Multiple user access for single file

Question

Dears ,
I am building a web-application that will send xml files to a web-service to perform several operation . Multiple users will need to access a single xml file and they will need to ( READ , WRITE ) on the same file , so if a user is writing and another user is reading from the same file we will have a big error in this manner . So please assest me in this issue .
 

thank you ,