[openstack-dev] [openstack][nova] Streamlining of config options in nova
sean at dague.net
Mon Jul 27 14:35:56 UTC 2015
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
>>> 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
>>> Implementations of this refactoring can be done piece by piece where piece
>>> 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
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