Re: [Qemu-devel] [PATCH 5/7] QMP: Update spec file

[Markus Armbruster <armbru@redhat.com>]

Luiz Capitulino <lcapitulino@redhat.com> writes:

> On Fri, 18 Dec 2009 18:20:52 +0100
> Markus Armbruster <armbru@redhat.com> wrote:
>
>> Luiz Capitulino <lcapitulino@redhat.com> writes:
>> 
>> [...]
>> > -4. Notes to Client implementors
>> > --------------------------------
>> > +4. Compatibility Considerations
>> > +--------------------------------
>> >  
>> > -4.1 It is recommended to always start the Server in pause mode, thus the
>> > -    Client is able to perform any setup procedure without the risk of
>> > -    race conditions and related problems
>> > +In order to achieve maximum compatibility between versions, the following
>> > +changes are forbidden in newer versions of the Server:
>> >  
>> > -4.2 It is recommended to always check the capabilities json-array, issued
>> > -    with the greeting message, at connection time
>> > +- Removal of commands
>> > +- Removal of command arguments
>> > +- Addition of extra mandatory arguments for commands
>> > +- Modification of arguments types
>> > +- Modification of arguments, commands, events or error names
>> > +- Modification of arguments in replies, events or errors
>> 
>> While I think these promises are appropriate for a mature version of the
>> protocol, I do not think we should make them for 0.12.
>> 
>> We've just dreamed up version 0.1 of the protocol.  It hasn't been used
>> in anger.  Yes, we put some serious thought in it, and we even have
>> prototype code using it in libvirt, but let's face it, we're not
>> infallible: we *will* have to evolve stuff.
>
>  While I agree with your arguments, I think this will happen to any
> QMP version or any stable protocol/API.
>
>  You have the point that QMP is immature at this point, but:
>
> 1. Given the current Monitor design, if you want QMP to be perfect to be
> useful then we're going to have it around QEMU version 10.0

No human creation will ever be perfect.

> 2. I don't think we're going to get any serious user until we
> declare it stable

You're quite right about that.  But right now QMP doesn't cover enough
ground for serious use anyway.  Once it does, we'll soon be compelled to
declare it stable, and live with the consequences.