[openstack-dev] RFC: Synchronizing hypervisor <-> nova state with event notifications

Sandy Walsh sandy.walsh at rackspace.com
Tue Jan 15 19:03:43 UTC 2013

On 01/15/2013 12:55 PM, Daniel P. Berrange wrote:
> On Tue, Jan 15, 2013 at 11:32:31AM -0400, Sandy Walsh wrote:
>> On 01/15/2013 09:24 AM, Daniel P. Berrange wrote:
>>> On Tue, Jan 15, 2013 at 12:04:31PM +0000, Mark McLoughlin wrote:
>>>> On Tue, 2013-01-15 at 10:52 +0000, Daniel P. Berrange wrote:
>>>>> The question is how to structure the processing of events. I don't
>>>>> think that the hypervisor drivers should be directly processing events.
>>>>> Instead I believe they need to pass along the event notifications to
>>>>> the manager.py class. So my current thought is to introduce a new API
>>>>> to nova.virt.api
>>>>>   register_event_notifier(self, callback)
>>>>> and have nova.compute.manager provide a callback impl to receive the
>>>>> events. Before I start coding on this, I want some kind of confirmation
>>>>> that this is an acceptable direction to go in, since there is no current
>>>>> callback based interactions between nova.compute.manager & nova.virt.api
>>>> That all sounds good to me, but can you go into more details about how
>>>> the event dispatching would work?
>>>> We don't have a mainloop to integrate this with, so presumably you're
>>>> talking about spawning a greenthread which would poll for events (using
>>>> virEventRunDefaultImpl()?) and then invoke the callbacks? Does this
>>>> greenthread need its own libvirt connection?
>>>> Does this new thread introduce any new concurrency issues? I guess not
>>>> since processing each RPC message and running periodic timers all happen
>>>> in different threads, so this wouldn't be much different.
>>> Yes, thanks to eventlet awfulness, we'd need to have a greenthread running
>>> the libvirt event loop.  The event notification callbacks from libvirt
>>> would thus obviously execute in the greenthread. What I'm not sure about
>>> is how to switch control back to the main eventlet thread before we call
>>> out into the manager.py's callback.
>>> I imagine a queue of events in the nova.virt.libvirt.driver object which
>>> is fed back the libvirt greenthread callbacks. Something in the main
>>> eventlet thread would then have to process the queue to dispatch to the
>>> manager. With a regular mainloop the way you'd do this is to schedule a
>>> timer to fire after zero seconds. I'm not familiar enough with eventlet
>>> yet to know how you'd do this.
>> Sounds like a good idea. My immediate concern is what happens if we miss
>> or drop the callback? How will the downstream consumers of the
>> notifications get the update?
> I'm not really sure what makes you think a callback could be missed
> or dropped. It is just another python method call in nova-compute.
> In any case the existing periodic task could still reconcille state
> even if events were used.

The event could get dropped from anything downstream. The issue is, once
gone, there's no recovery. At least with the existing mechanism, if it
did get dropped we'd recover on the next one. Keep the existing periodic
task is fine, but almost defeats the purpose of the whole exercise, no?

>> Beyond that, could the callback write to the rabbit queue directly,
>> either as a notification or as a new RPC call?
> The RPC layer isn't involved at all here. This is just about interaction
> between nova.compute.manager & nova.virt.api classes which are all in the
> same process.

Gotcha, I saw "event notifications" in the subject and was assuming that
the was end-goal.

> Daniel

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