Re: [Qemu-devel] [PATCH 19/22] qapi: add QMP put-event command

[Anthony Liguori]

On 03/10/2011 09:30 AM, Avi Kivity wrote:
> On 03/10/2011 04:24 PM, Avi Kivity wrote:
> > > > What would the wire exchange look like?
> > >
> > > > { 'execute': 'get-block-io-error-event' }
> > > < { 'return' : 32 }
> > > ...
> > > < { 'event': 'BLOCK_IO_ERROR', 'data': { 'action': 'stop', 'device': 
> > > 'ide0-hd0', 'operation': 'read' }, 'tag': 32 }
> > > ...
> > > > { 'execute': 'put-event', 'arguments': { 'tag': 32 } }
> >
> >
> > Well, I may be biased, but I prefer my variant.
> >
> > btw, it's good to decree that a subscription is immediately followed 
> > by an event with the current state (which means events have to 
> > provide state and be idempotent); so the subscribe-and-query pattern 
> > is provided automatically.
> >
> > btw2, I now nominate subscribe and unsubscribe as replacements for 
> > get and put.
>
> I also think it should be at the protocol layer:
>
> > { execute: some-command, id: foo, arguments: { ... } }
> < { result: { ... }, id: foo }
> > { subscribe: block-io-error, id: bar, arguments: { ... } }
> < { result: { ... } id: bar }
> < { event: block-io-error, id: bar, data : { ... } }
> > { unsubscribe: block-io-error, id: bar }
> < { result: { ... } id: bar }
>
> So events are now protocol-level pieces like commands, and the use of 
> tags is uniform.

Maybe for QMPv2, but for QMPv1, this is going to introduce an extremely 
incompatible change.

Actually, we missed the json-rpc boat in designing QMP.  It should look 
like:

> { method: some-command, id: foo, params: { ... } }
< { result: { ... }, id: foo, params: { ... }, error: null }
> { method: connect-block-io-error, id: bar, params: { ... } }
< { result: { ... }, id: bar, error: null }
< { method: block-io-error, id: null, params: { ... } }

Keys are different and null is passed instead of not including a tag.  
Events are sent exactly like methods but id is null.  A result is never 
sent to the server for an event.

One of the good things about using a code generator and IDL though is 
that we can add a -qmp dev,protocol=json-rpc that encodes 
appropriately.  We can probably do this translation at the QObject level 
really.  But in the future, we can also add -qmp dev,protocol=xml-rpc, 
-qmp dev,protocol=rest, or -qmp dev,protocol=asn1-rpc

Regards,

Anthony Liguori