On 6/06/2009 8:19 AM, Nikolaus Rath wrote:
> John Machin <sjmac...@lexicon.net> writes:
>>> Now I'm confused. I want to know if it will be sufficient to wrap my
>>> last_insert_rowid() call between BEGIN .. and END in order to make it
>>> return the rowid that was last inserted by the same thread even if
>>> multiple threads are using the same connection (but different cursors).
>>>
>>> As I understand Nuno, he is saying that this is sufficient. Igor OTOH is
>>> saying that it's not sufficient, I need to use additional mechanism.
>> As Igor pointed out, if you have multiple threads using the same
>> connection, you ALREADY need mutexes or whatever to maintain atomicity.
>> If you don't have that, yes you need to "use additional mechanism" BUT
>> this constitutes an EXISTING bug in your code. Perhaps Nuno should have
>> added a rider like "(presuming your existing code is not stuffed)".
>
> Are you saying that I need to use mutexes or whatever in the following
> program?
>
> def thread1():
> cur = conn.cursor()
> for i in range(500):
> cur.execute("INSERT INTO tab1 (no) VALUES(?)", i)
>
> def thread2():
> cur = conn.cursor()
> for i in range(500):
> cur.execute("INSERT INTO tab2 (no) VALUES(?)", i)
>
> threading.Thread(target=thread1).start()
> threading.Thread(target=thread2).start()
Somebody needs to use mutexes somewhere. You have obscured the question
by introducing two unknowns: (1) Which Python wrapper are you using
(sqlite3 module or the apsw module)? (2) What does it do under the
covers? Try asking the relevant guru for whatever you are using.
>>> Where am I wrong?
>> In wasting time on semantic confusion instead of implementing it and
>> testing the bejaysus out of it.
>
> When you are working with a multithreaded program, you can't even hope
> to test a fraction of the possible state trajectories. Finding the
> proper implementation by trial & error is therefore even more hopeless
> than in a single threaded program.
If you can't test it, then how will you know whether *any*
implementation is "proper", let alone *the* "proper" one?
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