Matthew T. Adams schrieb:
Don't forget that once JPOX starts throwing exceptions during this behavior,
the exception should contain exceptions for each of the violating detached
objects (the ones that are to be inserted in the new datastore instead of
merged). You should be able to handle this in a catch clause, making them
transient, then persisting the transient instances.
This won't work with datastore identity, as the ids will be lost upon
makeTransient(), and one cannot rely on the second datastore assigning
the same datastore identity to the new instance as did the original
datastore. That's because of different states and behaviours of
sequences in different datastores (potentially from different vendors).
That way, you can
continue to merge detached objects across datastores, and still insert new
ones in a portable way. There would be some issues, however, with regard to
object references, but I'd expect that you could handle them during
exception handling.
Please see below...
It's also not the most efficient design, however, as you're almost expecting
a rollback for the violating detached objects:
public void replicateGraph(Object root, boolean retry) {
try {
pm.makePersistent(root); // pm is for destination datastore
return; // happy path
}
catch (JDOUserException x) { // I'm not sure of the exact exception type
here
if (!retry) {
throw x;
}
// else
// for each nested exception in x
// get the deleted detached object
// make it transient
We cannot makeTransient() on a detached instance, Table 2 specifies an
error for that.
// replace references in the root graph with the newly transient
objects (might be hard)
We could try to find the original instances in the first PM for the
failed detached instances, and making those original instances
transient. After calling makePersistent() for them, the makePersistent()
for the detached graph shouldn't fail anymore.
// end for
// now recurse (only once)
replicate(root, false);
}
}
Just a thought...
Thanks for that thought!
--matthew
-----Original Message-----
From: Jörg von Frantzius [mailto:[EMAIL PROTECTED]
Sent: Thursday, March 09, 2006 10:33 AM
To: [email protected]
Subject: Re: makePersistent detached instance deleted on database
Hi Craig,
thanks for pointing me to the new makeTransient(Object, useFetchPlan)
method, that totally has escaped me. It's support for MaxFetchDepth,
DETACH_LOAD_FIELDS, and DETACH_UNLOAD_FIELDS should suffice for
replication. Do I understand it right that those parameters
are not only
applied when loading the "pc or
pcs parameter instance(s)", but also for loading reachable instances
according to the fetch plan?
I'm afraid that JPOX currently supports this only on detaching, though.
Detachment for replication is already used in production with an older
JPOX version, that still has attachCopy() and insertion of new
instances, by the way.
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!
