如何防止数据库更新的并发问题？

Question

I have the following sequence which may be executed by more than one instance of my program at any time:

Step 1: Check to see whether bill is unpaid

Step 2: If bill is unpaid, then mark bill as paid

I am worried that Instance 1 and Instance 2 of my program will both do Step 1 at almost exactly the same time and therefore proceed to both do Step 2. How can I prevent this happening?

Answer 1

Using a (serializable) TRANSACTION . More details depend on your database platform

See http://en.wikipedia.org/wiki/Database_transaction for details

Answer 2

It's hard to be concrete about this without seeing code, but the obvious way to do this is to combine steps 1 and 2.

So, instead of:

$invoice_status = call_database(
select INVOICE_STATUS
from   INVOICES
where  INVOICE_ID = $invoice_id)

if ($invoice_status = UNPAID){
   call_database (
      UPDATE INVOICES 
      SET INVOICE_STATUS = PAID 
      WHERE INVOICE_ID = $invoice_id)
}

You would do something like

call_database(
   update invoices
   set    invoice_status = PAID
   where  invoice_id = $invoice_id
   and    invoice_status = UNPAID)