How can I assure message ordering in SwitchYard?

Issue

• We want to be able to have a service that receives requests asynchronously, but keep the order of requests that have the same group key. In other words, requests with the same group key should execute synchronously in the same order they were received.

• For example, if there are 3 requests to a service, where 2 of them are for the same customer:

  #r1 - customer=1
  #r2 - customer=2 
  #r3 - customer=1
  

• then we want the third request (r3) to wait the first one (r1) finishes before it starts (because they are with the same group key - customer = 1), but at the same time the second request (r2) to happen asynchronously.

• What is the best way to implement that in SwitchYard?