[Qemu-devel] Re: [PATCH 3/8] Add QBuffer

[Anthony Liguori <anthony@codemonkey.ws>]

On 05/17/2010 02:45 AM, Avi Kivity wrote:
> On 05/17/2010 10:40 AM, Jan Kiszka wrote:
>>
>>> The alternative is to have a schema.  Sun RPC/XDR doesn't carry any 
>>> type
>>> information (you can't even distinguish between a number and text) 
>>> yet C
>>> clients have to problem extracting typed information from it.
>>>
>>> Having __class__ everywhere means we're carrying the schema in every
>>> message instead of just once.
>> The device_show command is already an example where you don't have a
>> predefined schema. It is derived from the data stream the encodes the
>> vmstate fields. So far we have no collision between base64-encoded
>> buffers and real strings, but this may actually change when we start
>> annotating the fields with symbolic constants.
>
> What is the receiver to do with it?
>
> If it doesn't know the schema (and there is no schema), then all it 
> can do is display the key/values.  If it does know the schema, then 
> __class__ is unnecessary.
>
> My worry is that __class__ will make the protocol more ad-hoc.
>
>> I really don't see the problem with __class__. Being a text protocol,
>> JSON is already fairly verbose.
>
> The problem is not the verbosity, it's that information is carried too 
> late.  Many clients want to know this information at compile time or 
> initialization time, and we are providing it at object instantiating 
> time.

Whether a protocol is self-describing is orthogonal to whether it's well 
defined (ala a schema).  A self-describing protocol is very convenient 
for dynamic languages like Python.  We should also provide a formal 
schema though for languages that require IDL to generate bindings (like C).

Regards,

Anthony Liguori