John Machin <sjmac...@lexicon.net> writes:
> 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.

It's made for apsw and it shouldn't do anything under the hood. Consider
the same example in C if it helps.

>>>> 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?

Of course you have to test once you have defined proper behavior. But my
question in this thread is essentially what the "proper" behavior is.

Best,

   -Nikolaus

-- 
 »Time flies like an arrow, fruit flies like a Banana.«

  PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6  02CF A9AD B7F8 AE4E 425C

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