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Re: Gflags, Settings, Dependency Injection


So I know I haven't convinced everyone to love bzr yet ... but as they
are a large python project with command line and config file options -
and plugins - perhaps looking at the infrastructure/design they use
might be a good idea?

Also, the work derks did with cement might be of help.

I believe both are designed to do things similar to how you are
discussing them below (although different, of course - we're all python
devs, there's no way we're going to actually do things the same. :) )


(what Eric is saying makes sense to me - but I don't have a whole bunch
of stake either way here- I am a fan of reusing solutions that exist
where possible though of course)

On 07/28/2010 01:24 PM, Eric Day wrote:
> Hi Vish,
> If we want to keep things modular and have runtime module selection
> like you mention, we probably need to rethink flags. Using gflags
> may not be an option unless we can somehow make 'undefok=' a global
> option. In other project (that was not in Python, so no code to help),
> the flow is:
> * Enforce the use of module names in the options. For example, for
>   generic queue module options use --queue.*, for libvirt module
>   options, use --libvirt.*. If we want to make this seamless, we
>   would probably need to use something else instead gflags or create
>   a wrapper to enforce the required behavior.
> * Import the core option manager, first thing that happens when
>   starting a binary.
> * Parse all options, separating each out into the modules they belong
>   to. We don't know what is valid yet, but we can at least group by module.
> * Load any required modules via normal 'import' lines. They can verify
>   options for their module space.
> * Have some core flags that specify which modules to load, for example,
>   use rabbit vs fakerabbit. Then 'import' the selected optional modules.
> * As optional modules load, let them verify the module namespace
>   options just like the required modules did.
> * Any options for modules that were not loaded are just ignored.
> Thoughts on this? It has worked out quite well in the other C++ project
> for me, and with Python it would be even easier to put together. :)
> -Eric
> On Wed, Jul 28, 2010 at 11:13:40AM -0700, Vishvananda Ishaya wrote:
>>    I'm having some annoyances with gflags which I'd like to air out here.
>>     Maybe we can come to a consensus about how to move forward with them.  I
>>    find gflags annoying in the following ways:
>>    a) flags are irritating for global settings.  Settings that apply to the
>>    project as a whole have to be set in multiple places so that the binaries
>>    all get them properly.  This can be fixed somewhat by a shared flagfile.
>>     For example:
>>    /etc/nova/nova-manage.conf:
>>      --flagfile=/etc/nova/nova-common.conf # shared settings
>>      --otherflag=true #manage specific settings
>>    The problem here is that the shared settings can only include settings
>>    that are imported by EVERY binary, or one of the binaries will choke.  So
>>    if you have a flag that 4 of 5 binaries use, you either have to set it in
>>    four flagfiles or put it in common with an ugly undefok= line.  This all
>>    seems nasty to me.  Other possibilities include moving truly
>>    common/settings related flags into the common flags.py so that they are
>>    available to all binaries.  It all seems a bit hackish.
>>    b) including files for flags only
>>    There are places where we need access to a flag, but we aren't actually
>>    making calls in the file.  Pyflakes and pylint complain about unused
>>    imports.  Perhaps we fix this by moving these flags into common flagfile?
>>    c) dependency injection
>>    This is related to the issue above.  If we are dynamically loading
>>    specific drivers (for example the auth driver or a datastore backend) as
>>    specified by a flag, the import is often done later than the parent file
>>    is imported.  Therefore using flags to configure settings for the driver
>>    will fail, because the binary recognizing the flags is dependent on the
>>    file that contains the flags being imported.  Workarounds here include
>>    finding a different method for dependency injection, hacking flags to
>>    search for flags in injected dependencies somehow, or configuring drivers
>>    differently than the rest of the system.
>>    So I see 3 options for moving forward
>>    1) ditch gflags completely and use a different method for specifying
>>    settings
>>    2) use a combination of some kind of settings file for general
>>    configuration, and flags for specific runtime settings/hacks
>>    3) find good standard practices/workarounds for the above issues
>>    Thoughts?
>>    Vish
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