Re: [Qemu-devel] [PATCH v3 2/3] Add migrate_incoming

[Markus Armbruster <armbru@redhat.com>]

Eric Blake <eblake@redhat.com> writes:

> On 02/20/2015 01:18 AM, Markus Armbruster wrote:
>> I'd like Eric's opinion on on encoding configuration tuples as URIs
>> rather than JSON in QMP.
>> 
>
>>> +{ 'command': 'migrate-incoming', 'data': {'uri': 'str' } }
>>> +
>>>  # @xen-save-devices-state:
>>>  #
>>>  # Save the state of all devices to file. The RAM and the block devices
>> 
>> Eric, what's your take on this?
>> 
>> The general rule in QMP is "no ad hoc encoding of tuples in strings, use
>> JSON objects instead".
>
> Yes, it would be nice to be type-safe, and have a QAPI union with a
> discriminator of different URI types along with appropriate accompanying
> data.  But it is also new code (both in qemu to parse it, and in libvirt
> to generate it), and we already have existing code that knows how to
> generate the string-encoded tuple for the -incoming command line
> argument that can be reused as-is with the new QMP command as proposed.
>  So even though it is less safe, I'm okay with this particular command
> using an overloaded string argument.
>
> Down the road, if we WANT to add type-safety, we can make this command
> take an anonymous union, where a 'str' value is the compact old style,
> and where a dictionary value is the new discriminated union style.  So
> we aren't completely locked into non-type-safe string forever, but we
> probably won't add a type-safe variant until (if) we ever add a new
> migration format that just looks too ugly as a URI based on the
> parameters it requires.

I can accept this, but I want the rationale in a comment, so this
exception to the QMP rule serves less well as bad example.