
else, in a non-containment relationship.
[Blaise] In the SDO Type to XML Schema algorithm containment relationships
in SDO translate to nesting relationships in XML schema. As such types
can definitely be orphaned, the solution to this is to have the complex
type corresponding to the root type "adopt them" although this
relationship is represented in the XML schema it is never realized as a
property on the root type. Instances can also be orphaned, the algorithm
makes the same assumption as SDO 2.1 and JAXB that all the necessary
containment relationships are in place, if this is not sufficient then the
root complex type will need to be adapted to always expect orphans.
[SAP10]I?m trying to imagine your use-case. Is it something like this?
You have something like a DAS with methods like getAddress(),
getEmployee(), etc. And even though the SDO representation of the results
are the same, you want to have a different schemas for each result type.
Is that correct? This is an extremely cool idea!
[Blaise] That is the exact use case we have in mind. The core data model
is derived from the data source, then data is requested based on a
particular SDO type. The XML representation of this data should have the
queried type as the root type.
[SAP11]Traversal path is not sufficiently defined. It seems to mean, if
I?ve already seen the type, I don?t generate the element. Is this right?
In your examples, every time a reference is thrown out based on this rule
it happens to be the back-pointer to a containment relation. I agree that
in this case, we can throw the data out of the XML. However, this does
not hold in general.
[Blaise] If the SDO type has already been seen and an XML complex type has
already been generated then don't generate another XML complex type for
it. When converting properties to elements then if the property is
containment create a nested element, otherwise create an element
representing a FK.
Imagine a node that has a containment relationship to itself (say,
Person.children). That?s of course, a trivial example, but you could have
more complex models, where this would be impossible to analyse.
[Blaise]Not sure if the answer to the above point addresses this, but a
SDO type can have a containment property of the same type as the owning
type.
I think this algorithm makes the fundamental mistake of thinking that
containment is a feature of the metadata, where it?s really a part of the
data.
[Blaise] Can you comment further on containment being part of the data?
See also the following comments.
[SAP12]Lets imagine adding ?Department? to this model. Department has a
relationship to Employee, and has a non-containement relationship to
address.
The type ?Address? won?t be an orphan, but the XML would contain
unresolved references.
[Blaise] See my response to [SAP13]
Maybe there should be a property ?orphans? that has type ?Object?, rather
than a set of elements?
[Blaise] During the marshalling process "orphans" can be determined, there
is no need to explicity track them as a property value. Also the orphans
change depending on the type of the root object.
[SAP13]Same problem. In a general case, the list of employees that is
property of this object might not be the complete list that you want to
have in your orphans. In such cases, this is broken.
[Blaise] This algorithm does make the assumption that if there are
containment relationships defined for a SDO type then they will hold all
the instances that will be referenced by non-containment relationships.
This is assumption is also made by SDO 2.1 and JAXB 2.0. This assumption
could be removed by adding elements to the root complex type to hold onto
all orphaned instances.
[SAP14]How do we know when to apply this algorithm, and when to apply the
algorithm specified in SDO 2.1? I think there might be backwards
compatibility issues here. Maybe we need a flag to determine this.
[Blaise] I do not see the algorithm specified here as being a second
algorithm. Instead it is an ammendment to the 2.1 algorithm. When
converting SDO 2.1 compliant metadata to XML schema the algorithm will
produce the same XML schema. Of course the algorithm also supports the
proposed metadata.
-Blaise
----- Original Message -----
From: "Barack, Ron" <ron.barack@sap.com>
To: <blaise.doughan@oracle.com>; <sdo@lists.oasis-open.org>
Sent: Thursday, April 03, 2008 6:22 PM
Subject: Comments on Blaises Proposal
Hi Everyone,
Much as I like aspects of this proposal, there are several fundamental
problems with it. In particular, Blaise's claim that "makes use of
containment properties when they are present and handles things when they
are not" is false. The fundemtental problem is that what Blaise calls
"orphans" can not be determined by analysing the metadata, you have to
look at the actual instances. Using the proposed algorithm, it is still
possible to generate documents with unresolved references.
I want to point out that this, or any other proposal regarding containment
within a context is somewhat orthogonal to the issue 66. Issue 66 solves
the more general problem of moving data between contexts in which the
definition of the types varies slightly. Both containment and instance
class are our first targets, but certainly we believe this to be a a way
in which other problems, such as versioning, can be addressed.
Nonetheless, let's compare, at a very high level, the approach to
containment of the two approaches.
One of the problems we've got when dealing with different application
using different sources of metadata is that the definition have to
sync-up. In standard SDO 2.1, metadata comes basically from XSD. The
standard defines how to convert this to Java, and it's expected that the
users of the java interfaces take what they get.
Blaise's proposal at least allows a second route. It allows the data to
be defined from Java, and any XML clients have to take what they get.
That we've tried to achieve with Issue 66 is to "meet-in-the-middle". That
is, for instance, some aspects of the model can be defined in Java, the
things that effect XML can be defined in XSD. Both sides can be kept a
little happy.
In other words, the project method allows the XML structure to be added in
after the Properties are already defined, without touching the application
that is defining the properties. None the less, the XML client has a
large degree of flexibility in defining the document he wants.
Please find my detailed comments to Blaise's proposal in the attached
document.
Best Regards,
Ron
-----Ursprüngliche Nachricht-----
Von: blaise.doughan@oracle.com [mailto:blaise.doughan@oracle.com]
Gesendet: Donnerstag, 3. April 2008 22:05
An: sdo@lists.oasis-open.org
Betreff: [sdo] Groups - Proposal - Containment and Enterprise Data Models
(SDO-EnterpriseDataModel.doc) uploaded
This is the containment proposal I mentioned during the conference call on
April 1, 2008.
-- Mr. Blaise Doughan
The document named Proposal - Containment and Enterprise Data Models
(SDO-EnterpriseDataModel.doc) has been submitted by Mr. Blaise Doughan to
the OASIS Service Data Objects (SDO) TC document repository.
Document Description:
The SDO spec to date has primarily concerned itself with deriving SDO
metadata from XML schema. As such containment has come to represent the
concept of nesting as it relates to XML elements. We prefer to think of
containment as a type of "privately owned" concept. For the association
"residence" between types "Employee" and "Address" if instances of
"Employee" may not share references to instances of "Address" then it is a
containment relationship:
Containment: employeeDO1.get("residence") ==
employeeDO2.get("residence");
// This can never be true
Non-Containment: employeeDO1.get("residence") ==
employeeDO2.get("residence"); // This can be true
Using the above interpretation of containment it becomes easy to derive
SDO
metadata from other sources, such as JPA entities, JAXB objects,
relational
databases, etc. (the doc provides an example of deriving SDO metadata from
a relational database). These sources may not have a concept of nesting,
but they are aware of data sharing rules. Of course DataObjects require
an
XML representation, and containment has been an important part of that.
The attached proposal contains an algorithm that makes use of containment
properties when they are present and handles things when they are not.
View Document Details:
http://www.oasis-open.org/apps/org/workgroup/sdo/document.php?document_id=27848
Download Document:
http://www.oasis-open.org/apps/org/workgroup/sdo/download.php/27848/SDO-EnterpriseDataModel.doc
PLEASE NOTE: If the above links do not work for you, your email
application
may be breaking the link into two pieces. You may be able to copy and
paste
the entire link address into the address field of your web browser.
-OASIS Open Administration
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