Discriminator Pattern

This is a Control flow pattern in Workflow Patterns.

Suppose that

  • two processes are running in parallel
  • merge is activated when one of them ends
  • the result of the other is just ignored


Example 1:

  • to speed up a query we send it to two databases
  • once the first answer arrives we proceed
  • and just ignore the second answer


There are 6 kinds of Discriminator pattern [1]:

  • the Structured Discriminator (WCP9),
  • the Blocking Discriminator (WCP28),
  • the Cancelling Discriminator (WCP29),
  • the Structured Partial Join (WCP30),
  • the Blocking Partial Join (WCP31) and
  • the Cancelling Partial Join (WCP32).

Canceling Discriminator

This is a discriminator that cancels the other process [2]

In YAWL this is achieved:

  • with one Parallel Split (AND-Split)
  • a Cancellation Region for the two processes
  • one Simple Merge (XOR-Join) that is also the cancel task for the cancellation region


What if we don't want to interrupt the 2nd activity?

  • for example, in a hospital we want to do two tests
  • we start treating the patient as soon as the first result arrives
  • but we cannot discard the results of the second result
  • so we do the full analysis when we have both results


  • this way we start treatment as soon as possible
  • but start the full diagnosis only when the second arrive

We may as well add a timing constrain

  • we wait for the result only for 24 hours
  • and then do the diagnosis based only on partial information


if $A$ has finish, but $B$ hasn't

  • case 1:
    • timeout task times out
    • then it fires and takes token from $B$
    • and also puts a token to the input place for full diagnosis
    • full diagnosis can start
  • case 2:
    • timeout starts to count down
    • but $B$ finishes
    • full diagnosis starts
    • it takes the token from the timeout task so it is no longer enabled