On 10/20/2009 12:16 PM, Steve Ebersole wrote:
See my reading of P2 though is more than just ensuring a repeatable
read.
If T1 does lock(entity, LockModeType.PESSIMISTIC_READ) we need to make
sure T2 cannot update or delete that row b4 T1 completes. That's
blocking, not simply repeatable read.
I guess we get into how repeatable read isolation level is implemented
and whether it could be implemented without at least a read lock on the
database table row. Or did you mean something else?
I'm fine with requiring at least a database read lock (if everyone is
happy with that).
On Tue, 2009-10-20 at 17:55 +0200, Emmanuel Bernard wrote:
> On 20 oct. 09, at 17:46, Steve Ebersole wrote:
>
>
>> Just wanted to point out that "checking" the isolation level may not
>> be
>> accurate. HSQLDB, for example, lets you set any isolation you want
>> and
>> dutifully reports it back even though it (used to, at least) only
>> supports READ UNCOMMITTED. But, I personally think we need to just
>> assume that we are running in at least READ COMMITTED (HSQLDB in a
>> single user environment would behave as if READ COMMITTED). READ
>> UNCOMMITTED has to be universally considered evil in real practice :)
>>
> I think the spec assumes Read committed isolation at minimum, so we
> should be good.
>
>
>> WRT PESSIMISTIC_READ, it really comes down to the intent. Is this
>> supposed to stop other writers from writing to the given data until we
>> are done with it? Aka, would this be an intention lock (I intend to
>> update this data so don't let other writers in here right now)? Or
>> merely a guarantee of repeatable read?
>>
> Here is the contract
> If transaction T1 calls lock(entity, LockModeType.PESSIMISTIC_READ) or
> lock(entity, LockModeType.PESSIMISTIC_WRITE)on an object, the entity
> manager must ensure that neither of the following phenomena can occur:
> • P1 (Dirty read): Transaction T1 modifies a row. Another
> transaction T2 then reads that row and obtains the modified value,
> before T1 has committed or rolled back.
> • P2 (Non-repeatable read): Transaction T1 reads a row. Another
> transaction T2 then modifies or deletes that row, before T1 has
> committed or rolled back.
>
> Any such lock must be obtained immediately and retained until
> transaction T1 completes (commits or rolls back).
>
>
>
>