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
Examples
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
Types
There are 6 kinds of Discriminator pattern [http://www.workflowpatterns.com/patterns/control/index.php]:
- 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 [http://www.workflowpatterns.com/patterns/control/new/wcp29.php]
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