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On Tue, Nov 27, 2012 at 4:49 AM, Julien Danjou <julien at danjou.info> wrote:

> On Mon, Nov 26 2012, Doug Hellmann wrote:
>
> > I understand Julien's reluctance to have the pollsters emitting computed
> > data, but tracking values inside the pollster so we can send a delta
> > instead of the cumulative value we have now seems like a relatively
> > straightforward fix for this.
>
> I may have a better idea.
> The problem with doing that in the pollster, is that:
> 1. it's bad bad bad bad
> 2. it's inefficient since you may lose values on restart
>    you won't know what the value was before
>

We will if we persist them on the compute node.


> 3. it's not sure all publishers targets will want delta
>

True. We may need to send gauge and delta events (or combine the values in
a single event).


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

That doesn't work if different collector instances receive events from the
same meter on the same resource in parallel (which could happen easily if
the queue gets backed up).


>
> 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
> counters.
>
> Thoughts?
>
> --
> Julien Danjou
> ;; Free Software hacker & freelance
> ;; http://julien.danjou.info
>
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