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Another thing which makes it hard to understand the impact of the config
options is, that it's not clear how the interdependency to other config 
options is. As an example, the "serial_console.base_url" has a 
dependency to "DEFAULT.cert" and "DEFAULT.key" if you want to use 
secured websockets ("base_url=wss://..."). Another one is the option
"serial_console.serialproxy_port". This port number must be the same
as it is in "serial_console.base_url". I couldn't find an explanation to
this.

The three questions I have with every config option:
1) which service(s) access this option?
2) what does it do? / what's the impact? 
3) which other options do I need to tweek to get the described impact?

Would it make sense to stage the changes?
M cycle: move the config options out of the modules to another place
         (like the approach Sean proposed) and annotate them with
         the services which uses them
N cycle: inject the options into the drivers and eliminate the global
         variables this way (like Daniel et al. proposed)

Especially for new contributors like me who didn't start in any of the
early releases and didn't have the change to grow with Nova and its
complexity, it would really help me a lot and enable me to contribute
in a better way.

As a side note:
The "nova.flagmappings" file, which gets generated when you want to 
build the configuration reference manual, contains 804 config options
for Nova. Quite a lot I think :)

Sean Dague <sean at dague.net> wrote on 07/27/2015 04:35:56 PM:

> From: Sean Dague <sean at dague.net>
> To: openstack-dev at lists.openstack.org
> Date: 07/27/2015 04:36 PM
> Subject: Re: [openstack-dev] [openstack][nova] Streamlining of config 
> options in nova
> 
> On 07/27/2015 10:05 AM, Daniel P. Berrange wrote:
> > On Fri, Jul 24, 2015 at 09:48:15AM +0100, Daniel P. Berrange wrote:
> >> On Thu, Jul 23, 2015 at 05:55:36PM +0300, mhorban wrote:
> >>> Hi all,
> >>>
> >>> During development process in nova I faced with an issue related 
with config
> >>> options. Now we have lists of config options and registering options 
mixed
> >>> with source code in regular files.
> >>> From one side it can be convenient: to have module-encapsulated 
config
> >>> options. But problems appear when we need to use some config option 
in
> >>> different modules/packages.
> >>>
> >>> If some option is registered in module X and module X imports module 
Y for
> >>> some reasons...
> >>> and in one day we need to import this option in module Y we will get
> >>> exception
> >>> NoSuchOptError on import_opt in module Y.
> >>> Because of circular dependency.
> >>> To resolve it we can move registering of this option in Y module(in 
the
> >>> inappropriate place) or use other tricks.
> >>>
> >>> I offer to create file options.py in each package and move all 
package's
> >>> config options and registration code there.
> >>> Such approach allows us to import any option in any place of nova 
without
> >>> problems.
> >>>
> >>> Implementations of this refactoring can be done piece by piece where 
piece
> >>> is
> >>> one package.
> >>>
> >>> What is your opinion about this idea?
> >>
> >> I tend to think that focusing on problems with dependancy ordering 
when
> >> modules import each others config options is merely attacking a 
symptom
> >> of the real root cause problem.
> >>
> >> The way we use config options is really entirely wrong. We have gone
> >> to the trouble of creating (or trying to create) structured code with
> >> isolated functional areas, files and object classes, and then we 
throw
> >> in these config options which are essentially global variables which 
are
> >> allowed to be accessed by any code anywhere. This destroys the 
isolation
> >> of the various classes we've created, and means their behaviour often
> >> based on side effects of config options from unrelated pieces of 
code.
> >> It is total madness in terms of good design practices to have such 
use
> >> of global variables.
> >>
> >> So IMHO, if we want to fix the real big problem with config options, 
we
> >> need to be looking to solution where we stop using config options as
> >> global variables. We should change our various classes so that the
> >> neccessary configurable options as passed into object constructors
> >> and/or methods as parameters.
> >>
> >> As an example in the libvirt driver.
> >>
> >> I would set it up so that /only/ the LibvirtDriver class in driver.py
> >> was allowed to access the CONF config options. In its constructor it
> >> would load all the various config options it needs, and either set
> >> class attributes for them, or pass them into other methods it calls.
> >> So in the driver.py, instead of calling 
CONF.libvirt.libvirt_migration_uri
> >> everywhere in the code,  in the constructor we'd save that config 
param
> >> value to an attribute 'self.mig_uri = 
CONF.libvirt.libvirt_migration_uri'
> >> and then where needed, we'd just call "self.mig_uri".
> >>
> >> Now in the various other libvirt files, imagebackend.py, volume.py
> >> vif.py, etc. None of those files would /ever/ access CONF.*. Any time
> >> they needed a config parameter, it would be passed into their 
constructor
> >> or method, by the LibvirtDriver or whatever invoked them.
> >>
> >> Getting rid of the global CONF object usage in all these files 
trivially
> >> now solves the circular dependancy import problem, as well as 
improving
> >> the overall structure and isolation of our code, freeing all these 
methods
> >> from unexpected side-effects from global variables.
> 
> How does that address config reload on SIGHUP? It seems like that
> approach would break that feature.
> 
> > Another significant downside of using CONF objects as global variables
> > is that it is largely impossible to say which nova.conf setting is
> > used by which service. Figuring out whether a setting affects 
nova-compute
> > or nova-api or nova-conductor, or ... largely comes down to guesswork 
or
> > reliance on tribal knowledge. It would make life significantly easier 
for
> > both developers and administrators if we could clear this up and in 
fact
> > have separate configuration files for each service, holding only the
> > options that are relevant for that service.  Such a cleanup is not 
going
> > to be practical though as long as we're using global variables for 
config
> > as it requires control-flow analysis find out what affects what :-(
> 
> Part of the idea that came up in the room is to annotate variables with
> the service they were used in, and deny access to in services they are
> not for.
> 
>    -Sean
> 
> -- 
> Sean Dague
> http://dague.net
> 
> 
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