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

[Jan Kiszka <jan.kiszka@siemens.com>]

Avi Kivity wrote:
> On 05/17/2010 03:12 AM, Anthony Liguori wrote:
>> On Sun, May 16, 2010 at 12:38 PM, Jamie Lokier<jamie@shareable.org> 
>> wrote:
>>   
>>> Anthony Liguori wrote:
>>>     
>>>> Instead of encoding just as a string, it would be a good idea to encode
>>>> it as something like:
>>>>
>>>> {'__class__': 'base64', 'data': ...}
>>>>        
>>> Is there a benefit to the class indirection, over simply a keyword?:
>>>
>>> {'__base64__': ...}
>>>
>>> __class__ seems to suggest much more than it's being used for here.
>>>      
>> Yes.  The problem with JSON is that it's based on JavaScript and
>> JavaScript is goofy :-)
>>
>>    
> 
> I suggest completely ignoring JavaScript.  JSON is simply an encoding
> for numbers, strings, arrays, and key-value stores.  Where's the goofiness?
> 
>> JavaScript's object mechanism doesn't map well to most other languages
>> since it's prototype based.  What we're calling QDict's are really
>> objects in JavaScript and they carry mostly no type information.  With
>> JS, it's very simple to treat a generic object as a specialized class
>> after instantiation which means objects don't need type information.
>>
>> For non-prototype languages, which is the vast majority of clients,
>> it's necessary to have type information at instantiation time since
>> monkey patching is awkward at best.  That's why we need a special,
>> reserved, object member to carry type information.  The remainder of
>> the object members represent the serialized state of the object.
>>    
> 
> 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.

I really don't see the problem with __class__. Being a text protocol,
JSON is already fairly verbose.

Jan

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