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Re: [Qemu-devel] ANN: QEMU Monitor Protocol git tree
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] ANN: QEMU Monitor Protocol git tree |
Date: |
Thu, 24 Sep 2009 15:28:40 +0200 |
User-agent: |
Gnus/5.11 (Gnus v5.11) Emacs/22.3 (gnu/linux) |
Luiz Capitulino <address@hidden> writes:
> On Thu, 24 Sep 2009 00:37:53 +0200
> Markus Armbruster <address@hidden> wrote:
>
>> Luiz Capitulino <address@hidden> 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.