openstack team mailing list archive

[Eoghan Glynn <eglynn@xxxxxxxxxx>]

> > 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.

Good point, previously IIUC there was an implicit assumption that
any prev time caching would be done in-memory, hence lost across
process restarts.

But as you point out, these data could be persisted locally by the
compute agent.

What would be the best way to achieve this? A small sqlite DB 
per-agent, or even simpler just a pickled dict? The latter would
avoid the complexity of DB versioning and migration.

Cheers,
Eoghan