RE: [OJB-developers] Clarification of Persistence Broker Storage

[Matthew B. <ma...@so...>]

Great find!

Post the test case to the list and I'll see what I can do, and put the test
case in the suite to make sure we don't regress. There are not near enough
proxy tests in there yet.

Thanks again for the contribution.

regards,
Matthew 

-----Original Message-----
From: Morgan Troke
To: obj...@li...
Sent: 6/12/02 11:03 AM
Subject: [OJB-developers] Clarification of Persistence Broker Storage

Hi,

    I have a few questions about the way in which the Persistence Broker
stores objects that I was hoping could be cleared up.

    First, when storing an object which contains a collection (a 1:N
mapping), it appears that even when proxies are used for the elements in
the
collection, when the object is stored each of the elements in the
collection
are materialized from the database. For example, within
test.ojb.broker.PersistenceBrokerTest I have added the following test
which
retrieves a test.ojb.broker.ProductGroup and stores it again (all XML
and
.properties files have been left unmodified from the 0.9 release):

public void testRetrieval()
{
    try
    {
       ProductGroup pg = new ProductGroup();
       pg.setId(8);
       Identity pgOID = new Identity(pg);

       pg = (ProductGroup) broker.getObjectByIdentity(pgOID);
       broker.store(pg);

    catch (Throwable t)
    {
        fail(t.getMessage());
    }
}

 When analyzing the SQL statements which are generated, it appears that
all
of the elements in the ProductGroup's collection are materialized when
the
broker.store(pg) line is executed. I traced through the code and tracked
it
to the calls to the logger, such as the one in store(Object obj) within
ojb.broker.singlevm.PersistenceBrokerImpl.java:

     logger.debug("store " + obj);

which calls the toString() method on the object being stored, in this
case
the ProductGroup, which returns the String values of all its fields. But
since allArticlesInGroup is a Vector, this in turn calls toString() on
each
of the proxy elements in the collection, which then materializes all of
the
elements. This means that a potentially large number of calls to the DB
are
being made when objects with a collection are being stored. Is this
behaviour intended, and if so is there anyway to modify it (perhaps in
the
XML files?) such that the calls to the DB are minimized?


    Second, when storing objects (again with PersistenceBroker), it
appears
that two calls to the DB are made - one to retrieve the object, and one
to
update it - regardless of whether or not the object has been modified.
This
has particular implications again with objects that contain collections.
Using ProductGroup again as an example, when the elements of the
collection
have already been materialized (eg. by iterating over and calling a
method
on each of them), and the ProductGroup is stored, each of the elements
in
the collection is materialized (even though they already have been), and
then updated. This results in a large number of calls to the DB being
made
when the ProductGroup is stored, even if nothing has been changed.


    In both of the above cases, have I used PersistenceBroker to
retrieve
and store objects correctly, as well as interpreted the calls made to
the DB
correctly? If so, is this behaviour intended and is there anyway to
change
it to minimize the number of calls made?

    Thanks in advace for any help,

    Morgan






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