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I think the 4th of the options you proposed would be the best. We don't
want to give agents direct access to the agent object or else we will run
the risk of breaking extensions all of the time during any kind of
reorganization or refactoring. Having a well defined API in between will
give us flexibility to move things around.

On Fri, Sep 25, 2015 at 1:32 AM, <thomas.morin at orange.com> wrote:

> Hi everyone,
>
> (TL;DR: we would like an L2 agent extension to be able to call methods on
> the agent class, e.g. OVSAgent)
>
> In the networking-bgpvpn project, we need the reference driver to interact
> with the ML2 openvswitch agent with new RPCs to allow exchanging
> information with the BGP VPN implementation running on the compute nodes.
> We also need the OVS agent to setup specific things on the OVS bridges for
> MPLS traffic.
>
> To extend the agent behavior, we currently create a new agent by mimicking
> the main() in ovs_neutron_agent.py but instead of instantiating instantiate
> OVSAgent, with instantiate a class that overloads the OVSAgent class with
> the additional behavior we need [1] .
>
> This is really not the ideal way of extending the agent, and we would
> prefer using the L2 agent extension framework [2].
>
> Using the L2 agent extension framework would work, but only partially: it
> would easily allos us to register our RPC consumers, but not to let us
> access to some datastructures/methods of the agent that we need to use:
> setup_entry_for_arp_reply and local_vlan_map, access to the OVSBridge
> objects to manipulate OVS ports.
>
> I've filled-in an RFE bug to track this issue [5].
>
> We would like something like one of the following:
> 1) augment the L2 agent extension interface (AgentCoreResourceExtension)
> to give access to the agent object (and thus let the extension call methods
> of the agent) by giving the agent as a parameter of the initialize method
> [4]
> 2) augment the L2 agent extension interface (AgentCoreResourceExtension)
> to give access to the agent object (and thus let the extension call methods
> of the agent) by giving the agent as a parameter of a new setAgent method
> 3) augment the L2 agent extension interface (AgentCoreResourceExtension)
> to give access only to specific/chosen methods on the agent object, for
> instance by giving a dict as a parameter of the initialize method [4],
> whose keys would be method names, and values would be pointer to these
> methods on the agent object
> 4) define a new interface with methods to access things inside the agent,
> this interface would be implemented by an object instantiated by the agent,
> and that the agent would pass to the extension manager, thus allowing the
> extension manager to passe the object to an extension through the
> initialize method of AgentCoreResourceExtension [4]
>
> Any feedback on these ideas...?
> Of course any other idea is welcome...
>
> For the sake of triggering reaction, the question could be rephrased as:
> if we submit a change doing (1) above, would it have a reasonable chance of
> merging ?
>
> -Thomas
>
> [1]
> https://github.com/openstack/networking-bgpvpn/blob/master/networking_bgpvpn/neutron/services/service_drivers/bagpipe/ovs_agent/ovs_bagpipe_neutron_agent.py
> [2] https://review.openstack.org/#/c/195439/
> [3]
> https://github.com/openstack/neutron/blob/master/neutron/plugins/ml2/drivers/openvswitch/agent/extension_drivers/qos_driver.py#L30
> [4]
> https://github.com/openstack/neutron/blob/master/neutron/agent/l2/agent_extension.py#L28
> [5] https://bugs.launchpad.net/neutron/+bug/1499637
>
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-- 
Kevin Benton
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