Can squid get the updated data from the master web for the slave web when the clients access the slave web?

 Someone told me Squid can do the following,
 but I don't know how, could anyone help me
 on this?
 
 The following is two web servers
 WEB M is the master web server,
 it will be updated by other source all the time.
 
 WEB S is the salve web server
 it will be updated only if
 when the clients access the WEB and find the data
 IS NOT UP-TO-DATE AS the WEB M.
 
 Someone told me the Squid can handle this.
 Thanks a lot.
    ________ ________
   | | | |
   | WEB M |<--------| WEB S |
   | | | Squid |
   |________| |________|
                                      |
                                      |
                                      |
                                 Clients