[openstack-dev] [nova] nova hooks - document & test or deprecate?

Chris Friesen chris.friesen at windriver.com
Mon Feb 29 19:19:08 UTC 2016

On 02/29/2016 12:22 PM, Daniel P. Berrange wrote:

> There's three core scenarios for hooks
>   1. Modifying some aspect of the Nova operation
>   2. Triggering an external action synchronously to some Nova operation
>   3. Triggering an external action asynchronously to some Nova operation
> The Rdo example is falling in scenario 1 since it is modifying the
> injected files. I think this is is absolutely the kind of thing
> we should explicitly *never* support. When external code can arbitrarily
> modify some aspect of Nova operation we're in totally unchartered
> territory as to the behaviour of Nova. To support that we'd need to
> provide a stable internal API which is just not something we want to
> tie ourselves into. I don't know just what the Rdo example is trying
> to achieve, but whatever it is, it should be via some supportable API
> and not a hook.,
> Scenaris 2 and 3 are both valid to consider. Using the notifications
> system gets as an asynchronous trigger mechanism, which is probably
> fine for many scenarios.  The big question is whether there's a
> compelling need for scenario two, where the external action blocks
> execution of the Nova operation until it has completed its hook.

Even in the case of scenario two it is possible in some cases to use a proxy to 
intercept the HTTP request, take action, and then forward it or reject it as 

I think the real question is whether there's a need to trigger an external 
action synchronously from down in the guts of the nova code.


More information about the OpenStack-dev mailing list