Re: [core] [T2TRG] Review of CoRAL

Christian M. Amsüss <christian@amsuess.com> Mon, 05 November 2018 12:01 UTC

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Date: Mon, 5 Nov 2018 13:01:10 +0100
From: Christian =?iso-8859-1?B?TS4gQW1z/HNz?= <christian@amsuess.com>
To: Klaus Hartke <hartke@projectcool.de>
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Subject: Re: [core] [T2TRG] Review of CoRAL
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Hello Klaus,

(recipients reduced as CoRE indicated interest in adoption)

On Sun, Nov 04, 2018 at 03:56:28PM +0100, Klaus Hartke wrote:
> > * How are compressed URIs expressed in CBOR? Flat [6, "foo", 6, "bar"]
> >   or nested [[6, "foo"], [6, "bar"]]?
> 
> should be flat: [6, "foo", 6, "bar"].

Very well (saves ~10% message size in the 6690 example), now fixed in
micrurus.

> If it improves clarity, the Python code could be updated to operate
> on flat lists instead.

The grouper version is probably more concise than the note that "the
Python code expects base and href not to be input in the trivial
deserialization of a CBOR array, but as a list of (option number, option
value) pairs" that should otherwise be there.


> body.py:
> >                if ':' in i.relation:
> >                    rel = i.relation
> >                else:
> >                    rel = 'http://www.iana.org/assignments/relation/' + i.relation
> 
> When a relation is provided as a string, the intention is that the
> string always contains the IRI. I don't think the current set of
> IANA-registered link relation types is relevant enough in the context
> of IoT that it should get special treatment.

Indeed. I don't know where the mistake came from -- my mental model
after my current reading of the document was yours, just my the code and
example from March did not match it.

> body.py:
> >            elif isinstance(i, BaseDirective):
> >                base = base / i.iri
> 
> The draft currently specifies that a base IRI is resolved against the
> current context IRI, not the previous base IRI.

Thanks; likewise, I missed updating the current relation number. Both
being fixed.

> > * Is there a difference between a CoRAL registry and a CoRAL profile?
> >   Both terms are used.
> 
> The draft uses the "profile" media type parameter defined in
> <https://tools.ietf.org/html/rfc6906> to indicate the CoAL registry,
> which I think makes more sense to define a new "registry" parameter
> with essentially the same semantics. So "profile" should appear only
> as the parameter name. I've replaced all instances where "profile"
> appeared without this meaning with "registry".

Having read up on them, a "profile" has a no-changes-to-interpretation
condition:

  A profile MUST NOT change the semantics of the resource representation
  when processed without profile knowledge, so that clients both with
  and without knowledge of a profiled resource can safely use the same
  representation

Given that a client without knowledge of the profile can't process a
representation, the parameter should probably be called something else.

> In general, I think URNs would make more sense for identifiers than
> HTTP URIs, as the latter always look like they could be successfully
> dereferenced. But it seems that HTTP URIs are more popular in the
> Semantic Web...

It's a matter of style; my take: Devices that dereference URIs on a whim
will cause trouble in some form anyway, better let that show on expected
input and not only when under attack. And as a developer, I'd rather
have a link to follow than know where to look up values in
subregistries.


Thanks
Christian

-- 
To use raw power is to make yourself infinitely vulnerable to greater powers.
  -- Bene Gesserit axiom