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Subject: Re: AW: [sdo] ISSUE 119: Projection and Keys


Binding is a very overloaded term these days.

From the SCA view, right now a binding is effectively a transport protocol and its associated headers.  SCA has also identified the need to be able to describe the conversion between the format of data on the wire (which is strongly influenced by the binding) and the format needed by a component implementation (this is sometime called a data binding).  Independent of all of this are interface definitions and SCA is not addressing the specifics of complex data types, deferring to things like SDO or JAXB.

Having said that, I view the role of SDO as a storage and technology independent way of representing data in memory and as a way for different implementation technologies to access the same data structure.  The project idea we adopted in issue 66 is very consistent with this view since cooperating local services are very likely to be developed at different time and with compatible but independent definitions of the data model. Adding keys to this in a constrained manner makes sense.  The constraint is that like any other projection, it happens explicitly within a service, i.e. a service projects between the type used in the interface and the type used internally.  It makes sense because there are clearly cases where you want to share an object that references other objects, but not share the referenced objects explicitly.  Using the course registration example, the main registration service may want to share course enrollment information with another service, but that other should not have access to full student data - it could access data for a specific student via a request to the DAS (providing proper credentials).  So the main service would project from the full representation to a representation using keys before providing an object over a service interface.

This keeps us away from SDO being used for interface compatibility or implicit mapping over an SCA wire.  Of course if the main service only exposes the fully inflated set of objects, then an ESB transformation would be added to do the explicit mapping using the two separate SDO type definitions.

Bryan Aupperle, Ph.D.
STSM, WebSphere Enterprise Platform Software Solution Architect

Research Triangle Park,  NC
+1 919-254-7508 (T/L 444-7508)
Internet Address: aupperle@us.ibm.com


Blaise Doughan <blaise.doughan@oracle.com>

10/07/2008 11:40 AM

To
"Barack, Ron" <ron.barack@sap.com>
cc
radu.preotiuc-pietro@oracle.com, Frank Budinsky <frankb@ca.ibm.com>, sdo@lists.oasis-open.org
Subject
Re: AW: [sdo] ISSUE 119:  Projection and Keys





Hi Ron, Frank, Radu, and All,

Below are a couple of use cases I have to express my opinions.  

Use Case #1 - SOA Message Compatibility Beyond Identical WSDLs

This is a very compelling use case.  Speaking as someone who represents a SOA binding product (EclipseLink MOXy), this could make service development easier, reducing the amount of transformation necessary to link services.  However I would rather not have this issue addressed as part of the SDO spec.  I would suggest having a separate binding spec.  In this way SDO and JAXB perspectives on the topic could both be represented, instead of trying to give all this responsibility to SDO.   Also Frank mentions that additional mapping functionality is required as soon as different names are used for properties, the addition of a new spec could be expanded to handle these scenarios.  


Use Case #2 - Projection and Keys and Undefined States

Assume in this scenario SDO is being used to deal with a persistent data store such as a large database.  The data model is very connected with no containment relationships and the SDO type model has been designed accordingly.  Working in a SOA I want to respond to queries with messages, that are pruned down versions of the data.  To implement this I could use the "projection and keys" approach.  Wrt projection we have said a data object is in an undefined state until the data is projected back.  Ideally I could carve off many pruned messages to respond to service requests without waiting for the previous request to respond (if it ever does), but it is not clear this can be done (am I allowed to project an object that is already in an undefined state?).  Obviously there are mechanisms that could be put in place such as optimistic locking to ensure that the changes made by multiple projections are compatible, but here I agree with Radu that this aspect of projection and keys feels like a DAS responsibility.

-Blaise


Barack, Ron wrote:
Hi,

Each company of course has its own perspective on what SDO 3 should be, and I hope everyone will speak out at today's meeting.  I just want to point out that it is not a choice between DAS oriented and SOA oriented architectures - these two functionalities are very much complimentary to each other.  In a SOA architecture, the data has to come from somewhere, and there is even more reason that usual to for client not to care about the technology of the datasource:  DAS's fit very well into a SOA.  Conversely, DAS technology becomes even more important when there are more consumers of the data:  SOA expands the client base for DAS's.

We already resolved to support the project method, at the F2F as a concept, and then in the resolution to SDO-66. To me, that means SDO is in the business of moving data between contexts, therefore, we are already in the business of specifying the rules about when this is possible.  All this proposal adds to the picture is the question of whether or not the data must always be the transitive closure, the reachability graph, or whether SDO will allow is to "prune" the graph.  I would argue that such pruning is a necessary part of moving data between object oriented and document oriented technologies.  And that is central to the SOA and the SDO stories.

Guess Obama and McCain won't be the only debate today.

Best Regards,
Ron


-----Ursprüngliche Nachricht-----
Von: Radu Preotiuc-Pietro [mailto:radu.preotiuc-pietro@oracle.com]
Gesendet: Dienstag, 7. Oktober 2008 04:19
An: Barack, Ron; Frank Budinsky; sdo@lists.oasis-open.org
Betreff: RE: AW: AW: [sdo] ISSUE 119: Projection and Keys

Hi Frank, Ron,

First, I can confirm what Frank said about me worrying about the "multiple incompatible views" scenario. If you do have a universe which can be represented conceptually as a set of interconnected Java interfaces and you want to "slice" it into "views", then the solution presented works, even though it feels like DAS-level functionality to generate the data graphs. If your universe is anything else, then the solution doesn't work because it requires extra mapping, which is why I think in last week's call, none of the participant companies indicated that this was a feature that they would actually make use of.

I also have given some thought to Ron's closing comment that passing off a "projected" or live copy of a DataObject to another data service is like a pass-by-reference of data; in fact, the idea of a change summary that gets passed between DAS and client and which records changes is like an "optimistic", loosely coupled pass-by-reference. And if the contract between two DASes is WSDL and Schema-driven, SDO's XML serialization mechanisms (which need to be better specified by the way, to ensure change summary interoperability between vendors) fit perfectly as a conceptual model. Everybody understands that if the two services are co-located, ample optimization is possible, but from a conceptual point of view, SDO already fits the bill.

I agree that access to the same data by different technologies (like POJOs without containment and JAXBs with containment) is an interesting problem, but aren't we actually starting to talk about modifying the data now for different users? That's the slippery slope of going down the route of mapping. The other slippery slope is to move to a "connected" model for SDO where calling a set to an SDO object backed by a JPA object might trigger an update right then and there, before getting the chance to complete the modifications. I think we are taking a bit of risk of promoting SDO for a use-case it might not be well-suited for, thus eroding its credibility, and that's a worry for me. Especially since the proposal does not seem to fit most companies' use cases for SDO, as evidenced by the fact that it doesn't have enthusiastic support from at least one other member. As much as I hate having to disagree with Ron on this issue that he worked so dilligently on and made such good jo
b of presenting, I do disagree that this is a problem that SDO needs to solve.

Thanks,
Radu

 
-----Original Message-----
From: Barack, Ron [mailto:ron.barack@sap.com]
Sent: Monday, October 06, 2008 4:35 PM
To: Frank Budinsky; sdo@lists.oasis-open.org
Subject: AW: AW: [sdo] ISSUE 119: Projection and Keys

Hi Everyone,

The question Frank poses is whether or not the project
operation is simply an optimized copy operation, adding
convenience and perhaps reusing memory, sort of an optimized
CopyHelper.  I could argue that this optimization would be
difficult for someone outside of the SDO implementation to
provide, but I think there's a more important misunderstanding here.

Forget, for a moment, the implementation.  What's most
important here is what we are defining.  SDO projection
*defines* the when 2 types are compatible.  This is a central
problem when building any SOA.  Imagine a service that exposes

        Customer getCustomer();

Under what circumstances does this match a WSDL with XSD

       <complexType name="Customer"/>

Does it match only when JAXB says it would, namely, when the
structure of the Customer object match the containment
structure of the XSD type.  Are we going to impose the
clients containment structure on our service's data model?  
Or are we going to allow a looser definition, one that will
allow the data representations to differ in some ways.
Projection, the definition of projection, is intended to
bring us to the point where SDO type compatibility will give
the SOA world this definition.  

In order for SOA to work, especially in the case of SCA, but
also SOA generally, one of the main points is defining
"compatibility of interfaces".  Right now, standard SOA
declares that the WSDLs have to match (BTW, it's not clear
exactly what this means).  SCA allows different interfaces to
be used on the client and server side of the wire, but the
argument types must match exactly.  This is a major
obsticle...actually, I always felt that data representation
is the first of the SOA problems that needs solving, and that
the SCA groups really needed to address this first, not last,
but that's another rant.  The point is, SDO is the spec I
know of that is looking at data from a SOA perspective.  It's
fallen to us to solve this problem, and in fact, SDO, as a
technology neutral data representation is uniquely positioned
to actually solve it well.

And of course, SDO will not only define projection, but
implementations will provide it.  So SDO becomes an important
part of SOA infrastructure, providing the functionality to
move data between representations.

One of the most important ways that data representations
differ is the containment structure.  I've written somewhere
on this thread that the proposed projection of keys is a
fundamental part of moving data from document to object based
representations.

Obviously, in this functionality, we are not talking about an
arbitrary client calling an arbitrary service, with
absolutely no agreement on how the data is represented.  
That's a problem you can only solve with some kind of mapping
framework.  What we are defining are the cases where mapping
will not be necessary (and avoiding mapping has major
benefits).  Getting two types to match closely enough that
they will be projectable is not something I suppose will
happen by accident.  I imagine either the XSD or the object
model will come first, and one will be derived from the
other.  We can't do magic, and we can't replace a mapping
framework.  We *can* allow all components to use a data
representation that is natural to their technology, and we
*can* provide very efficient communication between them.  And
that's not a small thing.

Best Regards,
Ron



________________________________

Von: Frank Budinsky [mailto:frankb@ca.ibm.com]
Gesendet: Mo 06.10.2008 22:56
An: sdo@lists.oasis-open.org
Betreff: Re: AW: [sdo] ISSUE 119: Projection and Keys



Hi guys,

Given that the discussion of this topic consumed almost all
of the previous call, I was hoping to see some email
discussion. Given the lack of input, let me summarize my own
thoughts and where I think last weeks discussion left off.

One thing that is still not clear to me is how would one
implement Ron's scenario if we didn't allow key projection? I
think the answer is that it would need to be done by the SCA
binding that would manage the keys and do a
mapping/translation step (copying objects). If that's true,
then having
project() do it is really just an optimization (no copy), as
opposed to a critical function (not that I don't personally
think it would be a good thing regardless). The second
question is, how convinced are we that this function will be
usable in a typical scenario. Isn't it possible that the
different "views" will usually be incompatible anyway
(different naming, for example), so some higher level way of
mapping the views (e.g., in an SCA binding) will often be
needed anyway?

I think these are the kinds of things that Radu was also
getting at. I was hoping that Ron could answer these
questions, after which we'd all understand exactly what key
projection is good for and what it isn't good for.

Thanks,
Frank.




Frank Budinsky/Toronto/IBM@IBMCA
09/19/2008 06:21 PM

To
sdo@lists.oasis-open.org
cc

Subject
Re: AW: [sdo] ISSUE 119:  Projection and Keys






Hi guys,

I think I agree with Ron, that it is important to spec out
the behavior when projecting between keys and entities, but I
also think that this may be an easier sell if all SDO
implementations were not required to support it. I think it's
a good example of something that should be an optional
compliance point, since it may involve a lot of work to
handle scenarios that some implementations may not be
interested in supporting.

That said, I think Ron's proposal for how key projection works (if
supported) is good. If an implementation doesn't support key
projection, I

would say that it should throw UnsupportedOperationException (or maybe
InvalidArgumentException) when called with arguments that
have key types, rather than do something that simply ignores
the key metadata. Otherwise, as Ron mentioned, there could be
two incompatible (non error) behaviors that are both
considered compliant.

Frank.




"Barack, Ron" <ron.barack@sap.com>
09/10/2008 04:20 AM

To
<sdo@lists.oasis-open.org>
cc

Subject
AW: [sdo] ISSUE 119:  Projection and Keys






Hi Everyone,

As suggesting in yesterday's meeting, I'm sending out some
mails, refereshing everyone's mind on the state of these discussions.

There was actually some discussion of 119 during the call on
Sept. 2, and maybe it is a good idea  to have the discussion
reflected in an email thread, so I will repeat it here.

There question was (from Blaise on behaf of Radu), why we
need to specify this behavior, why can't it be left as a
vendor extension?  After all, the

resolution to SDO-66 defined strict compatibility rules, but
allowed implementations to weaken them.  Can't the
compatibility between keys and entities be allowed under this
clause, without requiring vendors to implement this behaviour?

I would argue the answer is "no", and I'll give 2 reasons.  
First of all, the proposal includes more than just the rule
that entities are compatible

with their key types, there is a certain amount of behavior
that goes along with it, eg, changes to the values of keys do
not change the value of the key field in the underlying
entity, but change the target of there reference.  It's
difficult to imagine that this behaviour would be
"compatible" to the behavior of an implementation that did
not support projection between keys and entities.

The second reason is that this behavior is necessary for
projection to achieve its primary goal, allowing different
containment structures to be imposed on object oriented
models.  For instance, consider the m:n relationship between
students and courses in the example.  When generating

the XML, we want to produce a document in which Student
contains (a reference to) Course.  The projection works
exactly because of the special

behavior associated with keys.  If Student contained the
entity "Course", then we'd get an illegal graph, because each
"Course" would have multiple containers.

Moving data between object-oriented and document-oriented
applications, for instance, between JEE and BPEL, is perhaps
*the* central problem we are looking at SDO 3 to solve.  
Achieving this goal requires that the proposal (or one like
it) be accepted.  As such, the proposal is too important to
be left as a vendor extension.


Ron


Von: Barack, Ron [mailto:ron.barack@sap.com]
Gesendet: Dienstag, 26. August 2008 11:21
An: sdo@lists.oasis-open.org
Betreff: [sdo] ISSUE 119: Projection and Keys

Hi Everyone,
Moving a discussion from another thread to this thread, for
clarity, and so we can remember the issues:
Blaise asked:
* What is the impact of relaxing the SDO name uniqueness
requirement on projection?
* What is the impact of SDO-82 on projection?
Right now, the projection algorithm matches by name only. It
could be that

we need to make this a little more precise when considering
that property names may not necessarily be unique, and type
names (in the case of XSD anonymous types) may not even be
defined. It's also possible that we can say that projection,
like static SDO generation, has some additional constraints,
such as name uniqueness These are all valid questions to
consider, but what exactly is the relationship with the
proposal being discussed here?  Does having the compatibility
rule regarding keys and entities make the above issues any
worse or better?
* Is SDO-119 useful without an identity concept? Currently my
thinking is that identity management is required here.
There actually is some identity management within the
proposal, the scope in which the identity is meaningful is
the graph being projected.  SDO doesn't have any sense of a
scope that is smaller than the HelperContext yet bigger than
a DataGraph.  Other frameworks, notably JPA and similar
persistence frameworks, do.  For example, in JPA, you have
the users persistence context, in which the scope of the
mapping between objects and

identity can be managed on a per-thread basis.  In SDO,
however, the next larger scope for the identity is the
context, and I think we agree that the context can shared
between threads, at the enterprise application level.  It
could be that we need to add something like a persistence
context to SDO, but I've been trying to avoid this, and
instead to allow the user to inject his own flavor of entity
management by specifying an EntityResolver... please see my
emails to Bryan in this thread, and my
earlier proposals in SDO-125.   The current proposal
regarding keys and
projection is meant to define a default behavior and a
baseline on which
the future EntityResolver functionality can be built.   Would
you be more
willing to resolve this issue if I added the EntityResolver back in?
* Alignment with SDO-135. I believe the spirit of SDO-119 &
projection in general is to enable alignment with other
standards. I believe these issues should be worked on in
parallel, for example the Binder proposal could be enhanced
to unify SDO-135 and projection.
There were some proposals in this direction, which we moved
away from based on your previous wish that the
functionalities be separated.  For the earlier discussion,
please see
http://www.oasis-open.org/apps/org/workgroup/sdo/email/archive
   
s/200808/msg00001.html
 
http://www.oasis-open.org/apps/org/workgroup/sdo/email/archive
   
s/200807/msg00034.html
 
The question where I was hanging is whether or not Binder is
allowed to wrap the POJO or DOM structure, or must copy
involve a copy step.  Up to now, we've always though of
projection as not requiring a copy step.  If we want to bring
the concepts together then we must either say Binder allows
wrapping, or Projection does not allow wrapping.  I'd be willing to

further pursue these proposals, but I need some guidence from
the TC as to

which direction is appropriate.  I actually started work on
this proposal in 119 once I had the impression that we had a
better chance of making progress by separating Binding and
Projection.  Now the opposite opinion is being expressed.  I
usually do not propose something without first prototyping,
so that it is a significant effort to make a proposal.  It is

difficult to make progress without stakes in the ground, such
as this one,

that provide foundations upon which futher proposals can be based.
That said, opinions on wrapping/ copying are more than
welcome (but in the

appropriate thead, please)

Ron


Von: Bryan Aupperle [mailto:aupperle@us.ibm.com]
Gesendet: Freitag, 15. August 2008 17:40
An: sdo@lists.oasis-open.org
Betreff: Re: AW: AW: AW: [sdo] ISSUE 119: Projection and Keys


I would be happy to address it separately since I think it
opens some interesting questions that have DAS implications,  
(Questions like: Does SDO know what DAS is associated any
given type?  Does SDO retrieve entities from the DAS
directly, or is that the responsibility of client code? )

Perhaps the best way to handle it would be to make sure we
open a separate

issue as we resolve this one.

Bryan Aupperle, Ph.D.
STSM, WebSphere Enterprise Platform Software Solution
Architect Master Inventor

Research Triangle Park,  NC
+1 919-254-7508 (T/L 444-7508)
Internet Address: aupperle@us.ibm.com


"Barack, Ron" <ron.barack@sap.com>
08/15/2008 11:05 AM


To
Bryan Aupperle/Raleigh/IBM@IBMUS, <sdo@lists.oasis-open.org> cc

Subject
AW: AW: AW: [sdo] ISSUE 119:  Projection and Keys








Hi Bryan,

In th current proposal, the implementation would create one
entity for each distinct key value.  The entity would have
all default values, except

for the key properties, which would of course match the key.

In earlier versions of the proposal, I defined a second
projection method that took a second argument.  I called this
argument an "EntityResolver"
and the idea was exactly to allow the client (probably in
this case a DAS)

to provide some special logic to manage the reference
objects.  In the SAP

prototype, for instance, we have an implementation that
basically wraps a JPA EntityManager.

This is certainly something I want to address before we go
final, but I don't think we necessarily have to define it as
part of this issue.  It seems to me that the current proposal
is clear enough that it can be accepted as is...we've been
unable to make progress on this proposal for weeks now, and I
thought it might go easier if we added things piece by piece.

One of the things holding me back from making a proposal with
an EntityManager is that even though I think the
functionality is clear and clearly needed, I'm not sure about
how the API should look.  Specifically,

depending on how ISSUE 134 is resolved, we may get some kind
of "Projector" object, which would be a much better place it
parameterize projection...with an entity manager, or anything
else.  I'm not very enthusiastic about having a second
"project" method.

It also sort of disturbed me in our implementation that the
application would get back a graph in which the entities at
the edges were attached, but the real SDO content was still
detached.  It might be better to allow other hooks into the
projection process, eg, one for normal objects, so that the
implementation can call "merge" or something.

It seems you agree we need an EntityResolver, do you need to
see it as part of the resolution of this issue, or do you
think we can solve it seperately?


Ron


Von: Bryan Aupperle [mailto:aupperle@us.ibm.com]
Gesendet: Freitag, 15. August 2008 16:38
An: sdo@lists.oasis-open.org
Betreff: Re: AW: AW: [sdo] ISSUE 119: Projection and Keys


Ron,
I must not have been sufficiently clear about the scenario I
am concerned about.
I understand the case where you start with the context with
entities and project to the context with keys.  The mapping
between entities and keys can be created and maintained.  I
agree there is no problem projecting back tot he context with
entities.
Now consider the opposite case.  Start with an object
containing keys and the mapping between entities and keys has
not yet been created  (perhaps the object was received via a
service call).  Now project to a context with entities.  
Those entities have to be retrieved from the data store.
Or am I missing something here?

Bryan Aupperle, Ph.D.
STSM, WebSphere Enterprise Platform Software Solution Architect

Research Triangle Park,  NC
+1 919-254-7508 (T/L 444-7508)
Internet Address: aupperle@us.ibm.com

"Barack, Ron" <ron.barack@sap.com>
08/15/2008 09:58 AM


To
Bryan Aupperle/Raleigh/IBM@IBMUS, <sdo@lists.oasis-open.org> cc

Subject
AW: AW: [sdo] ISSUE 119:  Projection and Keys










Hi Bryan,

good to know that someone is reading the proposals...

I didn't mean located within the backend store, I meant
within the graph reachable from Oe, called Gk in the text.

Projection maintains identity (one reason why I think it can
be unified with binding).  That is,  if I have an object O in
HelperContext hc1, and I project it to HelperContext hc2, then

O == hc1.project( hc2.project( O ))

What I really what to say in the proposal is that the same
must hold true for keys.  That is, if the O has a property p,
that is a DataObject in hc1, and a key in hc2, then

o.get(p) == (hc1.project( hc2.project ( O ))) .get(p);

Moreover, whereever I set the value of a property in the
transitive closure reachable from hc2.project(O) (Gk in the
text), to the value of hc2.project(O).get(p), then, when I
project back into hc1, I will always get the same instance,
namely O.get(p).

It's actually easy, it's just hard to say it.

Basically, it means that when you project from a context with
entities to a context with keys, the user has implicitly
defined a map from entities to keys based on the objects in
the projection.  This map is not universal... it's based
solely on the graph being transfered.

Did that help or make things worse?

Ron


Von: Bryan Aupperle [mailto:aupperle@us.ibm.com]
Gesendet: Freitag, 15. August 2008 15:25
An: sdo@lists.oasis-open.org
Betreff: Re: AW: [sdo] ISSUE 119: Projection and Keys


This is a tangential question, and perhaps one that I should
already know the answer to, but here goes.  In your example
with context Ce and Ck, if I have object Ok and project it
into Ce, then for each key in Ok the corresponding entity has
to be located.  Since we have agree that entity management is
a DAS responsibility and not an SDO library responsibility,
are we laying any new requirements on DASs, or has the DAS
group already agreed that there has to be a standard API for
returning an entity given a

key?

Bryan Aupperle, Ph.D.
STSM, WebSphere Enterprise Platform Software Solution Architect


Research Triangle Park,  NC
+1 919-254-7508 (T/L 444-7508)
Internet Address: aupperle@us.ibm.com
"Barack, Ron" <ron.barack@sap.com>
08/14/2008 07:20 AM


To
<sdo@lists.oasis-open.org>
cc

Subject
AW: [sdo] ISSUE 119:  Projection and Keys












Hi Everyone,

I've modified the proposal a bit, based on Frank's suggestion
that projection from keys to entities be made more
intelligent.  In fact, I think that the behavior is now
intelligent enough so that a second project

method giving a "restoreContext" is no longer required.  New
text is in blue.

Comments of course are very welcome.

Ron

Use Cases for Keys and Projection
We begin with a motivating use case.
Domain models tend to be larger and complex than the views
used by individual clients. Moreover, when going from a large
server-side model, it is necessary to define the scope of the
data which should be transmitted to a remote client. The
boundaries of the data which should be

transmitted in a single packet must somehow be defined. On
the other hand,

the door must be left open for clients to further explore the
model in succeeding calls to the server.
Imagine a service that returns an Employee object. In the
domain model, Employee has a refence to Department, and
Department in turn has a list of

all Employees. If we define the transmission packet to be the
transient closure reachable from Employee, then we can never
send a single employee,

we will always send the complete list of employees in the department.
Our approach is to use keys to represent the boundary points
in a transmission packet. It is assumed that clients that
wish to explore the model beyond these boundaries will be
able to use the keys to somehow perform lookups, but the API
through which this is done is out-of-scope for SDO (being
more of a DAS issue).
As an example, we can imagine that the server has a SDO type
system in which Employee has a property with type Department.
In the client's type system, however, the corresponding
property has type {commonj.sdo}string.
This value would represent the identity of the Department.
The basic functionality we wish to achive is to allow
projection between these two HelperContexts.
Keys and Projection
A type and its keyType are compatible with each other.  That
mean, an object with a reference to an entity in one context
can be projected onto an object with a reference to the key
type in another context.
We illustrate this with an example.
HelperContext #1

Type Employee
* property - id (Integer) - KEY
* property - name (String)
* property - direct-report (List of Employee) is compatible
with the metamodel HelperContext #2

Type Employee
* property - id (Integer) - KEY
* property - name (String)
* property - direct-report (List of Integer) Here we see the
type of the direct-report property has been replaced in the
second HelperContext with the corresponding key type.
The projection from an entity to its key has some important
semantic differences when compared with projection between
entities.  Projection between entities creates a new view of
the same underlying data.  By contrast, a key does not
represent a view, but rather a reference to an entity in the
underlying data model.  When working with different views of

the same underlying data, it is natural that changes in one
view are reflected in all other views.  By contrast, changing
the value of a key changes the target of the reference rather
than the value of the key property in the referenced data
object.  When projecting from an entity to

a key, a new instance of the key is always created.  By
contrast, when we project from entity to entity, we get the
same number of objects in the target HelperContext that we
had in the source HelperContext.
Conversely, when projecting from a key to an entity then for
each distinct

key value within the graph being projected, all references to
that key must resolve to the same entity.
In many scenarios data will round-trip between contexts,
including between

contexts in which entity map to keys.  Let us consider two
context Ce and Ck, representing the entity and the key
context, respectively, and a DataObject Oe, in context Ce.  
Projecting Oe into Ck returns a DataObject,

Ok.  The transitive closure reachable from Ok is Gk.  Every
key value in Gk maps to an single entity in Ce, and it is
this entity is that is found when Ok (and effective, all of
Gk) is projected back into Ce.  In cases where the user has
set a key property to a value that is not found in Gk, then,
as a result of projecting Ok into Ce, a new entity will be created.
The created entity has default values for all properties
other than the key fields.
We illustrate with an example.  Note that the example uses
containment relationships in both contexts.  This is done for
clarity, since it allows

an XML representation of the data.  The use-case, however, is
stronger when the contexts have different (or perhaps no)
containment structures.
Imagine the following data in HelperContext #1 <employee id="11">
    <name>Foo Bar</name>
    <direct-report id="21">
            <name>Jane Doe</name>
    </direct-report>
    <direct-report id="31">
            <name>Jim Jones</name>
    </direct-report>
    <direct-report id="22">
            <name>John Smith</name>
    </direct-report>
</employee>
After projecting to HelperContext #2, we have the following
data <employee id="11">
    <name>Foo Bar</name
    <direct-report>21</direct-report>
    <direct-report>31</direct-report>
    <direct-report>22</direct-report>
</employee>
If we imagine the client changes the list of direct reports,
so that the second item in the list has value ?41? instead of
?31?, then the meaning of the change is not that the employee
with name ?Jim Jones? now has a new

ID, but that ?Jim Jones? has been replaced by another employee.
On projecting from context 2 back into context 1, the
instance of Employee

with id="22" would be detached from the Employee with
id="11".  Also a new

Employee with id="41" would be created and added as a direct
report to the

Employee with id="11".  If the graph in context 1 were in
scope of a ChangeSummary, then because the "direct-report"
property in context1 is a containment property, the change
would be tracked as a delete or a create.

If the relationship were non-containment, the change would
be tracked as a modification to the employee with id=11, not
as a delete to the employee

with id=22 or as the creation of a new employee with id=41;
semantically, it is only the reference to entity 22 that has
changed, the entity itself
still exists, and is unaltered.   Projecting this data from context 2:
<employee id="11">
    <name>Foo Bar</name
    <direct-report>21</direct-report>
    <direct-report>41</direct-report>
    <direct-report>22</direct-report>
</employee>
Into context 1 yields:
<employee id="11">
    <name>Foo Bar</name>
    <direct-report id="21">
            <name>Jane Doe</name>
    </direct-report>
    <direct-report id="41"/>
    <direct-report id="22">
            <name>John Smith</name>
    </direct-report>
</employee>
Projection between entities and keys becomes more powerful
when the relationship through which the entities tie into the
graph are non-containment.  The following example shows how a
complex model that lacks containment relationships can be
projected onto a context that requires a specific XML
serialization, which implicitly prunes the orignal

domain to the requirements of a specific client.
HelperContext #1

Type School
* property - name (String) - KEY
* property - students (Student) - many=true
* property - courses (Course) - many=true  Type Student
* property - name (String) - KEY
* property - courses (Course) - many=true, containment=false,
opposite=students
* property - school (School) - containment=false,
opposite=students Type Course
* property - name (String) - KEY
* property - students (Students) - many=true,
containment=false, opposite=courses
* property - school (School) - containment=false,
opposite=courses Notice the m:n relationship between Student
and Course.  If we imagine a service that should expose this
domain model to clients, it is possible that some clients
will wish to obtain the list of students participating in a
particular course, while other clients may wish to obtain the list of

courses in which a particular student is enrolled.  The
application cannot

determine based on the structure of the data which of the two
possible containment structures is "correct".  Notice also
that returning the transitive closure would return all the
data associated with the entire school.
In this example the client wants the data structured
according to the following XSD <?xml version="1.0"
encoding="UTF-8"?> <schema
xmlns="http://www.w3.org/2001/XMLSchema" targetNamespace=
"http://projection <http://projection/> "
xmlns:tns="http://projection <http://projection/> "
elementFormDefault="qualified">
 <complexType name="School">
       <sequence>
           <element name="students" type="tns:Student"
maxOccurs="unbounded"/>
       </sequence>
       <attribute name="name" type="string"/>
 </complexType>
 <complexType name="Student">
       <sequence>
             <element name="courses" type="string"
maxOccurs="unbounded"/>
       </sequence>
       <attribute name="name" type="string"/>
 </complexType>

 <element name="school" type="tns:School"/> </schema> Notice
that we are imposing a containment structure on the original
context, as well as pruning it by replacing the course entity
by the corresponding key.  The following code illustrates the
behavior of the project method.
  DataObject cal =
_helperContext.getDataFactory().create(School.class);
// Create the SDO graph
cal.set("name","Berkeley");
DataObject billy = cal.createDataObject("students");  
billy.set("name", "Billy Bear");  DataObject bob =
cal.createDataObject("students");  bob.set("name", "Bobbie
Bear");  DataObject basketWeaving =
cal.createDataObject("courses");  basketWeaving.set("name",
"Basket Weaving");  DataObject algol =
cal.createDataObject("courses");  algol.set("name", "Algol");
DataObject revolution = cal.createDataObject("courses");  
revolution.set("name", "Revolution");  // hook things up  
billy.getList("courses").add(basketWeaving);
billy.getList("courses").add(algol);
bob.getList("courses").add(basketWeaving);
bob.getList("courses").add(revolution);
// Create a second context defined by an XSD  HelperContext
hc2 = HelperProvider.getNewContext();

hc2.getXSDHelper().define(getClass().getClassLoader().getResou
rceAsStream("com/sap/sdo/testcase/internal/pojo/ex/projection.xsd"),

null);
// Project from the java context to the XSD context  
DataObject projection = hc2.getDataFactory().project(cal);
// Produce XML based on the XSD
String xml = hc2.getXMLHelper().save(projection,
"http://projection <http://projection/> ", "school");  // I'm
imagining here that sending the XML out over the wire (eg,
using it as a response to a  // WebService request. On the
client side, we go from the XML back to SDO. We use the
context  // based on the XSD.
DataObject projection2 =
hc2.getXMLHelper().load(xml).getRootObject();
// We can make some changes. We can add a new course...
projection2.getList("students.0/courses").add("Fortran and
You");  // So, now the trip back to the server?
//I'm skipping the XML step, and simply projecting the modified (XML
oriented) data back into
// my java context
DataObject cal2 =
_helperContext.getDataFactory().project(projection2);

// Test that there is one entity per key value  DataObject
billy2 = (DataObject)cal2.getList("students").get(0);
DataObject basketWeavingBill = (DataObject)billy2.getList("courses")
                                              .get(0);  
DataObject bob2 = (DataObject)cal2.getList("students").get(1);
DataObject basketWeavingBob =
(DataObject)bob2.getList("courses").get(0);


assertSame(basketWeaving2, basketWeavingBob);



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