SQLAlchemy(Postgresql) - Race Conditions

Question

We are writing an inventory system and I have some questions about sqlalchemy (postgresql) and transactions/sessions. This is a web app using TG2, not sure this matters but to much info is never a bad.

1. How can make sure that when changing inventory qty's that i don't run into race conditions. If i understand it correctly if user on is going to decrement inventory on an item to say 0 and user two is also trying to decrement the inventory to 0 then if user 1s session hasn't been committed yet then user two starting inventory number is going to be the same as user one resulting in a race condition when both commit, one overwriting the other instead of having a compound effect.

2. If i wanted to use postgresql sequence for things like order/invoice numbers how can I get/set next values from sqlalchemy without running into race conditions?

EDIT: I think i found the solution i need to use with_lockmode, using for update or for share. I am going to leave open for more answers or for others to correct me if I am mistaken.

TIA

Answer 1

If two transactions try to set the same value at the same time one of them will fail. The one that loses will need error handling. For your particular example you will want to query for the number of parts and update the number of parts in the same transaction.

There is no race condition on sequence numbers. Save a record that uses a sequence number the DB will automatically assign it.

Edit:

Note as Limscoder points out you need to set the isolation level to Repeatable Read.

Answer 2

Setup the scenario you are talking about and see how your configuration handles it. Just open up two separate connections to test it.

Also read up on FOR UPDATE For Update and also on transaction isolation level Isolation Level