[OJB-developers] access paths and "object graph sectioning"

[Mike H. <me...@mi...>]

Hi,

I have just posted this to jdocentral.  Please forgive the cross posting.

I got quite excited about JDO because I figured it would allow me persist
data on the serverside without the muckiness of Entity Beans.  I also figure
JDO objects could act as my value objects - that I could return them to the
presentation layer from my session beans.  I see that there are arguments
for and against that idea on this site and others.  But lets leave that
argument aside and assume I want to return my JDO instances as value objects
from session beans.

The problem is of course that if I return the head of a graph of objects,
the whole graph will get serialized back to the client.  The only way around
that right now that I can see if judicious placement of nulls to cut the
graph.

But it seems to me JDO is well positioned to do this for me.  Lets assume
the JDO runtime loads data for an object only when the reference to that
object is navigated (the root object is a special case).  This will allow me
surf my object graph to my hearts content in my session bean, loading data
as I go.

When I have done the surfing I want to do, I currently have to insert nulls
to cut a section out of the object graph so I can serialize it back to the
client.  Would it not be possible to tell the persistence manager NOT to
load data for any further references that are navigated, but to return null
instead.  Then I can simply do persistenceManager.cut(), and serialize a
section of the graph back to the client - java serialization will be bound
to the area of the graph I surfed.

Now, this might very well remove the opportunity to do preloading of data
for performance reasons.  But it would be really cool if these access paths
were implemented in 2.0.  Then I could load an object from the db and tell
the PM what the expected access path is, with one access path per use case.
This will allow me load data in a performant way while still sectioning the
graph.

One extra nice extension would be a means to indicate what access paths
follow a given access path.  For example, if I am executing the "login" use
case, I would tell the PM to work with the "login" access path.  But I could
also inform it (at deployment time probably) that after the "login" use
case, the "order item" use case or the "logout" use case can follow.  To the
JDO runtime, this will mean the "order item" or the "logout" access path
will follow, and it can preload both if it chooses, or, depending on their
relative frequence, load one of them.

Cheers,
Mike