Order of Execution - Process Builder

Question

I have the following process triggering when an Opp generates an Order:

1. Opp -> Order (Draft status)
2. Platform Event Trigger (based on Order Status change)
3. PE Flow Subscriber: Sets Order Status to Order Received
4. Process Builder A (IF Order Status = Order Received) - Check if Order should be automated and tick 'Auto' checkbox if is.
5. Process Builder B (IF 'Auto' field = TRUE) - Auto process the order

The problem I have is that everything is fine up to 4), however, 5) is being checked once and not actioning. If I activate recursion on 5) then it works. And it also works if I convert 4) into a platform event subscriber...

But I want to understand why it's not working, please. I've checked a number of 'Order of Execution' guides but it's still not helping. Thanks for your help.

Answer 1

Each phase of automation (Workflow, Flows, and Process Builders) are designed in a way so as to simulate parallelism (e.g. as if all elements executed simultaneously). That means that each Process Builder gets the same, pre-modified version of the record, even if they "modify" the record.

More specifically, each element is first evaluated, any actions that are met will be queued for execution, and then all actions are executed only after all elements are done.

This pseudocode attempts to explain how this looks internally:

ProcessBuilderAction[] queue = new ProcessBuilderAction[0];
for(ProcessBuilder element: getProcessBuilders()) {
  for(ProcessBuilderStep step: element.getSteps()) {
    if(step.shouldExecute()) {
      queue.add(step.actionsToExecute());
    }
 }
 for(ProcessBuilderAction action: queue) {
   action.execute();
 }

This is why you need the recursion feature enabled to make it work. Without it, the second Process Builder doesn't get to see the changes.

Answer 2

It's not working because you can't guarantee that process builders will fire in any particular order. That's why it's always recommended that there is only one process builder per object.