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On Fri, 15 Mar 2019, Ben Nemec wrote:

> According to [0], the monkey-patching wasn't done in nova/__init__.py because 
> it was supposed to be able to run without eventlet under WSGI. If that's not 
> the case, then it's probably fine to do it there.
>
> 0: 
> http://specs.openstack.org/openstack/openstack-specs/specs/eventlet-best-practices.html
>
> "Monkey patching should also be done in a way that allows services to run 
> without it, such as when an API service runs under Apache. This is the reason 
> for Nova not simply monkey patching in nova/__init__.py."

Yeah, I think several people have run through the eventlet
initialization at various times, including me, with various results,
none of them quite good enough.

The mod_wsgi/uwsgi side of things strived to be eventlet free as it
makes for weirdness, and at some point I did some work to be sure it
never showed up in placement [1] while it was still in nova. A side
effect of that work was that it also didn't need to show up in the
nova-api unless you used the cmd line script version of it. At the
same time I also explored not importing the world, and was able to
get some improvements (mostly by moving things out of __init__.py in
packages that had deeply nested members) but not as much as would
have liked.

However, we later (as mentioned elsewhere in the thread) made
getting cell mappings parallelize, bring back the need for eventlet,
it seems.

>> TL;DR I want to move monkey patching to nova/__init__.py because, for
>> now at least, we always require it anyway. When we eventually remove
>> the requirement for wsgi services to monkey patch we can move monkey
>> patching back to nova/cmd/__init__.py.
>
> This sounds fine to me, but it's been a loooong time since I looked at this 
> and I'm not familiar with some of the more recent changes you talked about, 
> so take my opinion with a grain of salt.

This does seem like the expedient solution (assuming it works), but
if we continue to use eventlet anywhere in the api service it will
continue to introduce difficulties and inflexibility with deployment
choices as it will interact poorly with at least some of the
web-servers that could be chosen to run the nova API.

[1] Recently we managed to get placement to a state where not only
does it not use eventlet, it doesn't even use anything that has it
as an unused dependency. This made me very happy.

-- 
Chris Dent                       ٩◔̯◔۶           https://anticdent.org/
freenode: cdent                                         tw: @anticdent