On 09/22/2014 07:24 AM, Daniel P. Berrange wrote:
On Mon, Sep 22, 2014 at 11:03:02AM +0000, Day, Phil wrote:
Hi Folks,

I'd like to get some opinions on the use of pairs of notification
messages for simple events.   I get that for complex operations on
an instance (create, rebuild, etc) a start and end message are useful
to help instrument progress and how long the operations took. However
we also use this pattern for things like aggregate creation, which is
just a single DB operation - and it strikes me as kind of overkill and
probably not all that useful to any external system compared to a
single event ".create" event after the DB operation.

A start + end pair is not solely useful for timing, but also potentially
detecting if it completed successfully. eg if you receive an end event
notification you know it has completed. That said, if this is a use case
we want to target, then ideally we'd have a third notification for this
failure case, so consumers don't have to wait & timeout to detect error.

There is a change up for review to add notifications for service groups
which is following this pattern (https://review.openstack.org/#/c/107954/)
- the author isn't doing  anything wrong in that there just following that
pattern, but it made me wonder if we shouldn't have some better guidance
on when to use a single notification rather that a .start/.end pair.

Does anyone else have thoughts on this , or know of external systems that
would break if we restricted .start and .end usage to long-lived instance
operations ?

I think we should aim to /always/ have 3 notifications using a pattern of

      ...notify start...

      ...do the work...

      ...notify end...
      ...notify abort...

Precisely my viewpoint as well. Unless we standardize on the above, our notifications are less than useful, since they will be open to interpretation by the consumer as to what precisely they mean (and the consumer will need to go looking into the source code to determine when an event actually occurred...)

Smells like a blueprint to me. Anyone have objections to me writing one up for Kilo?


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