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Excerpts from Simon Pasquier's message of 2013-09-17 05:57:58 -0700:
> Hello,
> 
> I'm testing stack updates with instance group and wait conditions and 
> I'd like to get feedback from the Heat community.
> 
> My template declares an instance group resource with size = N and a wait 
> condition resource with count = N (N being passed as a parameter of the 
> template). Each group's instance is calling cfn-signal (with a different 
> id!) at the end of the user data script and my stack creates with no error.
> 
> Now when I update my stack to run N+X instances, the instance group gets 
> updated with size=N+X but since the wait condition is deleted and 
> recreated, the count value should either be updated to X or my existing 
> instances should re-execute cfn-signal.

That is a bug, the count should be something that can be updated in-place.

https://bugs.launchpad.net/heat/+bug/1228362

Once that is fixed, there will be an odd interaction between the groups
though. Any new instances will add to the count, but removed instances
will not decrease it. I'm not sure how to deal with that particular quirk.

That said, rolling updates will likely produce some changes to the way
updates interact with wait conditions so that we can let instances and/or
monitoring systems feed back when an instance is ready. That will also
help deal with the problem you are seeing.

In the mean time, cfn-hup is exactly what you want, and I see no problem
with re-running cfn-signal after an update to signal that the update
has applied.