This is more of a PSA post rather than a question, but I thought I'd put it up here to a) help anyone else out who is confused by this and b) hopefully get Appian's attention to fix this (I will also make a support case for this issue).
This is regarding how the XOR node in the process modeler evaluates multiple conditions. Looking at how the Decision tab is laid out (with IF...ELSE IF...ELSE IF labels and the ability to change the order of the conditions) my first thought would be that the conditions evaluate in order from top to bottom.
However, in the documentation for this node it states:
But then in other places the same page states:
The actual behavior of the XOR node indicates that the order of the conditions does not matter, all are evaluated simultaneously. I hope that Appian changes the behavior of the node to execute conditions in order as this would be much more useful.
If I have misunderstood anything in the documentation, please feel free to correct me, because as of now I don't really see any value to an XOR node when compared to an OR node.
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I think this is more a mishap in the documentation, than a problem in the implementation. I never had an issue, but strictly follow the if..else top-to-bottom approach and never had an issue.
Peter Lewis , what are your thoughts?
The important thing to remember is that all the conditions will be evaluated (as opposed to one at a time until reaching a true condition). What that means is if there are two conditions, one which checks if a list is null, and the next that tries to remove a value of the list, an error will be thrown, since the second condition does not have a null check, even though the first path should be taken. Screenshots attached.
I ran into an issue with this today, but I believe I have figured out why. My problem was that condition 3 was throwing an error, but (in my mind) condition 3 should never have been evaluated because condition 2 was true (which prevents that error). Basically, I was expecting it to operate like an a!match statement. However, we're now thinking the node evaluates all conditions at the same time, and then takes the path of the first (topmost) true condition. This would make much more sense and explain why both "conditions aren't evaluated in order" and "condition order does matter" are present in the documentation. I think the documentation could make this a lot clearer though.
Yes, this was exactly my issue (getting an error for a condition that I didn't expect to run). I was then confused because, if order didn't matter in an XOR I didn't really see much difference between it and an OR. But I think I have a better understanding of how it works now (see my reply to Stefan).
So you mean something like that the code for all conditions is executed in batch, but to find the correct outgoing path, the resulting values are evaluated top-to-bottom?
Yeah it's unfortunate that it doesn't work like "and()", where conditions that might throw an error can be safely "hidden" underneath prior checks against that condition. I've just accepted this as one of those foibles you need to get used to - i assume it's in their "process modeler quality of life common sense enhacements" back-lot dumpster.
That's right! It is done that way for performance reasons.
It's actually done this way purposefully, so that if you have some expressions that take a while, we evaluate them in parallel.
James Lepone said:for performance reasons
That's a surprise to me though, because it seems like performance would generally be improved by allowing XOR to not have to calculate the evaluation expressions for lines it would never even get to.
Think of it this way. I have 4 conditions. The 4th one will be true. Each expression takes 2 seconds. Evaluating in a batch lets us have a result in 2 seconds, if we evaluated serially it would be 8 seconds before we could move on.