Concurrency Control on my Code

Question

I am working on an order capture and generator application. Application is working fine with concurrent users working on different orders. The problem starts when two Users from different systems/locations try to work on the same order. How it is impacting the business is, that for same order, application will generate duplicate data since two users are working on that order simultaneously.

I have tried to synchronize the method where I am generating the order, but that would mean that no other user can work on any new order since synchronize will put a lock for that method. This will certainly block all the users from generating a new order when one order is being progressed, since, it will hit the synchronized code.

I have also tried with criteria initilization for an order, but no success.

Can anyone please suggest a proper approach?? All suggestions/comments are welcome. Thanks in advance.

Answer 1

Instead of synchronizing on the method level, you may use block-level synchronization for the blocks of code which must be operated on by only one thread at a time. This way you can increase the scope for parallel processing of the same order.

Answer 2

On a grand scale, if you are backing up your entities in a database, I would advice you to look at optimistic locking.

Add a version field to your order entity. Once the order is placed (the first time) the version is 1. Every update should then come in order from this, so imagine two subsequent concurrent processes

a -> Read data (version=1)
     Update data
     Store data (set version=2 if version=1)

b -> Read data (version=1)
     Update data
     Store data (set version=2 if version=1)

If the processing of these two are concurrent rather than serialized, you will notice how one of the processes indeed will fail to store data. That is the losing user, who will have to retry his edits. (Where he reads version=2 instead).

If you use JPA, optimistic locking is as easy as adding a @Version attribute to your model. If you use raw JDBC, you will need to add the add it to the update condition

update table set version=2, data=xyz where orderid=x and version=1

That is by far the best and in fact preferred solution to your general problem.