Suppose you have an entity bean representing an account. This account is owned
by several people, each of them being allowed to perform operations on the
account.
Now suppose that two of these people start a transaction and try to access the
bean.
If pessimistic concurrency is used, then the first one who uses the account bean
will obtain a lock on this bean. The other one will have to wait until the first
transaction ends before to be able to perform an operation on the bean. That
way, you'll be sure that the information that you get in the bean is accurate
information, and that you'll read the latest state of the bean. On the other
hand, if the two persons just want to have the balance of the account (read-only
operation) and if the first transaction is long, then the second person will
have to wait for nothing.
Now if optimistic concurrency is used, then the two persons will be able to
access the bean simultaneously. There will be no lock on the bean itself. If the
operations they perform are read-only, no problem. Now if the operations they
perform update the state of the bean, it can lead to problems. For example, if
the two persons increment read the initial balance (5000) and then increment it
(+1000), instead of having a final balance of 7000, the account will have a
final balance of 6000. Now depending on the configuration of the database
transaction isolation level, and on other configuration options the container
might provide you, the second transaction could fail and rollback, and you might
have to retry the transaction.
Hope it was clear.
JB.
P.S. : to the maintainer of this mailing list : is it normal that I don't
receive the messages I post. It's rather annoying to have incomplete threads and
not to be able to easily find the reactions to your posts.
Asha Balasubramanyan wrote:
> Hi:
>
> In this forum in lot of disccusions , i heard Pessimistic/optimistic
> concurrency , can anyone tell me what this means?
>
> Thanks
> Asha
>
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