Handling of concurrent events

• Null server - no universal solution
  • its output may be blocked → null server stores entity
  • no problem, if an (unlimited) queue follows
• Careful with null server between queue and server
  • example timeshared/CPU
  • 
  • server busy → next entity sits in the null server
  • has to be taken into account, e.g. for computation of queue length!
• Null server workaround conceptually wrong
  • its purpose is not related to storage → easily forgotten
  • creates unexpected problems