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Subject: Re: [Qemu-devel] ANN: QEMU Monitor Protocol git tree
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	<87my4l1e4e.fsf@pike.pond.sub.org> <20090924093146.66191c37@doriath>
From: Markus Armbruster <armbru@redhat.com>
Date: Thu, 24 Sep 2009 15:28:40 +0200
In-Reply-To: <20090924093146.66191c37@doriath> (Luiz Capitulino's message of
	"Thu\, 24 Sep 2009 09\:31\:46 -0300")
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To: Luiz Capitulino <lcapitulino@redhat.com>
Cc: aliguori@us.ibm.com, qemu-devel@nongnu.org, avi@redhat.com

Luiz Capitulino <lcapitulino@redhat.com> writes:

> On Thu, 24 Sep 2009 00:37:53 +0200
> Markus Armbruster <armbru@redhat.com> wrote:
>
>> Luiz Capitulino <lcapitulino@redhat.com> writes:
>> 
>> [...]
>> > 2.2 Server Greeting
>> > -------------------
>> >
>> > Right when connected the Server will issue a greeting message, which signals
>> > that the connection has been successfully established and that the Server is
>> > waiting for commands.
>> >
>> > The format is:
>> >
>> > { "QEMU": json-string, "QMP": json-string, "capabilities": json-array }
>> >
>> >  Where,
>> >
>> > - The "QEMU" member contains the QEMU version
>> > - The "QMP" member contains the QMP version
>> > - The "capabilities" member specify the availability of features beyond the
>> >   baseline specification
>> 
>> What about capability negotiation?  Server offers capabilities, client
>> can accept or decline them.
>> 
>> [...]
>
>  I think the easiest way to have this would be to add a
> monitor command to disable capabilities. Like a command to
> disable async messages.

Greeting capabilities (for lack of a better word) are for the protocol.
Changing protocol capabilities while you use the protocol is awkward.
Better do it in an initial handshake.

Case in point: if you disable asynchronous messages with a command, you
still have to be prepared to receive one between initial handshake and
completion of the disable command.  If I have to ignore them anyway, why
bother with disabling them?

The problem becomes more serious if we ever want to add a capability
that isn't fully backward compatible.  Lack of feature negotiation
limits protocol evolvability.