Puppet puzzle : parametrized classes - common logic

gistfile1.txt

#First class = role web (calls logstash::common to setup directory structure)
class logstash::web(
   $version = '1.1.0beta8'
) {
   if !defined(Class['logstash::common']) {
      class {'logstash::common':
         version => $version
      }
   }
}

#Second class = role shipper (calls logstash::common to setup directory structure)
class logstash::shipper(
   $version = '1.1.0beta8'
) {
   if !defined(Class['logstash::common']) {
      class {'logstash::common':
         version => $version
      }
   }
}

# Now on the same node:
class {'logtash::web':}
class {'logtash::shipper':}

#---> this will tell me I'm redefining class 'logstash::common'. Fair enough
# The solution I found is to use include logstash::common 
# But that doesn't allow me to use params? just include
# Is there something like virtualized classes ?
# Or maybe I need to move my classes to defines?

# A possible solution is to use `if !defined`.  You'll need to add a dependency
# if you want one to have precedence.