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[heat][neutron] improving extraroute support


On 15/04/19 11:01 AM, Bence Romsics wrote:
> On Mon, Apr 15, 2019 at 4:39 PM Rabi Mishra <ramishra at redhat.com> wrote:
>> I think this had been discussed on several occasions[1] before and one of the
>> suggestions from the neutron team was to use compare-and-swap update api [2]
>> to avoid race conditions.
>>
>> [1] http://lists.openstack.org/pipermail/openstack-dev/2018-June/131020.html
>>   [2] https://bugs.launchpad.net/neutron/+bug/1703234
> 
> Thank you for your quick answer. I did read that previous discussion
> and I did not properly understand why compare-and-swap was
> recommended. To my understanding compare-and-swap allows a client to
> detect a race with another, but it would not eliminate the races. All
> clients would have to retry until success and with the current
> OS::Neutron::ExtraRoute we do actively generate those races. That's
> why I went in the directions mentioned in my original mail. Am I
> missing something?

Yes, you have to do it in a loop. So instead of just:

* read the current status
* add the new route to the data
* write back the whole thing

you'd need to do:

* read the current status and remember the ETag
* add the new route to the data
* write back the whole thing with If-Match
* if the write is rejected with status code 412, go back to the beginning

IMHO the best place to implement this would be in 
neutronclient/openstacksdk and not Heat, because the problem is inherent 
to the API and exists everywhere. However, I wouldn't reject a fix in Heat.

cheers,
Zane.