[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]
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/archives/200808/msg00001.html
http://www.oasis-open.org/apps/org/workgroup/sdo/email/archives/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"; xmlns:tns="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().getResourceAsStream("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";,
"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);
[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]