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Posted to dev@openjpa.apache.org by Tim Holloway <ti...@mousetech.com> on 2008/07/23 17:32:29 UTC
QUERY: detach/merge behavior
This probably rates coverage in the docs, since it's not obvious and I
haven't seen anything published on the issue.
In fact, I'd consider it a bug except that I've seen similar behavior in
Kodo JDO, so I can only assume I missed something here.
The issue has to do with using the detach/attach behavior in a local web
application context. This is an area that doesn't get much press - all
the usual examples have to do with shipping out objects remotely,
getting back the modified objects, and merging them.
However, in some environments - notably JavaServer Faces - there is
theoretically an advantage in working with detached objects that are
stored in an HttpSession. A JSF conversation can typically present a
form backed (directly or indirectly) by a datamodel object (forgoing a
distinct DTO). That datamodel object can then be merged. So far well and
good.
The problem is, if that object - or the object returned from the merge -
is then later merged again, an OptimisticLockingException results and
the reasons aren't obvious.
The cure, as it turns out, is to discard the results of the merge,
re-fetch the object (e.g., by primary key) and then store that object in
the session so that it can be presented for the next set of changes.
Just to make things more interesting, the object returned from the merge
and the object fetched are often the same object, except that the fetch
appears to reset the "dirty" bits, which seems to be where the
OptimisticLockingException comes from.
Presumably, the overhead from a re-fetch is fairly low (cache hit), but
since merge is supposedly returning an updated object, the need isn't
obvious, so I'd like someone to tell me if that's the proper thing to
do.
The environment that works most naturally here (tiered architecture) has
some quirks. Because of all the back-and-forth between the different
levels of the JSF system, it's not really possible to attach and set up
a transaction at the form level. Hence the need for a merge. The actual
data operations are done in a transactionally-wrapped service layer
method and the merge is done inside that method.
Any insights, document references, etc. will be greatly appreciated.
Re: QUERY: detach/merge behavior
Posted by Tim Holloway <ti...@mousetech.com>.
On Fri, 2008-07-25 at 16:47 -0400, Tim Holloway wrote:
.....
OK, wait a minute - that looks like there's 2 decimal places of
sub-second time being included there. Something's not right. It looks
like OpenJPA and PostgreSQL aren't in sync on timestamp precision.
This is a real nuisance. The reason I added versioning was that I was
getting OptimisticLockingExceptions on differential-compare checking.
Now it looks like I can't use timestamp versioning either.
Re: QUERY: detach/merge behavior
Posted by Tim Holloway <ti...@mousetech.com>.
On Fri, 2008-07-25 at 11:42 -0700, Pinaki Poddar wrote:
> Hi,
> 1. As persistence context is TRANSACTION scoped, the context will terminate
> (by Spring or whomever) after every transaction and all managed instances
> will be detached. btw, OpenJPA does allow programmatic detach through
> extended OpenJPAEntityManager API. However, from the description, it seems
> instances are getting detached all right and as merging context is a new
> transaction-scoped context -- hence these areas are unlikely cause of the
> conflict.
>
> 2. Another angle: are you enhancing at run-time or build-time? Noticed few
> anomalies with runtime enhanced entities dirty state transitions. To narrow
> the source of error, it is advisable to enhance the entities at build-time.
>
I am enhancing at run-time. Off-topic, I also do that on the test case
for OPENJPA-571, which runs fine when I replace OpenJPA with Hibernate
JPA. Still hoping someone will look at that one.
> 3. Is it possible to switch to a long/int version field rather than
> timestamp? Again not permanently but to narrow down the cause.
>
Not easily. But I did tweak the schema to work with integral seconds
after I found out that PostgreSQL's default timestamp format is floating
point. The hard way.
> 4. If possible, post the error stack.
>
That's easily done. Look down. All I did was comment out the post-merge
re-fetch.
> 5. Are you flushing before queries?
>
No.
>
>
>
> Tim Holloway wrote:
> >
> > On Thu, 2008-07-24 at 15:18 -0400, Tim Holloway wrote:
> >
> > I've appended some additional info.
> >
> >> OK, here goes...
> >>
> >> On Thu, 2008-07-24 at 07:42 -0700, Pinaki Poddar wrote:
> >> > Hi,
> >> > > Unfortunately, that's rather murky in this context.
> >> > Sounds like it.
> >> >
> >> > But let us try to clear the murkiness. Answer to following questions
> >> might.
> >> >
> >> > 1. Let X be an instance managed in persistence context C1 in
> >> transaction T1
> >> > when X is detached.
> >> >
> >> > 2. what action triggered detachment of X?
> >> > Commit of T1?
> >> > Close of C1?
> >> > Clearing of C1?
> >> > Explicit programmatic detach?
> >> >
> >>
> >> Here's my understanding, occasionally muddled from exposure to similar
> >> platforms such as JDO:
> >>
> >> A. I didn't think that committing was supposed to be able to cause a
> >> detach
> >> B. Probably this, but
> >> C. Maybe this
> >> D. JDO2 supports explicit detach, but if JPA does, it's not obvious to
> >> me.
> >>
> >> The service class uses the following mechanism for injection of the
> >> EntityManager:
> >>
> >>
> >> @PersistenceContext
> >> public void setEntityManager(EntityManager entityManager) {
> >> this.entityManager = entityManager;
> >> }
> >>
> >> The service class methods only do queries, merge() and persist(). Any
> >> other operation is coming from Spring or some other external non-visible
> >> binding.
> >>
> >> > 3. was X dirty or clean when it was detached from C1?
> >> >
> >>
> >> In all cases, X should have been clean.
> >>
> >> > 4. is that detach trigger controlled explicitly by your application or
> >> is it
> >> > happening implicitly by the other management artifacts of the
> >> environment?
> >> > For example, X is getting detached because Spring is closing the
> >> > persistence context C1.
> >> >
> >>
> >> There is no explicit detachment here. As mentioned, anything above the
> >> CRUD primitives is coming from outside.
> >>
> >> > 5. There is no "hanging transaction". X gets modified out of any
> >> persistence
> >> > context, and then X is merged to persistence context C2.
> >> > C2 is definitely not the same as C1. Is that true?
> >> > Or is it that X gets merged to C1 but now C1 is running a
> >> different
> >> > transaction T2?
> >> >
> >>
> >> I haven't verified about the overall contexts, but I'm virtually certain
> >> that the service manager is application-scoped and the entityManager is
> >> injected once and only once in the life of the application.
> >>
> >> Yes, these are 2 separate transactions, coming from 2 separate HTTP
> >> requests. I have no actual need to detach at all, since these are all in
> >> the same JVM, although it would not be a good idea in case the
> >> HttpSession was serialized (e.g. load balance or cluster operations).
> >> And, in fact, I am not doing anything explicit to cause detachment,
> >> short of perhaps not fine-tuning framework options.
> >>
> >> > The document I referred addressed a different use case where
> >> modified X
> >> > gets merged to original (C1,T1).
> >> >
> >> > 6. Does class of X has a version field?
> >> >
> >>
> >> It does now. It didn't make any difference, however.
> >>
> >> > 7. > The problem is, if that object - or the object returned from the
> >> merge
> >> > -is then later merged again,
> >> > > an OptimisticLockingException results and the reasons aren't obvious.
> >> > Yes it is not obvious -- because repeated chain of detach and merge
> >> to a
> >> > series of different contexts is a supported use case.
> >> >
> >> > 8. What is the scope of persistence context -- TRANSACTION or EXTENDED?
> >> >
> >>
> >> TRANSACTION
> >> >
> >> >
> >>
> >> What I find most vexing is that if I work with the object returned from
> >> the merge(), I get the lock violation, but if I insert a
> >> "findByPrimaryKey" query right after that, the object I get from the
> >> query works just fine. By my understanding, the two objects should be in
> >> an identical state, but in practice, the return from the merge has its
> >> internal dirty field bits still set, but the results from the query do
> >> not. Worse, I *think* the return from the query is not merely an
> >> otherwise equivalent object - it's the exact same object. I think I'm
> >> going to go back and verify that.
> >
> > I can't find it in an FM yet, but the default operation of the Spring
> > transaction framework is to detach at the end of the transaction
> > according to several informal sources. This makes it consistent with
> > Hibernate and ensures that the objects are POJOS and need no DTOs. My
> > debugger verifies that at all times in my business layer the objects are
> > in fact detached.
> >
> > For those who prefer lazy-loading, there's a filter to prevent this, but
> > I didn't design the app to lazy-load anyway.
> >
> > I was wrong - the results of the find query are NOT the identical object
> > as what came back from the merge. However, I'm at a loss as to why the
> > merge isn't resetting the dirty flags and why the output from the merge
> > is considered as older than the persistent data.
> >
> > This is the versioning definition:
> >
> > @Basic
> > @Column(name="change_time", nullable=false)
> > @Version
> > private Timestamp changeTime;
> >
> > The merge returns an updated changeTime. The query returns the same
> > changeTime. Only the "dirty flags" distinguish them.
> >
> >
Failure log:
16:33:44,434 DEBUG openjpa.jdbc.SQL:72 - <t 16757535, conn 27534602> executing prepstmnt 7525605 SELECT t0.change_time, t0.description, t0.
inbound, t0.street1_block, t1.street_direction_id, t1.description,
t2.street_type_id, t2.description, t0.street2_block, t3.street_direction
_id, t3.description, t4.street_type_id, t4.description FROM
public.bus_stops t0 LEFT OUTER JOIN public.lk_street_directions t1 ON
t0.street
1_direction = t1.street_direction_id LEFT OUTER JOIN
public.lk_street_types t2 ON t0.street1_type = t2.street_type_id LEFT
OUTER JOIN publi
c.lk_street_directions t3 ON t0.street2_direction =
t3.street_direction_id LEFT OUTER JOIN public.lk_street_types t4 ON
t0.street2_type = t
4.street_type_id WHERE t0.stop_id = ? [params=(int) 4]
16:33:44,443 DEBUG openjpa.jdbc.SQL:72 - <t 16757535, conn 27534602> [8
ms] spent
16:33:44,447 DEBUG openjpa.jdbc.JDBC:72 - <t 16757535, conn 0> [0 ms]
close
16:33:44,450 DEBUG openjpa.jdbc.JDBC:72 - The batch limit is set to 0.
16:33:44,455 DEBUG openjpa.jdbc.SQL:72 - <t 16757535, conn 27534602>
executing prepstmnt 32638273 UPDATE public.bus_stops SET description =
?, inbound = ?, change_time = ? WHERE stop_id = ? AND change_time = ?
[params=(String) Citi Center Transfer Junction, (boolean) true, (Tim
estamp) 2008-07-25 16:33:44.45, (int) 4, (Timestamp) 2008-07-25
16:32:50.36]
16:33:44,466 DEBUG openjpa.jdbc.SQL:72 - <t 16757535, conn 27534602> [2
ms] spent
16:33:44,497 DEBUG openjpa.Runtime:76 - An exception occurred while
ending the transaction. This exception will be re-thrown.
<openjpa-1.2.0-SNAPSHOT-rexported nonfatal store error>
org.apache.openjpa.util.OptimisticException: Optimistic locking errors
were detecte
d when flushing to the data store. The following objects may have been
concurrently modified in another transaction: [com.mousetech.jta.pe
rsist.BusStops-4]
at
org.apache.openjpa.kernel.BrokerImpl.newFlushException(BrokerImpl.java:2160)
at
org.apache.openjpa.kernel.BrokerImpl.flush(BrokerImpl.java:2010)
at
org.apache.openjpa.kernel.BrokerImpl.flushSafe(BrokerImpl.java:1908)
at
org.apache.openjpa.kernel.BrokerImpl.beforeCompletion(BrokerImpl.java:1826)
at
org.apache.openjpa.kernel.LocalManagedRuntime.commit(LocalManagedRuntime.java:81)
at
org.apache.openjpa.kernel.BrokerImpl.commit(BrokerImpl.java:1350)
at
org.apache.openjpa.kernel.DelegatingBroker.commit(DelegatingBroker.java:877)
at
org.apache.openjpa.persistence.EntityManagerImpl.commit(EntityManagerImpl.java:512)
at
org.springframework.orm.jpa.JpaTransactionManager.doCommit(JpaTransactionManager.java:433)
at
org.springframework.transaction.support.AbstractPlatformTransactionManager.processCommit(AbstractPlatformTransactionManager.java
:662)
at
org.springframework.transaction.support.AbstractPlatformTransactionManager.commit(AbstractPlatformTransactionManager.java:632)
at
org.springframework.transaction.interceptor.TransactionAspectSupport.commitTransactionAfterReturning(TransactionAspectSupport.ja
va:314)
at
org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:117)
at
org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:166)
at
org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:204)
at $Proxy22.update(Unknown Source)
at
com.mousetech.jta.backing.BusStopsBackingBean.doCommitAction(BusStopsBackingBean.java:275)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.apache.el.parser.AstValue.invoke(AstValue.java:152)
at
org.apache.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:276)
at
com.sun.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:68)
at
javax.faces.component._MethodExpressionToMethodBinding.invoke(_MethodExpressionToMethodBinding.java:75)
at
org.apache.myfaces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:54)
at javax.faces.component.UICommand.broadcast(UICommand.java:121)
> >
>
Re: QUERY: detach/merge behavior
Posted by Pinaki Poddar <pp...@apache.org>.
Hi,
1. As persistence context is TRANSACTION scoped, the context will terminate
(by Spring or whomever) after every transaction and all managed instances
will be detached. btw, OpenJPA does allow programmatic detach through
extended OpenJPAEntityManager API. However, from the description, it seems
instances are getting detached all right and as merging context is a new
transaction-scoped context -- hence these areas are unlikely cause of the
conflict.
2. Another angle: are you enhancing at run-time or build-time? Noticed few
anomalies with runtime enhanced entities dirty state transitions. To narrow
the source of error, it is advisable to enhance the entities at build-time.
3. Is it possible to switch to a long/int version field rather than
timestamp? Again not permanently but to narrow down the cause.
4. If possible, post the error stack.
5. Are you flushing before queries?
Tim Holloway wrote:
>
> On Thu, 2008-07-24 at 15:18 -0400, Tim Holloway wrote:
>
> I've appended some additional info.
>
>> OK, here goes...
>>
>> On Thu, 2008-07-24 at 07:42 -0700, Pinaki Poddar wrote:
>> > Hi,
>> > > Unfortunately, that's rather murky in this context.
>> > Sounds like it.
>> >
>> > But let us try to clear the murkiness. Answer to following questions
>> might.
>> >
>> > 1. Let X be an instance managed in persistence context C1 in
>> transaction T1
>> > when X is detached.
>> >
>> > 2. what action triggered detachment of X?
>> > Commit of T1?
>> > Close of C1?
>> > Clearing of C1?
>> > Explicit programmatic detach?
>> >
>>
>> Here's my understanding, occasionally muddled from exposure to similar
>> platforms such as JDO:
>>
>> A. I didn't think that committing was supposed to be able to cause a
>> detach
>> B. Probably this, but
>> C. Maybe this
>> D. JDO2 supports explicit detach, but if JPA does, it's not obvious to
>> me.
>>
>> The service class uses the following mechanism for injection of the
>> EntityManager:
>>
>>
>> @PersistenceContext
>> public void setEntityManager(EntityManager entityManager) {
>> this.entityManager = entityManager;
>> }
>>
>> The service class methods only do queries, merge() and persist(). Any
>> other operation is coming from Spring or some other external non-visible
>> binding.
>>
>> > 3. was X dirty or clean when it was detached from C1?
>> >
>>
>> In all cases, X should have been clean.
>>
>> > 4. is that detach trigger controlled explicitly by your application or
>> is it
>> > happening implicitly by the other management artifacts of the
>> environment?
>> > For example, X is getting detached because Spring is closing the
>> > persistence context C1.
>> >
>>
>> There is no explicit detachment here. As mentioned, anything above the
>> CRUD primitives is coming from outside.
>>
>> > 5. There is no "hanging transaction". X gets modified out of any
>> persistence
>> > context, and then X is merged to persistence context C2.
>> > C2 is definitely not the same as C1. Is that true?
>> > Or is it that X gets merged to C1 but now C1 is running a
>> different
>> > transaction T2?
>> >
>>
>> I haven't verified about the overall contexts, but I'm virtually certain
>> that the service manager is application-scoped and the entityManager is
>> injected once and only once in the life of the application.
>>
>> Yes, these are 2 separate transactions, coming from 2 separate HTTP
>> requests. I have no actual need to detach at all, since these are all in
>> the same JVM, although it would not be a good idea in case the
>> HttpSession was serialized (e.g. load balance or cluster operations).
>> And, in fact, I am not doing anything explicit to cause detachment,
>> short of perhaps not fine-tuning framework options.
>>
>> > The document I referred addressed a different use case where
>> modified X
>> > gets merged to original (C1,T1).
>> >
>> > 6. Does class of X has a version field?
>> >
>>
>> It does now. It didn't make any difference, however.
>>
>> > 7. > The problem is, if that object - or the object returned from the
>> merge
>> > -is then later merged again,
>> > > an OptimisticLockingException results and the reasons aren't obvious.
>> > Yes it is not obvious -- because repeated chain of detach and merge
>> to a
>> > series of different contexts is a supported use case.
>> >
>> > 8. What is the scope of persistence context -- TRANSACTION or EXTENDED?
>> >
>>
>> TRANSACTION
>> >
>> >
>>
>> What I find most vexing is that if I work with the object returned from
>> the merge(), I get the lock violation, but if I insert a
>> "findByPrimaryKey" query right after that, the object I get from the
>> query works just fine. By my understanding, the two objects should be in
>> an identical state, but in practice, the return from the merge has its
>> internal dirty field bits still set, but the results from the query do
>> not. Worse, I *think* the return from the query is not merely an
>> otherwise equivalent object - it's the exact same object. I think I'm
>> going to go back and verify that.
>
> I can't find it in an FM yet, but the default operation of the Spring
> transaction framework is to detach at the end of the transaction
> according to several informal sources. This makes it consistent with
> Hibernate and ensures that the objects are POJOS and need no DTOs. My
> debugger verifies that at all times in my business layer the objects are
> in fact detached.
>
> For those who prefer lazy-loading, there's a filter to prevent this, but
> I didn't design the app to lazy-load anyway.
>
> I was wrong - the results of the find query are NOT the identical object
> as what came back from the merge. However, I'm at a loss as to why the
> merge isn't resetting the dirty flags and why the output from the merge
> is considered as older than the persistent data.
>
> This is the versioning definition:
>
> @Basic
> @Column(name="change_time", nullable=false)
> @Version
> private Timestamp changeTime;
>
> The merge returns an updated changeTime. The query returns the same
> changeTime. Only the "dirty flags" distinguish them.
>
>
>
--
View this message in context: http://n2.nabble.com/QUERY%3A-detach-merge-behavior-tp578652p583162.html
Sent from the OpenJPA Developers mailing list archive at Nabble.com.
Re: QUERY: detach/merge behavior
Posted by Tim Holloway <ti...@mousetech.com>.
On Thu, 2008-07-24 at 15:18 -0400, Tim Holloway wrote:
I've appended some additional info.
> OK, here goes...
>
> On Thu, 2008-07-24 at 07:42 -0700, Pinaki Poddar wrote:
> > Hi,
> > > Unfortunately, that's rather murky in this context.
> > Sounds like it.
> >
> > But let us try to clear the murkiness. Answer to following questions might.
> >
> > 1. Let X be an instance managed in persistence context C1 in transaction T1
> > when X is detached.
> >
> > 2. what action triggered detachment of X?
> > Commit of T1?
> > Close of C1?
> > Clearing of C1?
> > Explicit programmatic detach?
> >
>
> Here's my understanding, occasionally muddled from exposure to similar
> platforms such as JDO:
>
> A. I didn't think that committing was supposed to be able to cause a
> detach
> B. Probably this, but
> C. Maybe this
> D. JDO2 supports explicit detach, but if JPA does, it's not obvious to
> me.
>
> The service class uses the following mechanism for injection of the
> EntityManager:
>
>
> @PersistenceContext
> public void setEntityManager(EntityManager entityManager) {
> this.entityManager = entityManager;
> }
>
> The service class methods only do queries, merge() and persist(). Any
> other operation is coming from Spring or some other external non-visible
> binding.
>
> > 3. was X dirty or clean when it was detached from C1?
> >
>
> In all cases, X should have been clean.
>
> > 4. is that detach trigger controlled explicitly by your application or is it
> > happening implicitly by the other management artifacts of the environment?
> > For example, X is getting detached because Spring is closing the
> > persistence context C1.
> >
>
> There is no explicit detachment here. As mentioned, anything above the
> CRUD primitives is coming from outside.
>
> > 5. There is no "hanging transaction". X gets modified out of any persistence
> > context, and then X is merged to persistence context C2.
> > C2 is definitely not the same as C1. Is that true?
> > Or is it that X gets merged to C1 but now C1 is running a different
> > transaction T2?
> >
>
> I haven't verified about the overall contexts, but I'm virtually certain
> that the service manager is application-scoped and the entityManager is
> injected once and only once in the life of the application.
>
> Yes, these are 2 separate transactions, coming from 2 separate HTTP
> requests. I have no actual need to detach at all, since these are all in
> the same JVM, although it would not be a good idea in case the
> HttpSession was serialized (e.g. load balance or cluster operations).
> And, in fact, I am not doing anything explicit to cause detachment,
> short of perhaps not fine-tuning framework options.
>
> > The document I referred addressed a different use case where modified X
> > gets merged to original (C1,T1).
> >
> > 6. Does class of X has a version field?
> >
>
> It does now. It didn't make any difference, however.
>
> > 7. > The problem is, if that object - or the object returned from the merge
> > -is then later merged again,
> > > an OptimisticLockingException results and the reasons aren't obvious.
> > Yes it is not obvious -- because repeated chain of detach and merge to a
> > series of different contexts is a supported use case.
> >
> > 8. What is the scope of persistence context -- TRANSACTION or EXTENDED?
> >
>
> TRANSACTION
> >
> >
>
> What I find most vexing is that if I work with the object returned from
> the merge(), I get the lock violation, but if I insert a
> "findByPrimaryKey" query right after that, the object I get from the
> query works just fine. By my understanding, the two objects should be in
> an identical state, but in practice, the return from the merge has its
> internal dirty field bits still set, but the results from the query do
> not. Worse, I *think* the return from the query is not merely an
> otherwise equivalent object - it's the exact same object. I think I'm
> going to go back and verify that.
I can't find it in an FM yet, but the default operation of the Spring
transaction framework is to detach at the end of the transaction
according to several informal sources. This makes it consistent with
Hibernate and ensures that the objects are POJOS and need no DTOs. My
debugger verifies that at all times in my business layer the objects are
in fact detached.
For those who prefer lazy-loading, there's a filter to prevent this, but
I didn't design the app to lazy-load anyway.
I was wrong - the results of the find query are NOT the identical object
as what came back from the merge. However, I'm at a loss as to why the
merge isn't resetting the dirty flags and why the output from the merge
is considered as older than the persistent data.
This is the versioning definition:
@Basic
@Column(name="change_time", nullable=false)
@Version
private Timestamp changeTime;
The merge returns an updated changeTime. The query returns the same
changeTime. Only the "dirty flags" distinguish them.
Re: QUERY: detach/merge behavior
Posted by Tim Holloway <ti...@mousetech.com>.
OK, here goes...
On Thu, 2008-07-24 at 07:42 -0700, Pinaki Poddar wrote:
> Hi,
> > Unfortunately, that's rather murky in this context.
> Sounds like it.
>
> But let us try to clear the murkiness. Answer to following questions might.
>
> 1. Let X be an instance managed in persistence context C1 in transaction T1
> when X is detached.
>
> 2. what action triggered detachment of X?
> Commit of T1?
> Close of C1?
> Clearing of C1?
> Explicit programmatic detach?
>
Here's my understanding, occasionally muddled from exposure to similar
platforms such as JDO:
A. I didn't think that committing was supposed to be able to cause a
detach
B. Probably this, but
C. Maybe this
D. JDO2 supports explicit detach, but if JPA does, it's not obvious to
me.
The service class uses the following mechanism for injection of the
EntityManager:
@PersistenceContext
public void setEntityManager(EntityManager entityManager) {
this.entityManager = entityManager;
}
The service class methods only do queries, merge() and persist(). Any
other operation is coming from Spring or some other external non-visible
binding.
> 3. was X dirty or clean when it was detached from C1?
>
In all cases, X should have been clean.
> 4. is that detach trigger controlled explicitly by your application or is it
> happening implicitly by the other management artifacts of the environment?
> For example, X is getting detached because Spring is closing the
> persistence context C1.
>
There is no explicit detachment here. As mentioned, anything above the
CRUD primitives is coming from outside.
> 5. There is no "hanging transaction". X gets modified out of any persistence
> context, and then X is merged to persistence context C2.
> C2 is definitely not the same as C1. Is that true?
> Or is it that X gets merged to C1 but now C1 is running a different
> transaction T2?
>
I haven't verified about the overall contexts, but I'm virtually certain
that the service manager is application-scoped and the entityManager is
injected once and only once in the life of the application.
Yes, these are 2 separate transactions, coming from 2 separate HTTP
requests. I have no actual need to detach at all, since these are all in
the same JVM, although it would not be a good idea in case the
HttpSession was serialized (e.g. load balance or cluster operations).
And, in fact, I am not doing anything explicit to cause detachment,
short of perhaps not fine-tuning framework options.
> The document I referred addressed a different use case where modified X
> gets merged to original (C1,T1).
>
> 6. Does class of X has a version field?
>
It does now. It didn't make any difference, however.
> 7. > The problem is, if that object - or the object returned from the merge
> -is then later merged again,
> > an OptimisticLockingException results and the reasons aren't obvious.
> Yes it is not obvious -- because repeated chain of detach and merge to a
> series of different contexts is a supported use case.
>
> 8. What is the scope of persistence context -- TRANSACTION or EXTENDED?
>
TRANSACTION
>
>
What I find most vexing is that if I work with the object returned from
the merge(), I get the lock violation, but if I insert a
"findByPrimaryKey" query right after that, the object I get from the
query works just fine. By my understanding, the two objects should be in
an identical state, but in practice, the return from the merge has its
internal dirty field bits still set, but the results from the query do
not. Worse, I *think* the return from the query is not merely an
otherwise equivalent object - it's the exact same object. I think I'm
going to go back and verify that.
Re: QUERY: detach/merge behavior
Posted by Pinaki Poddar <pp...@apache.org>.
Hi,
> Unfortunately, that's rather murky in this context.
Sounds like it.
But let us try to clear the murkiness. Answer to following questions might.
1. Let X be an instance managed in persistence context C1 in transaction T1
when X is detached.
2. what action triggered detachment of X?
Commit of T1?
Close of C1?
Clearing of C1?
Explicit programmatic detach?
3. was X dirty or clean when it was detached from C1?
4. is that detach trigger controlled explicitly by your application or is it
happening implicitly by the other management artifacts of the environment?
For example, X is getting detached because Spring is closing the
persistence context C1.
5. There is no "hanging transaction". X gets modified out of any persistence
context, and then X is merged to persistence context C2.
C2 is definitely not the same as C1. Is that true?
Or is it that X gets merged to C1 but now C1 is running a different
transaction T2?
The document I referred addressed a different use case where modified X
gets merged to original (C1,T1).
6. Does class of X has a version field?
7. > The problem is, if that object - or the object returned from the merge
-is then later merged again,
> an OptimisticLockingException results and the reasons aren't obvious.
Yes it is not obvious -- because repeated chain of detach and merge to a
series of different contexts is a supported use case.
8. What is the scope of persistence context -- TRANSACTION or EXTENDED?
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Re: QUERY: detach/merge behavior
Posted by Tim Holloway <ti...@mousetech.com>.
Unfortunately, that's rather murky in this context. The service layer is
responsible for commit or rollback, so there shouldn't be a "hanging
commit" - which is what I read this excerpt to be addressing.
Just to make life more fun, in JSF, it isn't as obvious where the detach
is being done. In Struts, I knew definitively that I was going to
release the connection at the end of the application logic (action) and
before the View referenced the object graph, but the JSF lifecycle is
more complex and I haven't tried to formally map it out. All I know is
that by the next trip in (HTTP Request), the session datamodel objects
have been detached for me.
Urg. Just remembered. I'm using Spring to do the transaction management
so it's supposed to handle the commit/rollback for me. Just to make
things even more subtle.
On Wed, 2008-07-23 at 12:25 -0700, Pinaki Poddar wrote:
> Hi,
> Following excerpt from Chapter 11 [1] of the manual may be relevant in
> this context:
>
> "When detaching an instance that has been modified in the current
> transaction (and thus made dirty), the current transaction is flushed. This
> means that when subsequently re-attaching the detached instances, OpenJPA
> assumes that the transaction from which they were originally detached was
> committed; if it has been rolled back, then the re-attachment process will
> throw an optimistic concurrency exception.
>
> You can stop OpenJPA from assuming the transaction will commit in the
> following ways :
>
> * Invoke EntityTransaction.setRollbackOnly prior to detaching your
> objects. Setting the RollbackOnly flag prevents OpenJPA from flushing when
> detaching dirty objects; instead OpenJPA just runs its pre-flush actions
> (see the OpenJPAEntityManager.preFlush Javadoc for details).
>
> This allows you to use the same instances in multiple
> attach/modify/detach/rollback cycles.
>
> * Make your modifications outside of a transaction (with
> NontransactionalWrite enabled) before detaching.
>
> * Set flushBeforeDetach to false (see Compatibility.setFlushBeforeDetach
> Javadoc ). This option is similar to the first option, but does not affect
> the current transaction."
>
> [1]
> http://openjpa.apache.org/docs/latest/manual/manual.html#ref_guide_detach
>
Re: QUERY: detach/merge behavior
Posted by Pinaki Poddar <pp...@apache.org>.
Hi,
Following excerpt from Chapter 11 [1] of the manual may be relevant in
this context:
"When detaching an instance that has been modified in the current
transaction (and thus made dirty), the current transaction is flushed. This
means that when subsequently re-attaching the detached instances, OpenJPA
assumes that the transaction from which they were originally detached was
committed; if it has been rolled back, then the re-attachment process will
throw an optimistic concurrency exception.
You can stop OpenJPA from assuming the transaction will commit in the
following ways :
* Invoke EntityTransaction.setRollbackOnly prior to detaching your
objects. Setting the RollbackOnly flag prevents OpenJPA from flushing when
detaching dirty objects; instead OpenJPA just runs its pre-flush actions
(see the OpenJPAEntityManager.preFlush Javadoc for details).
This allows you to use the same instances in multiple
attach/modify/detach/rollback cycles.
* Make your modifications outside of a transaction (with
NontransactionalWrite enabled) before detaching.
* Set flushBeforeDetach to false (see Compatibility.setFlushBeforeDetach
Javadoc ). This option is similar to the first option, but does not affect
the current transaction."
[1]
http://openjpa.apache.org/docs/latest/manual/manual.html#ref_guide_detach
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