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[Loic Dachary <loic@xxxxxxxxxxxx>]

> - The interesting metadata for a resource may depend on the type of
> resource. Do we need separate "tables" for that or can we normalize
> somehow?
> - How do we map a resource to the correct version of its metadata at
> any given time? Timestamps seem brittle.
> - Do we need to reflect the metadata in the aggregation API?
>
Hi,

I started a new thread for the "metadata" topic. I suspect it deserves it. Although I was reluctant to acknowledge that the metadate should be stored by the metering, yesterday's meeting made me realize that it was mandatory. The compelling reason ( for me ;-) is that it would make it much more difficult to implement a billing system if the metering does not provide a simple way to extract metadata and display it in a human readable way (or meaningfull to accountants ?) .

I see two separate questions :

a) how to store and query metadata ?
b) what is the semantic of metadata for a given resource ?

My hunch is that there will never be a definitive answer to b) and that the best we can do is to provide a format and leave the semantic to the documentation of the metering system, explaining the metadata of a resource.

Regarding the storage of the metadata, the metering could listen / poll events creating / updating / deleting a given resource and store a history log indexed by the resource id. Something like:

{ meter_type: TTT,
resource_id: RRR,
metadata: [{ version: VVVV,
timestamp: TIME1,
payload: PAYLOAD1 },
{ version: VVVV,
timestamp: TIME3,
payload: PAYLOAD2 }]
}

With PPP being the resource dependant metadata that depends on the type of the resource. And the metadata array being an ordered list of the successive states of the resource over time. The VVV version accounting for changes in the format of the payload.

The query would be :

GET /resource/<meter_type>/<resource_id>/<TIME2>

and it would return PAYLOAD1 if TIME2 is in the range [TIME1,TIME3[

I'm not sure why you think "timestamp is brittle". Maybe I'm missing something.

Cheers

-- 
Loïc Dachary         Chief Research Officer
// eNovance labs   http://labs.enovance.com
// ✉ loic@xxxxxxxxxxxx  ☎ +33 1 49 70 99 82