AW: [Objectbridge-developers] Querying databases (including local transactions changes)

[Mahler T. <tho...@it...>]

Hi Geir Ove,


Hello all!

I really haven't started using ObjectBridge yet, but I have some
experience already using TopLink and Castor JDO.

I've been able to successfully use TopLink in the project
doing. Unfortunately TopLink is extremely expensive and the licensing
is not really very flexible for the use I'm interested in. Secondly
being able to peek at the source code whenever I'm stuck is
_extremely_ useful to say the least.

So, I tried switching to Castor JDO, which I have the impression is
quite a nice framework. From what I've seen so far - it
is. Unfortunately my project rely heavily on querying the object model
during transactions (before commiting), so the query results must be
conformed with the local transaction changes somehow.

I'm not quite sure if I get you mean by "conformed with". Do you mean the
following? say you have a class Articles with an attribute price. You
perform a query with price < 100. Then you operate on the found articles and
for some articles you set the price to 250. Then you perform the query again
(within the same tx) and want to have the objects with price set to 250 NOT
included in the query result? This is a thing OJB can't do as all queries
are translated to SQL and go to the database (and do not work on in-memory
objects).

Or do you mean the following: during a transaction you change an article A
to A' and then perform (within same tx) a query that returns also article A.
You want of course to see your modified article A' and not the original A.
This works with OJB due to its caching algorithms.



TopLink has a feature called conformInUnitOfWork, which lets you do
this. There were some missing parts, but I where able to tweak
everything to make this work. TopLink doesn't actually modify the
database until commit at all (except for sequence counters that
is). So TopLink was up to the task.

In OJB there are also NO database write until a commit ist performed.
(except for sequence counters...) 



Then I tried Castor which actually modifies the database whenever
objects are created (inserts a row with the default values), and on
commit (of course), but not when they are modified. Castor does not,
like TopLink, have support for conforming query results in-memory.

It does also not have support for something like a savepoint where the
changes up till the current point in the transaction is actually
stored in the database without the transaction being commited. This
would allow the query results to be conformed automatically by the
database.


The OJB ODMG implementation supports checkpoints. Currently they have the
same semantics as commits. :-( But this could easily be changed.

HTH

Thomas



I guess savepoint would also be the most efficient way of supporting
such a feature - at least if there were support to actually only store
modifications that are relevant to the query you've about to
execute. In TopLink this is often not very efficient, since it is very
hard upfront knowing how to store the changes to efficiently sync the
database result with the inmemory-result.

Does ObjectBridge have support for conforming queries either in-memory
or in the database via savepoints?

All the best,
Geir O.

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