Re: [Qemu-devel] [PATCH 01/11] QMP: Introduce specification file

[Avi Kivity]

On 06/23/2009 09:32 PM, Anthony Liguori wrote:
> Avi Kivity wrote:
> > An RPC makes it a lot easier for clients to parse, there's no need 
> > for a regular expression per command, or to bother with line 
> > continuations etc.  You just focus on the data.
> >
> > We have an opportunity to make a clean break, let's make use of it.
>
> If we structure things internally correctly, by having a flow that 
> basically looks like:
>
> int <monitor_command>(Monitor *mon, <args>)
> {
>   // process args
>   if (success) {
>       monitor_print_data(mon, <highly structured output>);
>    } else {
>       record_extended_error(<message>);
>       return error_code;
>    }
> }
>
> Then it that works well for both the human monitor and the computer 
> monitor.  It's an RPC though.

It looks brittle to me.  What if you want to add an array?  Some 
structure that contains a nested structure?

> Should we support other types of RPCs?  I don't know.  I'd think that 
> you could create a computer monitor -> jsonrpc bridge easily enough.

No doubt, but I look at jsonrpc as a way of reducing the effort in 
qemu.  Not so much interested in it for its own sake.

> If we implement this correctly, then an RPC interface could just be 
> another option to see the computer monitor mode implemented as a 
> "module".

This is deep into overkill territory IMO.  We should pick some format 
that is good enough and has implementations for a variety of languages.  
Luiz' RFC was well done, but the subject is complex enough to be 
difficult to get right.  And as far as I could tell, all the issues are 
addressed by jsonrpc.  The only downside I can see is that jsonrpc is 
some kind of ajax-kiddie web-2.0 script dom thingie while we are a 
serious UNIX style daemon that complies with RFC 123456.


> > > I don't want to invent Xen-API here and go through that pain in QEMU.
> >
> > I don't see the parallel.
>
> Xen-API was the "clean break" for Xend.

Anything can be screwed up.  Normally I'm very much against rewrites and 
totally for incremental progress, but when it comes to protocols, you 
have to get them right first shot.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.