--- In firebird-support@yahoogroups.com, Eugen Stoianovici
<eugen.stoianovici@...> wrote:

>
> How does firebird's versioning work on transactions? If i have..let's
> say a stocks application. At some point there are 10 items A in stock
> Users U1 and U2 both sell 8 A's roughly at the same time:
>
> 1. --U1 : begin transaction--
> 2. --U2 : begin transaction--
>
> 3. U1: Insert some sale info into sale table
> 4. U1: Check if there are 8 A's available (true at this poing)
> 5. U2: Insert some sale info into sale table
> 6. U1: Update stock for A to stockA-8 items available
> 7. U2: Check if there are 8 A'2 available (also true at this point?)
> 8. --U1: Commit transaction--
> 9. U2: Update stock for A to stockA-8 items available (which would set
> stock for A at -6)
> 10 --U2: commits--(does firebird accept this transaction?)
>
> From what i understood reading the docs, U2 (the second transaction)
> doesn't see what U1 does until U2 commits. Am i right? How does

firebird

> handle this issue? How should i handle this issue?
>
> Thank You
>

Hi Eugen,
Qty in stock is volatile information : you will not know if it goes
bellow zero until you process a sell. Create a before update trigger
on stock table and raise exception if new.cant < 0 the sell.