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On Thu, Jan 19, 2017 at 10:34 AM, Jay Pipes <jaypipes at gmail.com> wrote:
> On 01/19/2017 11:25 AM, Alex Schultz wrote:
>>
>> On Thu, Jan 19, 2017 at 8:27 AM, Matt Riedemann
>> <mriedem at linux.vnet.ibm.com> wrote:
>>>
>>> Sylvain and I were talking about how he's going to work placement
>>> microversion requests into his filter scheduler patch [1]. He needs to
>>> make
>>> requests to the placement API with microversion 1.4 [2] or later for
>>> resource provider filtering on specific resource classes like VCPU and
>>> MEMORY_MB.
>>>
>>> The question was what happens if microversion 1.4 isn't available in the
>>> placement API, i.e. the nova-scheduler is running Ocata code now but the
>>> placement service is running Newton still.
>>>
>>> Our rolling upgrades doc [3] says:
>>>
>>> "It is safest to start nova-conductor first and nova-api last."
>>>
>>> But since placement is bundled with n-api that would cause issues since
>>> n-sch now depends on the n-api code.
>>>
>>> If you package the placement service separately from the nova-api service
>>> then this is probably not an issue. You can still roll out n-api last and
>>> restart it last (for control services), and just make sure that placement
>>> is
>>> upgraded before nova-scheduler (we need to be clear about that in [3]).
>>>
>>> But do we have any other issues if they are not packaged separately? Is
>>> it
>>> possible to install the new code, but still only restart the placement
>>> service before nova-api? I believe it is, but want to ask this out loud.
>>>
>>
>> Forgive me as I haven't looked really in depth, but if the api and
>> placement api are both collocated in the same apache instance this is
>> not necessarily the simplest thing to achieve.  While, yes it could be
>> achieved it will require more manual intervention of custom upgrade
>> scripts. To me this is not a good idea. My personal preference (now
>> having dealt with multiple N->O nova related acrobatics) is that these
>> types of requirements not be made.  We've already run into these
>> assumptions for new installs as well specifically in this newer code.
>> Why can't we turn all the services on and they properly enter a wait
>> state until such conditions are satisfied?
>
>
> Simply put, because it adds a bunch of conditional, temporary code to the
> Nova codebase as a replacement for well-documented upgrade steps.
>
> Can we do it? Yes. Is it kind of a pain in the ass? Yeah, mostly because of
> the testing requirements.
>

<rant>
You mean understanding how people actually consume your software and
handling those cases?  To me this is the fundamental problem if you
want software adoption, understand your user. Know what you're doing
and the impact on them.  I was just raising awareness around how some
people are deploying this stuff because it feels that sometimes folks
just don't know or don't care.  So IMHO adding service startup/restart
ordering requirements is not ideal for the person who has to run your
software because it makes the entire process hard and more complex.
Why use this when I can just buy a product that does this for me and
handles these types of cases?  We're not all containers yet which
might alleviate some of this but as there was a push for the placement
service specifically to be in a shared vhost, this recommended
deployment method introduces these kind of complexities. It's not
something that just affects me.  Squeaky wheel gets the hose, I mean
grease.
</rant>

> But meh, I can whip up an amendment to Sylvain's patch that would add the
> self-healing/fallback to legacy behaviour if this is what the operator
> community insists on.
>
> I think Matt generally has been in the "push forward" camp because we're
> tired of delaying improvements to Nova because of some terror that we may
> cause some deployer somewhere to restart their controller services in a
> particular order in order to minimize any downtime of the control plane.
>
> For the distributed compute nodes, I totally understand the need to tolerate
> long rolling upgrade windows. For controller nodes/services, what we're
> talking about here is adding code into Nova scheduler to deal with what in
> 99% of cases will be something that isn't even noticed because the upgrade
> tooling will be restarting all these nodes at almost the same time and the
> momentary failures that might be logged on the scheduler (400s returned from
> the placement API due to using an unknown parameter in a GET request) will
> only exist for a second or two as the upgrade completes.

So in our case they will get (re)started at the same time. If that's
not a problem, great.  I've seen services in the past where it's been
a problem when a service actually won't start because the dependent
service is not up yet. That's what I wanted to make sure is not the
case here.  So if we have documented assurance that restarting both at
the same time won't cause any problems or the interaction is that the
api service won't be 'up' until the placement service is available
then I'm good.  I'm not necessarily looking for the 99.9999% uptime.
Just that it doesn't fall on it's face and we have to write extra
deployment code for this. :)

>
> So, yeah, a lot of work and testing for very little real-world benefit,
> which is why a number of us just want to more forward...
>

So for me it's a end user experience thing. I appreciate moving
forward and I think it should be done. But the code you don't want to
write has to be written elsewhere. This isn't something that doesn't
have to be handled. So all the code you don't want to handle gets
handled in tens or hundreds or thousands of different implementations
of the upgrade process.  So rather than just bulldozing through
because you don't want to do it, perhaps considering where the best
place to handle this is more appropriate.  In terms of OpenStack
services, we really don't have that many expectations of specific
ordering of things because the services should play nicely together
and it's better to keep it that way.

Thanks,
-Alex

> Best,
> -jay
>
>
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