[rest-practices] Read-only fields in a representation type

[Bob McWhirter]

>> Rather than silently discard read-only fields, though, I'd prefer  
>> us to
>> return an error. That way the semantics are clear.
>
> That's kinda the opposite of what we expect from clients - for  
> those, we
> want them to pick out what they know about, and ignore the rest. As  
> long
> as what they send is unambiguous, I don't see the harm in ignoring
> read-only fields.

In fact, one usage is to fetch XML, simply modify that document (using  
xpath) and returning it, in its entirety, back to the server.

There, I'd hope that server really only considers fields that've  
honestly *changed*.  If I replace a readonly value with another  
different value, I'd expect an error (Sorry, can't modify a read-only  
field), but otherwise, I'd expect it to notice what really got changed  
in the read/write attributes.

With REST, client and server are exchanging their views of the world.   
The client's not instructing the server to change particular fields.  
More "Hey, I now consider this object to look like this".

That can either be compatible, or incompatible (ie, changes to read- 
only fields) with the server's policy.

	-Bob