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

Title: 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.

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.

is compatible with the metamodel

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 id="31">
                <name>Jim Jones</name>
        <direct-report id="22">
                <name>John Smith</name>

After projecting to HelperContext #2, we have the following data
<employee id="11">
        <name>Foo Bar</name

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

Into context 1 yields:
<employee id="11">
        <name>Foo Bar</name>
        <direct-report id="21">
                <name>Jane Doe</name>
        <direct-report id="41"/>
        <direct-report id="22">
                <name>John Smith</name>

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">


              <element name="students" type="tns:Student" maxOccurs="unbounded"/>


          <attribute name="name" type="string"/>


    <complexType name="Student">


                <element name="courses" type="string" maxOccurs="unbounded"/>


          <attribute name="name" type="string"/>



    <element name="school" type="tns:School"/>


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


   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





   // 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")


   DataObject bob2 = (DataObject)cal2.getList("students").get(1);

   DataObject basketWeavingBob = (DataObject)bob2.getList("courses").get(0);

   assertSame(basketWeaving2, basketWeavingBob);


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