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

[Anthony Liguori]

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.

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.

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, 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.

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.

Regards,

Anthony Liguori

> While the current code is in really in bad shape currently, I'm not sure that
> having this disadvantage will pay off the new design.
>
>