[Kamailio-Devel] postgres queries overlapping

[Henning Westerholt]

On Wednesday 30 July 2008, Jason Penton wrote:
> >> one thing I noticed in the mysql module (may be similar in postrges) is
> >> that a static char array is used to build the sql query. This is however
> >> NOT thread safe as we saw on multiprocessor Solaris boxes. Basically 2
> >
> > Kamailio does not use threads, but processes. Is this also an issue in
> > multi-process applications?
>
> it seems it is an issued becuase under high load the mysql buffer was beign
> overwritten resulting in bad SQL queries. This continued until I put
> mutexes around the static SQLQuery buffer in the mysql module. Maybe the
> multi-porcess programmers can shed some light? I just thought that this may
> be a similar probem to what the orginal poster is experiencing.

Hi Jason,

i assume that you refer to the 1.3 branch (in 1.4 static buffer is placed in 
the core DB API). Every worker children of the server has each own buffer, so 
normally queries could not overlap, if every module behaves correctly. This 
structure (with the static buffer) is basically used since the fork of 
OpenSER.

If some module does not free some results correctly, for example in error 
conditions (see today thread on users), then this could cause the errors 
you've described. Another issue is wrong initialization (for example in an 
out of tree module), which caused the problems observed in past 
thread "db_mysql commands out of sync" on devel. In this problem several 
mysql connections shares the same thread_id, and thus queries overlapped.

The code in trunk could of course changed to set locks around the buffer, or 
make this stuff non-static. But i'm not assured this fix the real cause of 
the problem, it would also decrease performance. Even if the whole DB API was 
made a little bit more abstract recently, it still avoids superflous work, 
although if this could mean some lack of robustness in certain error 
conditions.

Do you could share some more details, for example error messages?

Cheers,

Henning