Hi, I develop telecommunication software and I have encountered problem with isolation level in Postgres. Our database receive special packets that informs about end of call, and from time to time (it happens when such packets arrive almost in the same time e.g 8ms difference) one call charge user account twice. Look at following diagram that clarify what exactly happen: http://gdn.superhost.pl/pub/RozjazdKontWyjasnienie.jpg I think problem is because we use default Read Commited isolation level. In presented example value of credit should be changed only if call_status<>FINS and first transaction after modification of credit value set call_status to FINS. This should prevent from second modification of credit (bacause call_status=FINS), but in our systems sometimes such protection does not work. I think that between check of call_status and update of credit is small window that cause that second transaction cannot see results of first transaction (=second transaction cannot 'see' that call_status=FINS) I think that switching to serializable level can fix this issue but we cannot do this because we afraid of side effects of such strict isolation level. Second thought is to use explicitly ACCESS EXCLUSIVE lock on CDR table but does it help? Michal http://blog.szymanskich.net
