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On 10/03/2014 11:38 AM, Chris Dent wrote:
> On Fri, 3 Oct 2014, Joe Gordon wrote:

>>> Many of those services expect[1] to be able to send notifications (or
>>> be polled by) ceilometer[2]. We've got an ongoing thread about the need
>>> to contractualize notifications. Are those contracts (or the desire
>>> for them) a form of dependency? Should they be?
>>>
>>
>> So in the case of notifications, I think that is a Ceilometer CAN-USE
>> Nova
>> THROUGH notifications
>
> Your statement here is part of the reason I asked. I think it is
> possible to argue that the dependency has the opposite order: Nova might
> like to use Ceilometer to keep metrics via notifications or perhaps:
> Nova CAN-USE Ceilometer FOR telemetry THROUGH notifications and polling.
>
> This is perhaps not the strict technological representation of the
> dependency, but it represents the sort of pseudo-social
> relationships between projects: Nova desires for Ceilometer (or at
> least something doing telemetry) to exist.

This may be quibbling, but I would suggest that it is the end-user that 
may want something doing telemetry to exist.

Nova proper doesn't really care about telemetry.  Nova exports telemetry 
because end-users want the information to be available for use by other 
services.  Nova itself doesn't actually make use of it or call out to 
services that make use of it.

Now something like Heat really depends on telemetry.  It wants to know 
if an instance didn't kick the watchdog timer, or if the webserver keeps 
crashing, or other information provided by telemetry.

> Ceilometer itself is^wshould be agnostic about what sort of metrics are
> coming its way. It should accept them, potentially transform them, store
> them, and make them available for later use (including immediately). It
> doesn't^wshouldn't really care if Nova exists or not.
>
> There are probably lots of other relationships of this form between
> other services, thus the question: Is a use-of-notifications
> something worth tracking? I would say yes.

Sure, because those notifications are a form of API and those should be 
versioned and tracked appropriately. Nova can't arbitrarily change the 
format of the notifications it sends out because that would break anyone 
that cares about the contents of those notifications.

Chris