Business logic locks and CMP.

Question

I’m trying to implement the following:
If a raw in the database (a CMP bean instance) was changed by a user then this
raw is “locked” for all other users (write lock) until the user decides to release
the lock.
The database raw has three fields: ID, UserLockID, and Data
The code in my stateless session bean façade that wraps the CMP will be:
ResourceHome home = (ResourceHome)EJBHomeFactory.getInstance().lookUpLocalHome(
“Resource” , ResourceHome.class );
Resource rsc = home.findByPrimaryKey(id);
If (rsc.getUserLockId() == null || rsc.getUserLockId() == user ){
rsc.setUserLockId(user);
rsc.setData(data);     
} else{
throw new LockException(“This raw is locked by another user”);
My questions are:
1) Do I need to worry about race conditions, for example two user accessing the
same raw at the same time and then overwriting each other changes (where the
last one wins)?
2) If the answer for 1. is Yes, should I use TransactionReadCommittedForUpdate
3) If the answer for 2 is Yes, on which methods should I define this isolation
level? The session bean method? The CMP method, which once ejbLoad? Getters and
setters?.
4) Last Question: Can I use TransactionReadCommittedForUpdate and an optimistic
concurrency strategy for the same bean?
Thanks in advance.

Answer 1

Why would not you just use database concurrency strategy and use dummy
update (or select for update) right before you are going to change the data?
If no one else have changed it, then the update will be successful and you
can continue with changing and committing. If someone else changed the data
(and committed it to database), you will get an exception. Looks pretty
simple and easy.
Or you can use exclusive concurrency strategy, but in this case you will not
be able to read field values from another transaction. I am not sure will
you be able even to find the bean or not. If I recall correctly, you will.
Michael Jouravlev.
"M.A." <[email protected]> wrote in message
news:3e95ca33$[email protected]..
>
I’m trying to implement the following:
If a raw in the database (a CMP bean instance) was changed by a user thenthis
raw is “locked” for all other users (write lock) until the user decides
to release
the lock.
The database raw has three fields: ID, UserLockID, and Data
The code in my stateless session bean façade that wraps the CMP will be:
ResourceHome home =(ResourceHome)EJBHomeFactory.getInstance().lookUpLocalHome(
“Resource” , ResourceHome.class );
Resource rsc = home.findByPrimaryKey(id);
If (rsc.getUserLockId() == null || rsc.getUserLockId() == user ){
rsc.setUserLockId(user);
rsc.setData(data);
} else{
throw new LockException(“This raw is locked by another user”);
My questions are:
1) Do I need to worry about race conditions, for example two useraccessing the
same raw at the same time and then overwriting each other changes (wherethe
last one wins)?
2) If the answer for 1. is Yes, should I useTransactionReadCommittedForUpdate
3) If the answer for 2 is Yes, on which methods should I define thisisolation
level? The session bean method? The CMP method, which once ejbLoad?Getters and
setters?.
4) Last Question: Can I use TransactionReadCommittedForUpdate and anoptimistic
concurrency strategy for the same bean?
Thanks in advance.