Re: DB uniqueness constraints and dealing with them
Re: DB uniqueness constraints and dealing with them
- Subject: Re: DB uniqueness constraints and dealing with them
- From: Q <email@hidden>
- Date: Fri, 4 Jul 2008 17:08:58 +1000
On 04/07/2008, at 1:01 PM, Florijan Stamenkovic wrote:
The save -> error check is exactly what I had in mind, sorry if I
was not clear about it. However, I would rather like to delete the
newly saved EO, and throw a validation exception. But this is only
possible if I can find the exactly same EO, once before it saved to
the db, and once after. Is that possible? Huh, never thought about
this.
To answer Q's question, yes, it is a race condition, but a rather
funky one. Because each thread, regardless in which app instance or
even which machine they are one, checks for the record uniqueness
after it has saved it. So, it can happen that there are two records
saved that violate the uniqueness, but it will be detected. And the
EO created in the thread that detected the uniqueness violation
should be deleted. As far as I can see, this can never result in
having the uniqueness violated. In some circumstances however it
could result in all attempts failing. That would be the race
condition in this setup... If both processes manage to save at
first, then the first process detects the violation, but the second
process kicks in before the first deletes it's EO, then the second
one will also detect a violation. Both fail. See what I mean?
Now try and think about the case for three simultaneous violations,
network, database or application level disruption and how to deal with
contention and retries and your head will start to really hurt.. I
know mine does.
In a word, don't try it, ok that's three words. Unless you can do it
atomically with either database level uniqueness, or a table level
locking you are going to just torture yourself.
But perhaps I do not see it clearly, and somehow uniqueness could be
violated? I've been trying to run multithreaded (non synchronized)
scenarios in my head, and I can never come to one in which the
uniqueness is violated and not detected.
There are probably plenty of reasons, they just aren't obvious. What
if the detection detects a conflict but the reversal fails?
--
Seeya...Q
Quinton Dolan - email@hidden
Gold Coast, QLD, Australia (GMT+10)
Ph: +61 419 729 806
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