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On Fri, Oct 31, 2014 at 7:49 PM, Tim Simpson <tim.simpson at rackspace.com>
wrote:

>  Hi Denis,
>
>  It seems like the issue you're trying to solve is that these 'prepare'
> messages can't be consumed by the guest.
>
Not only 'prepare' call. I want to point out that each RPC API call that
uses RPC 'cast' messaging type may remain for ever inside AMPQ service.

> So, if the guest never actually comes online and therefore can't consume
> the prepare call, then you'll be left with the message in the queue
> forever.
>
> Yes, it still may fail, but at least we can be sure that when we want to
'cast' something to guest it's alive and the only way to check if it's
alive to use 'call' messaging type, because if guest is down for some
reasons 'call' will fail as soon as possible.


>  If you use a ping-pong message, you'll still be left with a stray
> message in the queue if it fails.
>
> Ok, let's discuss it. What do you thing would give us confidence that
guest is really up and ready to consume?


>  I think the best fix is if we delete the queue when deleting an
> instance. This way you'll never have more queues in rabbit than are needed.
>
> I do agree that it may work. But as for me, it'll be more safe if
taskmanager will handle topic initializing(when instance gets created) and
canceling (when instance gets deleted).


>  Thanks,
>
>  Tim
>
>
>
Best regards,
Denis M.

>  ------------------------------
> *From:* Denis Makogon [dmakogon at mirantis.com]
> *Sent:* Friday, October 31, 2014 4:32 AM
> *To:* OpenStack Development Mailing List
> *Subject:* [openstack-dev] [Trove] Guest RPC API improvements. Messages,
> topics, queues, consumption.
>
>    Hello, Stackers/Trovers.
>
>
>
>  I’d like to start discussion about how do we use guestagent API that
> will eventually be evaluated as a spec. For most of you who well-known with
> Trove’s codebase knows how do Trove acts when provisioning new instance.
>
> I’d like to point out next moments:
>
>    1.
>
>    When we provision new instance we expect that guest will create its
>    topic/queue for RPC messaging needs.
>    2.
>
>    Taskmanager doesn’t validate that guest is really up before sending
>    ‘prepare’ call.
>
>   And here comes the problem, what if guest wasn’t able to start properly
> and consume ‘prepare’ message due to certain circumstances? In this case
> ‘prepare’ message would never be consumed.
>
>
>  Me and Sergey Gotliv were looking for proper solution for this case. And
> we end up with next requirements for provisioning workflow:
>
>    1.
>
>    We must be sure that ‘prepare’ message will be consumed by guest.
>    2.
>
>    Taskmanager should handle topic/queue management for guest.
>    3.
>
>    Guest just need to consume income messages for already existing
>    topic/queue.
>
>   As concrete proposal (or at least topic for discussions) i’d like to
> discuss next improvements:
>
> We need to add new guest RPC API that will represent “ping-pong” action.
> So before sending any cast- or call-type messages we need to make sure that
> guest is really running.
>
>
>  Pros/Cons for such solution:
>
>    1.
>
>    Guest will do only consuming.
>    2.
>
>    Guest would not manage its topics/queues.
>    3.
>
>    We’ll be 100% sure that no messages would be lost.
>    4.
>
>    Fast-fail during provisioning.
>    5.
>
>    Other minor/major improvements.
>
>
>
>  Thoughts?
>
>
>  P.S.: I’d like to discuss this topic during upcoming Paris summit
> (during contribution meetup at Friday).
>
>
>
>  Best regards,
>
>  Denis Makogon
>
>
> _______________________________________________
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>
>
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