How to handle requests sent to mock in parallel

Hi,

we have the following solution. Applicaiton based on Mule takes txt-file, parse it and sends WS-requests in parallel. I receive these these requests on mock and trying to count how many of them was sent. I put simple counter code in OnRequest section of mock. But since these requests are sent in parallel I have never got correct number. There is a situation happen when two requests come simultaneously and my counter is not updated properly. Please help what can I use to handle this kind of situation
Thanks in advance

Misja