Hello,
I'm using JBoss 4.2.2, and I have the following challenge: I have an entity
that keeps track of a "global" numeric value for my application, inside the db.
I need a session been that should be able to:
1. init the counter to 0
2. update it atomically, i.e when I call some 'getNext' method, it will return
the next number and increment the counter in one go.
Something like:
| @PersistenceContext EntityManager em;
|
| void init() {
| // internally, MyCounter's ID field is always 1. I just set the counter
value to 0:
| em.persist(new MyCounter(0));
| }
|
| Long getNext() {
| // this is the challenge....
| }
|
My question is: how do I implement such a thing in a cluster-aware application?
In a non-clustered environment, I would consider creating a separate
persistence unit just for this entity, which will use a db pool that serializes
transactions (highest isolation level). Since the transaction is very short
relative to transactions in the "normal" PU, it should have little effect on
performance.
This would simply require me to set the PU name on the session bean's 'em'
member, e.g.:
| @PersistenceContext(name="serializedPU")
| EntityManager em;
|
BUT: in a clustered environment, each appserver would have its own db pool,
there is no sync between them... I was thinking of a few options to resolve
this:
1. configure all appservers in the cluster to reference a specific appserver
for this specific bean, but this would create a single point of failure...
2. Instead of using a separate PU, I could configure the session bean's pool
size to max=1, and again, configure all appservers in the cluster to use this
bean only on one specific appserver. And again: single point of failure...
3. do something at the dbms level, but this will require knowledge of
dbms-specific technicalities, make my application's deployment a lot more
complex, etc.
Any better/other suggestions, please?
thanks...
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