[openstack-dev] [Nova] RPC Communication Errors Might Lead to a Bad State

Shinobu Kinjo shinobu.kj at gmail.com
Thu Apr 14 02:40:17 UTC 2016


Just coming from my curiosity (inline).

On Thu, Apr 14, 2016 at 12:34 AM, Dan Smith <dms at danplanet.com> wrote:
>>   * nova-api should receive an acknowledgement from nova-compute. It is
>>     unclear to me why today it uses a non-reply mechanism - probably to
>>     free the worker as fast as it can.
> Yes, wherever possible, we want the API to return immediately and let
> the action complete later. Making a wholesale change to blocking calls
> from the API to any other service is not a good idea, IMHO.
>>   * Change the task_state mechanism to prevent this kind of a stuck
>>     state to stay in the DB. nova-compute can be the one that writes the
>>     task_state to the DB, but this is not enough of course, but maybe
>>     there's another way?
> The task_state being set in the API is our way of limiting/locking the
> operation so that if the request is queued for a long time, a user
> doesn't reissue the command a bunch of time and add load to the API
> and/or jam up the queue with a thousand requests to do the same
> operation just because it's taking a while.
>>   * nova-api could start a timer for the action to complete. If the
>>     shelving operation hasn't completed in X seconds, it will clean it
>>     by itself and rollback\try-again.
> I have wanted to make a change for a while that involves a TTL on
> messages, along with a deadline record so that we can know when to retry
> or revert things that were in flight. This requires a lot of machinery
> to accomplish, and is probably interwoven with the task concept we've
> had on the back burner for a while. The complexity of moving nova to
> this sort of scheme means that nobody has picked it up as of yet, but
> it's certainly in the minds of many of us as something we need to do
> before too long.

Are you still thinking of this kind of mechanism deployment?
We need any kind of RPC handling mechanism at the end of the day.

> --Dan
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shinobu at linux.com
Life with Distributed Computational System based on OpenSource

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