[kepler-dev] event queuing on input ports

[ian.brown@hsbcib.com]

We've recently tracked down a subtle logic problem in one of our models. 
We had a strategy expressed as a modal model which we then re-implemented 
as a python actor for increased speed. The issue we had was that the 
results were different even though the logic looked the same.
We finally tracked it down to the fact that tokens queue up on the python 
actor's input ports whilst they don't on the modal model.
There's nothing wrong with this - it's just that we didn't realise that 
was what was happening until we investigated a bit more deeply. To 
illustrate the issue, I have attached a simple model which just uses a 
clock, a delay and an expression actor. In order for it to work as we 
expect, we need a sampler actor too. This would be a good example to have 
in the documentation because as a user, the detail of how this worked was 
not clear.

Now, generally, in my domain, if events queue between director firings 
then it is almost certainly an error in the model. What I would like to do 
is to be able to check during a test run that all of the input ports for 
all of my actors are empty between director firings. In other words, they 
should be empty when the model time is incremented but they can have 
tokens when the microstep is incremented. In this case, the check would 
flag that the top model in the example was in error.
I could just add some code to the DE Director to check for this condition, 
but that does not seem very elegant. I was thinking that it could be 
possible to add a model attribute (like the dependency highlighter) which 
would perform this check if it was present. Does that sound like a good 
idea? Does anyone have any better suggestions for implementation and has 
anyone solved any similar analysis problems?
One reason for using Ptolemy / Kepler for this work rather than pure Java 
is the theoretical ability to better check the 'correctness' of the models 
... and this is a step in that direction.



Ian


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