Hawley, Dan a écrit :
Hi,
I seem to need some help understanding how record locking works in
Derby because I can't seem to make it work for me.
[...]
Any help you can provide will be greatly appreciated. Thanks, Dan
Hi Dan,
Well, if "any" help will be appreciated, I take my chance!
That's quite a long time I haven't used Hibernate. So I'm a little bit
rusty. But, I am wondering if you don't mix up two things:
* Database locks
* Hibernate locks
As far as I know, database locks allow one to prevent other transactions
to access some rows from a table while you are holding it.
With Derby, you may achieve such exclusive access by using
a LOCK TABLE statement (among others). This is not recommended if you
intent to hold the lock for a long period of time.
Depending on your needs, you may be interested in more informations
about Derby's Locks:
http://db.apache.org/derby/docs/10.0/manuals/develop/develop74.html
In the other hand, as far as I remember, Hibernates locks are something
else: they will prenvent the modification of the same persistent objet
in two different contexts.
By default, Hibernate use optimistic locking. Only when a dirty object
(i.e.: an object you have modified) is write back to the DB, Hibernate
is able to say: "Hey, I can't do that: Someone has changed it in your
back!".
Concerning your specific problem, in theory,
'setHint("org.hibernate.readOnly", true)' will prevent Hibernate to
"persist back" the object to the DB. I don't think this will change the
SQL query in any way. Moreover, in your case, I guess your second query
will hit the cache, returning the same "updatable" objects than the
first one. Once again, this is only a guess. And I couldn't say if this
a bug or a feature.
At least, I think this is more of an Hibernate than a Derby issue.
Finally, maybe I totally missed the point here ;) So if you think this
is definitively a Derby problem, please log the SQL query issued by
Hibernate: this will give us a better understanding of what's going on
there.
Anyway, let us know if you solved your problem.
Hope this helps at least a little,
Sylvain.
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