I've never used the locking features of Postgres before, and now I
find that I need to use them. I read through the instructions, but
I've never taken a database theory course so I don't understand all
the terms they used.
Here's what I need to do: The database keeps accounts. If one process
is accessing an account, no other process should be able to access it
at the same time. I need to do the following logical sequence of
things:
1. Lock the account
2. Check to see if the change to the account is permitted
3. Make the change
4. Unlock the account
I need this so that there isn't a race condition. Ie, if there are
$10 in the account, and one backen says "withdraw $9" and the other
also says "withdraw $9" at the same time, I need to make sure they
they don't execute at the same time, which would result in a negative
balance.
I tried to do this with SELECT FOR UPDATE but I couldn't get that to
work.
Thanks