Re: [Qemu-devel] [PATCH v2 2/2] qapi: Allow setting default values for optional parameters

[Eric Blake]

On 05/05/2014 03:51 AM, Markus Armbruster wrote:

>> Also, is the default value allowed to change between qemu versions?
>> What are the back-compat ramifications if two different releases want to
>> tweak an omitted variable to different defaults?  The documentation
>> should mention the rule of thumb that we plan on enforcing during
>> reviews.  I could go either way: the wire format is unimpacted by what
>> default value is used when an argument is omitted, and management can
>> always explicitly specify a parameter if they don't trust the default;
>> on the other hand, if changing a default changes visible behavior, then
>> we have not maintained ABI compatibility.
> 
> We should use common sense.
> 
> Changing the schema in a way that alters the meaning of existing usage
> is a no-no, and that applies to defaults as much as to anything else.
> But not every change of a default necessarily does that.  Contrieved
> example: if the value defines a buffer size, and it's specified to
> default to a sensible size, then we can and should let the default value
> evolve along with our idea of what a sensible size is.

I agree - it's more important to declare that we are maintaining
documented semantics (omitting the parameter will always pick the best
size, even if that size differs over time) than precise values (omitting
the parameter will always pick 512 bytes, even if that is not the ideal
size for newer versions) - so it is somewhat a documentation game of
whether we document default values well enough to let them be flexible :)

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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