On Fri, May 11, 2012 at 3:55 PM, Loic Dachary <lo...@enovance.com
<mailto:lo...@enovance.com>> wrote:

> - 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.

Each set of metering data will need to be associated with the appropriate
metadata from the resource at the time the metering information was collected.
The rate of change of metadata and metering events are different, though, so the
timestamps of the metadata records are unlikely to match exactly with the values
in the metering records. Depending on the clock resolution, it would be possible
to have metadata changes and meter data with the same timestamp, resulting in an
incorrect association. Indeed, good point.

We can work around that by maintaining proper foreign key references using the
metadata version field as you describe in the schema above (so the resource id
and metadata version value point to the correct metadata record). It will make
recording the metering data less efficient because we will need to determine the
current version for the resource metadata, but we can optimize that eventually
through indexes and caching.

Aggregation will also need to take the metadata version into account, so
everywhere in the list of queries we say "by resource_id" we need to change that
to "by resource_id and version".