[OJB-developers] High Low Sequence

[<Joa...@tp...>]

[snip HighLowSequence objects being GCed, resulting in strange ID
sequences]

this doesn't result in incorrect behaviour, but I agree that it wastes
key-space unnecessarily, which is A Bad Thing.

> So there is a need to tell PB to cache instances of HighLowSequence
> forever without giving the GC a chance to collect them.
>
[snip MetaObjectCache solution]
>
> A better solution would be to implement a Set in the HighLowSequence
> where all new instances are added so that these objects are not
> eligible for GC even if the cache reference is only a SoftReference.

hm ... such a Set exists. To be more precise, there is a Hashtable in
SequenceManagerHighLowImpl that holds all HighLowSequence objects that the
SequenceManager has ever used, so they can't be GCed. So I can't see how
this behaviour could possibly happen (of course I don't doubt that you saw
it, I just can't explain why), unless the SequenceManagerHighLowImpl itself
get GCed and re-created (or there are several instances of it).

Maybe you're working with the client/server model. 'Cause then there are
several singlevm PersistenceBrokers on the server and you get to use a new
one each time you issue an command (unless you use and transaction). And
since each of these PersistenceBrokers has its own SequenceManager this
could result in equally confusing id-sequences (or even more confusing
ones, like 1,6,11,2,16,7,3,12,...)

Another possible problem with the SequenceManagerHighLowImpl (and possibly
the DefaultImpl as well) is that if several PersistenceBroker acessed the
same DB there could be race conditions, so we'd need to lock the obj_hl_seq
table (or at least one row). The question is wether it is possible/good
style to use the ODMG API (which seems the only way to do locking in OJB)
within a core class. (I've never actually seen such a race condition and
haven't run any tests, but I once noticed it and there don't seem to be any
pre-cautions for this; please correct me if I'm wrong).

> Regards Rainer

regards
Joachim Sauer