Re: [Qemu-devel] Re: [FOR 0.12 PATCH 18/18] QMP: add human-readable description to error response

[Markus Armbruster]

Luiz Capitulino <address@hidden> writes:

> On Tue, 8 Dec 2009 12:25:13 +0000
> "Daniel P. Berrange" <address@hidden> wrote:
>
>> On Tue, Dec 08, 2009 at 10:11:48AM -0200, Luiz Capitulino wrote:
>> > On Mon,  7 Dec 2009 21:37:16 +0100
>> > Markus Armbruster <address@hidden> wrote:
>> > 
>> > > -{ "error": { "class": json-string, "data": json-value }, "id": 
>> > > json-value }
>> > > +{ "error": { "class": json-string, "data": json-value, "desc": 
>> > > json-string },
>> > > +  "id": json-value }
>> > >  
>> > >   Where,
>> > >  
>> > >  - The "class" member contains the error class name (eg. 
>> > > "ServiceUnavailable")
>> > >  - The "data" member contains specific error data and is defined in a
>> > >    per-command basis, it will be an empty json-object if the error has 
>> > > no data
>> > > +- The "desc" member is a human-readable error message.  Clients should
>> > > +  not attempt to parse this message.
>> > >  - The "id" member contains the transaction identification associated 
>> > > with
>> > >    the command execution (if issued by the Client)
>> > 
>> >  As we've talked on irc, I don't agree with this change.
>> > 
>> >  Basically, adding 'desc' to the standard error message introduces all
>> > the problems we've discussed about free-form English strings.
>> > 
>> >  I feel that QError is becoming the worst of all proposals.
>> > 
>> >  I agree with you that it's not as easy as it should be to report errors,
>> > but as we're targeting on Clients I was convinced that we could not have 
>> > the
>> > best API internally but offer a good interface for Clients.
>> > 
>> >  Now, having 'desc' as part of the standard protocol is like not having
>> > the best API internally and offering a bad interface for Clients.
>> > 
>> >  Not to mention that those strings can't be modified when the protocol
>> > becomes stable and we're probably talking about dozens if not a hundred
>> > of strings. Ok, there isn't a reason to change them often, but it's
>> > still one more thing to maintain.
>> 
>> I think it is fine to declare that 'desc' strings are subject to 
>> arbitrary change. Even if QEMU includes this 'desc' I think in libvirt
>> will end up doing its own error code -> human string conversion,
>> simply because we need to translate the strings. So we'd only use
>> 'desc' in any logging calls if it were present.
>
>  That's what we would expect from clients, but I'm not convinced that
> they'll took the easy way and start using it the way they shouldn't.

Change a couple of common messages in every release, and they'll stop
doing that real fast ;)