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

sdo message

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?

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

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

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.

HelperContext #1

Type Employee
* property - id (Integer) - KEY
* property - name (String)
* property - direct-report (List of Employee)

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);