db-jdo-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Wes Biggs <...@tralfamadore.com>
Subject Re: Cache invalidation
Date Mon, 06 Feb 2006 22:53:45 GMT
I agree that it would be nice to change the method signatures to 
"evictById" for those that take OIDs in order to avoid confusion.

To clarify what I mean about persistent nontransactional objects, see 
section 5.6.1 of the spec:
"A persistent-nontransactional instance transitions to persistent-clean 
if any managed field
is accessed when a datastore transaction is in progress. The state of 
the instance in memory
is discarded and the state is loaded from the datastore."

If you are running with an optimistic transaction instead, you'll get an 
optimistic verification exception at commit time.  So I guess it is 
possible to read stale data from the instance in the PM cache under a 
couple of scenarios:

1. Reading previously loaded fields of a P-NT instance outside of a 
transaction.
2. Reading previously loaded fields of a P-NT instance inside an 
optimistic transaction.

In these cases, I think you're right that it would be necessary to 
hollow the instances in order to be absolutely sure that no stale data 
is read after a L2 cache evict().

On the other hand, if you're in an optimistic transaction, don't you 
want to retain the previously read values (they represent the ACID 
guarantee from the optimistic transaction)?  So the only case where it 
might make sense to me is #1 above, and that seems debatable to me.  Do 
most people using P-NT objects expect them to be consistent with the L2 
cache at all times?  Or are they expected to act like a limited form of 
an optimistic transaction?

I don't have a strong opinion about this, I'm just trying to fully 
articulate the question.

Wes

Joerg von Frantzius wrote:

> Hi Wes,
>
> thanks for your answer, please see my comments below.
>
> Wesley Biggs schrieb:
>
>> Joerg von Frantzius wrote:
>>
>>> The problem here is that either evict() accepts only PC objects, not 
>>> object ids, so we have to call PM.getObjectById() beforehand. If no 
>>> object for that id was present, we're instantiating a hollow object 
>>> here only to discard it afterwards, that's not very effective.
>>
>> I'm not quite parsing your "either" here, sorry.  But 
>> DataStoreCache.evict() accepts object IDs.  I'm not sure I see the 
>> necessity of calling PM.evict() as well, unless you have some 
>> particularly long-lived transactions.
>
> We're doing nontransactional reads on long-living objects, so I 
> guessed we needed to call PM.evict() to avoid accessing stale field data.
>
> You're of course right about DatastoreCache.evict() accepting IDs, 
> thanks for pointing that out. I had just seen the same method 
> signature, and so I assumed the parameter semantics also being the same.
>
> Calling it evictById() probably would be less misleading, even more so 
> as a mistake here won't show up immediately. Also, if you only have a 
> jar without sourcecode, the signatures are absolutely 
> indistinguishable (Which of course is not an excuse for not having 
> read the spec thoroughly enough ;)
>
>>> As we really want cache invalidation here, not eviction, this is 
>>> even worse. For this purpose, it would be far more convenient to 
>>> have some method like invalidateCachesFor(Object id) on 
>>> PersistenceManagerFactory. 
>>
>> That's the intention of DataStoreCache.evict().  The semantics are 
>> different than PM.evict().
>
> Only now I start understanding that I was misled by the word evict() 
> for the L2-cache: as the user never gets hold of an L2 cache object 
> anyway (a L1-cache object will be created for that), he shouldn't need 
> to care whether the L2 cache internally needs to throw away (evict) 
> some object in order to invalidate cached state. Spec says "/The evict 
> methods are hints to the implementation that the instances referred to 
> by the object ids are stale and should be evicted from the cache./" It 
> might be nit-picking, but I think it would be clearer if the method 
> was called invalidateByÍd(), which would be natural for some cache 
> interface, and if the explanation said "/that the object state 
> referred to by the object ids should be discarded/"
>
> Also, the spec doesn't say anything about DatastoreCache.evict() 
> having any impact on P-nontrans instances. So I still need to go to 
> every PM and evict there as well, which is very inconvenient.
>
> Or does the "evict" row in table 2 for P-nontrans really apply to 
> /both /evict() methods, not only PM.evict()!? The RI JPOX isn't doing 
> anything like that, by the way.
>
>>> To make our wish complete ;) this method would transition all 
>>> non-transactional instances to hollow for that id, for all the PMs 
>>> the PMF has given out. All transactional objects with that id should 
>>> be transitioned to hollow after their transaction has completed 
>>> (either with commit or rollback).
>>
>> Persistent nontransactional instances will have to be revalidated 
>> against the datastore (or cache thereof) before being re-enlisted in 
>> a transaction anyway.  The behavior you mention is a good way to 
>> implement that, but it doesn't need to be mandated (hollow is not a 
>> user-visible state).
>
> I'm not sure what you mean by mandating here? I'd just like to make 
> sure that invalidated non-transactional instances will reload state 
> upon next read access, without having to iterate all PMs. Also, I'd 
> rather not like a call to PM.getObjectById() afterwards returning a 
> new Java object for the same id, which I guess is the case after 
> calling PM.evict(PM.getObjectById(id)).
>
> If a method invalidateById() existed, I'd see the sense of evict() in 
> releasing the associated memory. evict() currently does two things at 
> same time: evicting and transitioning to hollow. For (distributed) 
> cache invalidation, I find it sensible to desire only the latter.
>
> Regards,
> Jörg



Mime
View raw message