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On Wed, Oct 31, 2012 at 1:39 PM, Julien Danjou <julien at danjou.info> wrote:

> On Wed, Oct 31 2012, Eoghan Glynn wrote:
>
> > Would we have also have some 'misses' with the cumulative approach
> > when the ceilometer agent was down?
>
> No, unless the counter resets several times while your agent is down.
> But delta has the same issue.
>
> > If I understood the (\Sigma local maxima)-first idea correctly,
> > the usage up to the first polling cycle would always be
> > discounted from any duration.
>
> No, because if you have:
>
> Time | Value
> 0    | 10
> 1    | 30
> 2    | 50
> 3    | 80
> 4    | 100
>
> If your delta-pollster is down at 1 and 2, you restart at 3, therefore
> at 4 you'll send "20" as usage (100 minus 80). So you miss the delta
>
between 10 (time 0) and 80 (time 3) (therefore 70 for free!).
> If you send right away 80 at time 3 when restarting, the API will be
> able to guess that between 0 and 3 the value went from 10 to 80.
> With delta approach, the API cannot guess that.
>

Sure it can, you just need to move where the caching is done. Using a local
cache to maintain the previous time a value was published you would know at
time 3 that the last published value was 10, and so send 70. So the total
will be correct.

Doug
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