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From David Lutterkort <>
Subject RE: normalised metric names?
Date Thu, 06 Sep 2012 22:43:23 GMT
On Sun, 2012-09-02 at 13:53 +1000, Koper, Dies wrote:
> Looking at the EC2 Cloudwatch sample Michal once shared with me, I'm
> still not sure how to interpret their data and how it should be modelled
> in DC.
> Let me copy and paste part of the XML he sent me:
> <?xml version="1.0" encoding="UTF-8"?>
> <metric href="http://localhost:3002/api/metrics/i-5cac1c24"
> id="i-5cac1c24">
>   <entity>i-5cac1c24</entity>
>   <properties>
>     <NetworkIn>
>     <sample>
>       <property name="timestamp" value="2012-07-09T12:38:00Z"/>
>       <property name="unit" value="Bytes"/>
>       <property name="minimum" value="42.0"/>
>       <property name="maximum" value="42.0"/>
>       <property name="average" value="42.0"/>
>       <property name="samples" value="1.0"/>
>     </sample>
>     <sample>
>       <property name="timestamp" value="2012-07-09T12:34:00Z"/>
>       <property name="unit" value="Bytes"/>
>       <property name="minimum" value="84.0"/>
>       <property name="maximum" value="84.0"/>
>       <property name="average" value="84.0"/>
>       <property name="samples" value="1.0"/>
>     </sample>
>     <sample>
> ...
> What I observe is:
> 1. The 'samples' value is always 1. I suppose that's why a sample's min,
> max and average are the same. In what case would samples not be 1?
> 2. Even though the 'samples' value is 1, clearly it returns multiple
> samples (=measurements?) over a period (every 6 min. judging from the
> timestamp).
> The only useful data I get is the 42 and 84 and their timestamps. FGCP
> returns that data too, it just doesn't call them average, min and max.
> Do we need to include it three times in the DC API?

The important thing to me in the XML format is to prepare the format for
when the backend does only produce aggregates for multiple measurements.
I am not sure under what circumstances EC2 produces metrics for more
than one sample, but I think the general idea is sound: allow the
backend to do some aggregation of samples, and only report certain
staistics for them.

That in the case of nsamples = 1 average, min, and max are redundant is
somewhat unfortunate, and I would be perfectly fine to say that we only
expect backends to report an average, with min and max being optional.

IOW, for FGCP, I'd be fine if the XML looked something like

  <sample timestamp="..." count="1" unit="...">
    <entry name="average" value="42.0"/>

That gives us the flexibility to report more staistics for count > 1.


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