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>> Whats the underlying problem here? nova notifications aren't 
>> versioned?  Nova should try to support ceilometer's use case so
>> it sounds like there is may be a nova issue in here as well.
> 
> Oh you're far from it.
> 
> Long story short, the problem is that when an instance is detroyed,
> we need to poll one last time for its CPU, IO, etc statistics to
> send them to Ceilometer. The only way we found to do that in Nova
> is to plug a special notification driver that intercepts the
> deletion notification in Nova, run the pollsters, and then returns
> to Nova execution.

Doesn't this just mean that Nova needs to do an extra poll and send an
extra notification? Using a special notification driver, catching the
delete notification, and polling one last time seems extremely fragile
to me. It makes assumptions about the order things happen internally
to nova, right?

What can be done to make Ceilometer less of a bolt-on? That seems like
the thing worth spending time discussing...

--Dan