Re: [apps-discuss] JSON Schema considered harmful

["Tschofenig, Hannes (NSN - FI/Espoo)" <[e-mail]>]

My 5 cents to this topic. 

 

Even though many folks, including Tim Bray, educated the community that
it is not necessary to produce XML schemas when writing specifications
that use XML, everyone did. They felt that the specification was not
complete without the schema.

 

The consequence was that 

*        The XML replaced text in specifications leaving important
semantic unspecified. Look at many of the OASIS document, for example
CAP, and you know what I am talking about. The authors somehow thought
that the schema is a replacement for a description of what the protocol
actually does. Wrong! Interoperability problems are the consequence.

*        Sometimes the XML schema was provided in addition to a textual
description. Nice idea but then they often get out of sync. 

*        Extensibility thoughts were left behind. XML by itself meant
extensible, at least that's what many thought, and hence there was no
story about how the protocol would get extended in a smooth way and what
the policies are for extensions.

 

The worst thing was, however, that the entire protocol development was
guided by the (lack of) understanding of the XML schema language rather
than the requirements for the actual protocol. 

 

Funny enough the schema did not really help anyone: it did not help a
person reading the specification (who does not want to implement it)
since they typically care about an entire level of detail nor the guy
who wants to implements it. Just using the schema for an implementation
was never enough because you had to consider all the other special case
as well (which can typically only be found elsewhere in the
specification). If you managed to encode all the special cases into the
schema then it became unreadable. The instance validation against
concept was also useless once you included the proper extension points. 

 

In a nutshell, I think that XML (& Relax NG) schemas are harmful.
Unfortunately, it is already too late for fixing that in the IETF. I
believe we should not introduce schemas into JSON in the IETF.
Everything will be fine without using it. 

 

Ciao

Hannes