Re: [Qemu-devel] Re: [RFC] qapi: events in QMP

[Luiz Capitulino <lcapitulino@redhat.com>]

On Mon, 14 Feb 2011 08:39:11 -0600
Anthony Liguori <anthony@codemonkey.ws> wrote:

> On 02/14/2011 06:45 AM, Luiz Capitulino wrote:
> > So the question is: how does the schema based design support extending
> > commands or events? Does it require adding new commands/events?
> >    
> 
> Well, let me ask you, how do we do that today?
> 
> Let's say that I want to add a new parameter to the `change' function so 
> that I can include a salt parameter as part of the password.
> 
> The way we'd do this today is by checking for the 'salt' parameter in 
> qdict, and if it's not present, use a random salt or something like that.

You likely want to do what you did before. Of course that you have to
consider if what you're doing is extending an existing command or badly
overloading it (like change is today), in this case you'll want to add
a new command instead.

But yes, the use-case here is extending an existing command.

> However, if I'm a QMP client, how can I tell whether you're going to 
> ignore my salt parameter or actually use it?  Nothing in QMP tells me 
> this today.  If I set the salt parameter in the `change' command, I'll 
> just get a success message.

I'm sorry?

{ "execute": "change", "arguments": { "device": "vnc", "target": "password", "arg": "1234", "salt": "r1" } }
{"error": {"class": "InvalidParameter", "desc": "Invalid parameter 'salt'", "data": {"name": "salt"}}}

> Even if we expose a schema, but leave things as-is, having to parse the 
> schema as part of a function call is pretty horrible,

That's a client implementation detail, they are not required to do it
as part of a function call.

But let me ask, if we don't expose a schema, how will clients be able to
query available commands/events and their parameters?

> particularly if 
> distros do silly things like backport some optional parameters and not 
> others.  If those optional parameters are deeply nested in a structure, 
> it's even worse.

Why would they do this? I mean, if distros (or anyone else shipping qemu)
goes that deep on changing the wire protocol they are on their own, why
would we want to solve this problem?

Note that this is different from having a modular design where we can offer
the possibility of disabling features, say, in configure. That should be
possible, but it's different from choosing random parameters in commands.

> OTOH, if we introduce a new command to set the password with a salt, it 
> becomes very easy for the client to support.  The do something as simple as:
> 
> if qmp.has_command("vnc-set-password-with-salt"):
>      qmp.vnc_set_password_with_salt('foobar', 'X*')
> else:
>      window.set_weak_security_icon(True)
>      qmp.vnc_set_password('foobar')
> 
> Now you could answer, hey, we can add capabilities then those 
> capabilities can quickly get out of hand.

Adding one command per new argument has its problems too and it's even
worse with events, as clients will have to be changed to handle a
new event just because of a parameter addition.

Look, although I did _not_ check any code yet, your description of the QAPI
looks really exciting. I'm not against it, what bothers me though is this
number of small limitations we're imposing to the wire protocol.

Why don't we make libqmp internal only? This way we're free to change it
whatever we want.