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Hi all,

in feature/qos, we use ml2 extension drivers to handle additional
qos_policy_id field that can be provided thru API:

http://git.openstack.org/cgit/openstack/neutron/tree/neutron/plugins/ml2
/extensions/qos.py?h=feature/qos

What we do in qos extension is we create a database 'binding' object
between the updated port and the QoS policy that corresponds to
qos_policy_id. So we access the database. It means there may be some
complications there, f.e. the policy object is not available for the
tenant, or just does not exist. In that case, we raise an exception
from the extension, assuming that ml2 will propagate it to the user in
some form.

But it does not work. This is because _call_on_ext_drivers swallows
exceptions:

http://git.openstack.org/cgit/openstack/neutron/tree/neutron/plugins/ml2
/managers.py#n766

It makes me ask some questions:

- - first, do we use extensions as was expected? Can we extend
extensions to cover our use case?

- - second, what would be the right way to go assuming we want to
support the case? Should we just reraise? Or maybe postpone till all
extension drivers are called, and then propagate an exception top into
the stack? (Probably some extension manager specific exception?) Or
maybe we want extensions to claim whether they may raise, and handle
them accordingly?

- - alternatively, if we abuse the API and should stop doing it, which
other options do we have to achieve similar behaviour without relying
on ml2 extensions AND without polluting ml2 driver with qos specific cod
e?

Thanks for your answers,
Ihar
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