Hi Craig,
replication really is lost in a specification gap: makePersistent() on
transient instances won't update existing data, and on detached
instances it won't insert new. For replication, you need both behaviours
at the same time.
That's really misfortunate for such a nice feature! Even more so as it
is not just theory, but it proves to be working in production with JPOX'
old implementation of attachCopy().
Regards,
Jörg
Craig L Russell schrieb:
Hi Jörg,
Using detachment for replication is an interesting use case, and I'd
like to see more in-depth analysis of the issues that you encounter
once you've done with it.
The use-case for detachment is long-running optimistic transactions,
as you have noted below. We did add makeTransient(Object,
useFetchPlan) as a way to disconnect objects from one datastore that
could be used with another, but I really doubt that we are going to be
able to incorporate into the JDO API all the policy algorithms needed
by a general-purpose replication scheme.
Craig
On Mar 9, 2006, at 9:48 AM, Jörg von Frantzius wrote:
Craig L Russell schrieb:
Hi Jörg,
There are no tests planned for this behavior.
That's good ;-)
The issue is that it violates the contract of detachment. Detachment
is intended to provide a "long-running optimistic transaction" in
which conflicts are detected in a subsequent transaction.
I'd find it a little sad if a great feature like easy replication was
sacrificed in favor of that. Unless replication should be reserved
for JPOX (using a vendor extension), then maybe a future version of
the spec could have something along the lines of the solution
described by Marco in http://www.jpox.org/servlet/jira/browse/CORE-2741
That would be great.
Just for completeness, and maybe it's just me, but the only sentence
about detaching in general that I could find is
"These methods provide a way for an application to identify
persistent instances, obtain
copies of these persistent instances, modify the detached instances
either in the same JVM
or in a different JVM, apply the changes to the same or different
PersistenceManager,
and commit the changes."
It's not really talking about an equivalent to long-running
optimistic transactions, I find.
If an instance is detached and then the underlying datastore
instance is deleted, this is a consistency violation that should be
detected by the transaction semantics. For example, in an order
system, if a customer is in a long-running transaction with "groovy
beads" in the shopping cart, and the administrators decide that
"groovy beads" are no longer to be sold, you want the order that
contains "groovy beads" to be rejected when the shopping cart
arrives at checkout. You don't want that order to reinsert "groovy
beads" into the database.
I agree that this surely must be catered for.
Craig
On Mar 9, 2006, at 8:40 AM, Jörg von Frantzius wrote:
Hi Craig,
I was already afraid that "create a persistent instance" might only
apply to the PM cache, not the datastore (but only after second
read). However, would you say that JPOX is not JDO2 compliant if it
created missing instances in the datastore anyway? Will there be a
test in the TCK2 that expects an exception to be thrown if a
detached instances does not exist in the datastore?
And, most of all, what sense would it make to forbid the creation
of missing detached instances in the datastore? There is lots of
application for that behaviour, and at least I don't know of any
problem with it.
Regards,
Jörg
Craig L Russell schrieb:
Hi Jörg,
On Mar 9, 2006, at 1:43 AM, Jörg von Frantzius wrote:
Craig L Russell schrieb:
Also I find it confusing that the method most prominently used
for inserting new objects shouldn't do so for detached instances.
There is a bunch of history that you should look at, most of
which is in the jdo-dev archives. Bottom line, we used to have a
different API, attachCopy, but we looked at what it had to do
for transient and detached instances and decided that it wasn't
worth making a different API for attaching detached instances.
That particular behaviour of attachCopy() wasn't really
specified, but it was pleasant JPOX-specific behaviour, if I
remember correctly. I saw the discussion and I didn't see where
inserting the instances would be forbidden by the spec, and still
I don't see where it says that, especially in the light of
12.6.7. Please excuse my ignorance, where does it say that?
<spec>
These methods make transient instances persistent and apply
detached instance changes
to the cache.
...
For a detached instance, they locate or create a persistent
instance with the same JDO identity as the detached instance, and
merge the persistent
state of the detached instance into the persistent instance. Only
the state of persistent fields
is merged.
</spec>
This means that if there is already a persistent instance in the
cache with the same object id as the detached instance, the
detached state will be merged. If there is not a persistent
instance in the cache, a cache instance is created and the
detached state is merged with the persistent instance.
But there is no creation aspect of makePersistent on a detached
instance.
Craig
Regards,
Craig
Craig
On Mar 8, 2006, at 7:14 AM, Erik Bengtson wrote:
Hi,
What happens when we invoke makePersistent on a detached
instance that was
deleted by another isolated process? I suspect that we raise
an exception
instead of reinserting it for a second time. Is that right?
Maybe this can be clarified in the spec.
Regards,
Craig Russell
Architect, Sun Java Enterprise System
http://java.sun.com/products/jdo
408 276-5638 mailto:[EMAIL PROTECTED]
P.S. A good JDO? O, Gasp!
Craig Russell
Architect, Sun Java Enterprise System
http://java.sun.com/products/jdo
408 276-5638 mailto:[EMAIL PROTECTED]
P.S. A good JDO? O, Gasp!
Craig Russell
Architect, Sun Java Enterprise System
http://java.sun.com/products/jdo
408 276-5638 mailto:[EMAIL PROTECTED]
P.S. A good JDO? O, Gasp!
Craig Russell
Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
408 276-5638 mailto:[EMAIL PROTECTED]
P.S. A good JDO? O, Gasp!
Craig Russell
Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
408 276-5638 mailto:[EMAIL PROTECTED]
P.S. A good JDO? O, Gasp!
