[Zope] Counter Rentrancy

Hi, If I have a DTML document for generating and tracking say order numbers, is the code rentrant/Atomic? <dtml-call "REQUEST.set('counter', counter + 1)"> <dtml-call "manage_changeProperties( counter=REQUEST['counter'] )"> <dtml-return counter > What I'm really asking is how many threads run in parallel, and how would multiple threads accessing this code react. Do I need some form of transation or locking? Thanks in anticipation David Kyte