Hi David & Philip,
I have not had the time to pay attention to the use case it deserves --
but reading the case brings up certain aspects that I will like to share
with your experience.
This interesting use case can fail and is failing. But the issue it
reveals goes beyond locking semantics.
Behavior of lock is described at datum level -- the levels of warranty for
shared access/mutation to a *single datum* in a consistent way.
Transaction goes to the next stage and describes the level of warranty of a
set of datum as an atomic 'unit of work'.
But this test case demands a even higher level of warranty -- consistency or
invariance of a set-based property (in this case the odd-even parity of 3
instances), which is neither the property of an individual datum nor the
property of a unit of work.
Of course, optimism of optimistic transaction model results in weaker
warranty of set-based invariance. To ensure set-based property invariance, a
transaction must commit all 3 instances (with consistent odd-even parity) as
a unit of work, but what it does is that it reads {A,B,C} and writes only
{B, C}.
I will refrain from describing what flags of which OpenJPA configuration
property can be tweaked to get there because let me first hear your comments
on the expressed views in this posts.
Philip Aston wrote:
>
> Hi David,
>
> Thanks for confirming this. So to summarise where we are, we have:
>
> 1. A reasonable use case that can fail with some unlucky timing.
>
> 2. A technical test case demonstrating the problem that does not rely
> on unlucky timing.
>
> 3. A disagreement in our readings of whether 1 and 2 are spec.
> compliant. Personally, I don't share your reading of the spec. In my
> reading, read locks are safe and provide a concrete guarantee that if
> locked entity is changed by another transaction, the locking transaction
> will not complete.
>
> (This is a different QoS compared to a write lock - if a write lock is
> obtained and the pc flushed, the transaction knows that it will not fail
> due to another transaction updating the locked entity. Read locks are
> "more optimistic" and can support higher concurrency if there is minimal
> contention - many transactions can hold read locks, only one can hold
> right locks.).
>
>
> How can I convince you to change your interpretation of the spec? Anyone
> else have an opinion?
>
> FWIW, EclipseLink passes the test case.
>
> - Phil
>
> dezzio (via Nabble) wrote:
>> Hi Philip,
>>
>> Let's take a closer look.
>>
>> We have two bank accounts, Account[1] and Account[2], shared
>> jointly by customers Innocent[1] and Innocent[2]. The bank's
>> business rule is that no withdrawal can be made the draws
>> the combined total of the accounts below zero. This rule is
>> enforced in the server side Java application that customer's
>> use.
>>
>> At the start of the banking day, the accounts stand at:
>>
>> Account[1] balance 100.
>> Account[2] balance 50.
>>
>> Innocent[1] wants to draw out all the money, and asks the
>> application to take 150 from Account[1]. Innocent[2] also
>> wants to draw out all the money, and asks the application to
>> take 150 from Account[2]. By itself, either transaction
>> would conform to the bank's business rule.
>>
>> The application implements the withdrawal logic by doing the
>> following for each transaction.
>>
>> For Innocent[1], read Account[1] and Account[2]. Obtain a
>> read lock on Account[2]. Refresh Account[2]. Deduct 150 from
>> Account[1]. Verify business rule, result, sum of balances =
>> 0. Call JPA commit.
>>
>> For Innocent[2], read Account[1] and Account[2]. Obtain a
>> read lock on Account[1]. Refresh Account[1]. Deduct 150 from
>> Account[2]. Verify business rule, result, sum of balances =
>> 0. Call JPA commit.
>>
>> Within JPA commit, as seen over the JDBC connections, the
>> following time sequence occurs. (Other time sequences can
>> yield the same result.)
>>
>> Innocent[1]: Check version of Account[2]: passes.
>>
>> Innocent[2]: Check version of Account[1]: passes.
>> Innocent[2]: Update balance of Account[2], withdraw 150,
>> setting balance to -100: does not block.
>> Innocent[2]: commit: successful
>> Innocent[2]: Receives 150.
>>
>> Innocent[1]: Update balance of Account[1], withdraw 150,
>> setting balance to -50: does not block.
>> Innocent[1]: commit: successful.
>> Innocent[1]: Receives 150.
>>
>> After the two transactions:
>>
>> Account[1]: balance -50
>> Account[2]: balance -100
>>
>> Clearly the bank would not be happy. What's a developer to
>> do?
>>
>> I think the developer needs an education about what is meant
>> by the JPA spec. What JPA is guaranteeing is that when JPA
>> commit is called, the objects with read locks will have
>> their versions checked. The objects with write locks will
>> have their versions checked and changed. The objects that
>> have been modified will have their versions checked, their
>> information updated, and their versions changed. Clearly all
>> of these rules were enforced in the above example.
>>
>> If the developer had used write locks, both transactions
>> would not have succeeded. In fact, for the above example and
>> a similar time sequence, if write locks had been used in
>> place of read locks, there would have been deadlock.
>>
>> Now, if in fact, I'm wrong about my interpretation of the
>> JPA spec (and it wouldn't be the first time) then you have a
>> case. I'd be curious to know whether other JPA
>> implementations pass your elegant test case, and what they
>> are doing differently that makes it so.
>>
>> Also, if I am wrong about my interpretation, then the JPA
>> TCK needs a test case that will snag this failure, because
>> OpenJPA passes the current JPA TCK.
>>
>> Cheers,
>>
>> David
>>
>> Philip Aston wrote:
>>
>>> Oh yeah - my bad. Try this one instead:
>>>
>>> Suppose there are a set of Accounts, and a business rule that says
>>> that the net balance must be positive.
>>>
>>> Innocent wants to draw down on Account 1 as far as possible. It read
>>> locks the of Accounts, sums up the value, and and subtracts the
>>> positive total from Account 1. Innocent begins its commit, and its
>>> read locks are validated.
>>>
>>> Meanwhile InnocentToo does the same for Account 2, and commits.
>>>
>>> Innocent updates Account 1 and finishes its commit.
>>>
>>> The total in account summary is now negative, violating the business
>>> rule. If read locks worked as I think they should, Innocent would have
>>> received an OptimisticLockException.
>>>
>>>
>>>
>>> dezzio wrote:
>>>> Hi Philip,
>>>>
>>>> When two transactions read the same version of AccountSummary, both
>>>> cannot successfully update its sum. Only one will successfully commit.
>>>>
>>>> David
>
>
>
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