Re: [eclipselink-users] JPA locking

[cowwoc <cowwoc@xxxxxxxxxxxxxxxx>]

James,

How do you deal with high-contention applications where optimistic locking
is going to run into collisions with high frequency?

As for recovering from OptimisticLockException, I'll give you a better
use-case. Say you have an online shopping cart application where each
product has X inventory items and users should be able to place an order so
long as there is at least one item left. The problem is that between the
time the user adds the item to the cart to the moment he completes the order
form at least one item might have already gotten sold. Now, you want to use
optimistic locking over the Product but you don't care if the inventory
count decreased so long as there is at least one left. How do you implement
this? And more generally, how do you implement "decrement" in a
high-contention scenario without lost updates? That is, if 10 people order
the same item at the same time, I don't want to ask 9 people to repeat the
order process because of a OptimisticLockException.

Thanks,
Gili


James Sutherland wrote:
> 
> I'm not sure where the lock exception would occur in your use case.  But
> in general, whether something is valid or not depends on the application
> and use case, for some applications no locking at all is perfectly valid.
> 
> In JPA there is no pessimistic locking at all, so there is no correct way
> to use pessimistic locking.  The lock() API acquires an optimistic lock,
> not a pessimistic lock.  It means that the version will be checked, or
> updated on commit, it does not matter when it is called in the
> transaction, as the check occurs on commit.  The link gives a hack to
> attempt to simulate pessimistic locking using an update, it may work in
> some cases.  In EclipseLink you can just use the query hint to acquire a
> pessimistic lock.
> 
> 
> 
> cowwoc wrote:
>> 
>> Hi James,
>> 
>> I've been reading http://en.wikibooks.org/wiki/Java_Persistence/Locking
>> and it's a godsend... I've been looking for this kind of straight-forward
>> discussion for a long time now. So first of all, thank you for the
>> article, I really appreciate it! :)
>> 
>> Now, as for
>> http://en.wikibooks.org/wiki/Java_Persistence/Locking#Handling_optimistic_lock_exceptions
>> I was wondering, is it reasonable to automate handling of the
>> OptimisticLockException in the following case?
>> 
>> - Server holds a collection of images
>> - Client requests a random image
>> - Server queries the total number of images
>> - Server selects a random number
>> - Server queries image at index X but it turns out that the image was
>> deleted since the previous query
>> 
>> In such a case, the client really did nothing wrong. From it's point of
>> view the request is still valid (no stale data, nothing that would cause
>> it to change the request in any way). As such, would it be reasonable for
>> the server to automatically replay the request and select another image
>> randomly?
>> 
>> Secondly,
>> http://www.funofmath.com/weblog/2008/03/implementation-of-pessimistic-locking.html
>> indicates that the only way to lock pessimisticly (without race
>> conditions) is by doing:
>> 
>>     Object entity = entityManager.getReference(clazz, id);
>>     entityManager.lock(entity, LockMode);
>> 
>> I was wondering whether this is portable across all JPA vendors and
>> whether you could please add this to the Wiki page? Also, you probably
>> want to revise
>> http://en.wikibooks.org/wiki/Java_Persistence/Locking#Example_of_Using_the_Lock_API
>> as it seems to be open to a race condition (you should be locking the
>> Manager before reading his salary, not the other way around.)
>> 
>> Thank you,
>> Gili
>> 
> 
> 

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