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

[Jan Kiszka]

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.

There is a schema describing the fields (name, size, number of
elements), but their types (int, buffer, sub-field, array of X) are
derived from the JSON objects (ie. the JSON parser does this job).

> 
> 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.

What clients do you have in mind?

Jan

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