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Date: Mon, 21 May 2012 09:19:05 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] Proposal for extensions of block job commands in
 QEMU 1.2
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To: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, qemu-devel <qemu-devel@nongnu.org>, Federico Simoncelli <fsimonce@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Eric Blake <eblake@redhat.com>

On 05/21/2012 09:16 AM, Luiz Capitulino wrote:
> On Mon, 21 May 2012 09:10:40 -0500
> Anthony Liguori<anthony@codemonkey.ws>  wrote:
>
>> On 05/21/2012 08:59 AM, Luiz Capitulino wrote:
>>> On Fri, 18 May 2012 19:08:42 +0200
>>> Paolo Bonzini<pbonzini@redhat.com>   wrote:
>>>
>>>> Modified QMP commands
>>>> =====================
>>>
>>> As we have discussed on the ML, we're not going to extend QMP commands.
>>>
>>> I understand your reasoning, and since the beginning I thought this was
>>> something useful to do, but we've already settled for not doing this.
>>>
>>> I also think that we shouldn't have exceptions, as in practice this means
>>> we're extending commands anyway. So either, we do it or we don't.
>>
>> Well, I think we should ask ourselves the following question:
>>
>> How would a client figure out if the new options are available?
>>
>> This is the primary reason for not extending existing commands.
>
> Yes, I know. But if Paolo implements schema introspection, would you
> agree on extending commands.
>
> You seemed to be against even if we had schema introspection.

I'm not against it in principle, just in practice.  Today, checking whether a 
command exists is:

commands = qmp.query_commands()

if 'block-stream' in commands:
     # has block-stream

I have a hard time envisioning how schema introspection can be reasonably 
implemented in a client.

If we really feel it's important to extend commands, I'd prefer something like 
per-command capabilities.

Regards,

Anthony Liguori