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> What about doing this with both this approaches:
> - with a transformer if someone really wants to do this for a publisher

Yes, this is feasible through transformer, and two implementation details are, firstly we need identify the same counter, (same counter name, same resources_id, same instance_id etc), secondly we need keep a cache for compare, we need clean them when like instance destroyed.

I'm considering to implement a cached_transformer base class, which can be used for cpu_utilization, summary several counter for CW's network in/out etc.

> - for ceilometer-collector, so it inside the collector: it will always
>   know what the last value is, since it's stored in the database. So it
>   just has to fetch the last stored value from the db if it doesn't know
>   it, compute the delta, and stores it.
> 
> My main concerns in this, is that it'll be stored as a delta. We'll have
> to be clear that polling this kind of things, like delta in general
> actually, can't be done multiple times in parrallel. Something that is
> easily done -- though maybe not generally wanted -- with cumulative

I assume the data from same pollster in such situation. If two counter for same source comes from two pollster, the transformer can raise exception if needed.

--jyh