W3C home > Mailing lists > Public > public-hydra@w3.org > January 2015

Re: remove hydra:Resource and hydra:Class (ISSUE-90)

From: Ruben Verborgh <ruben.verborgh@ugent.be>
Date: Tue, 13 Jan 2015 09:51:35 +0100
Cc: public-hydra@w3.org
Message-Id: <B7B0462D-5439-44AC-BC98-06605D6D7D03@ugent.be>
To: Markus Lanthaler <markus.lanthaler@gmx.net>
Hi Markus,

Thanks for summarizing.

I'll just react on two things that IMHO are not technically correct.

> Efficiency and performance improvements on the client and reduced load on
> the server. As Tomasz says
> On 12 Jan 2015 at 21:26, Tomasz Pluskiewicz wrote:
>> However the reason for hydra:Class as described in the specification
>> is the without guidance the client would have to blindly try
>> dereferencing everything.
> On 12 Jan 2015 at 22:22, Ruben Verborgh wrote:
>> That reason is incorrect.
> I don't think so.
>> A client's need to dereference something is not influenced in any way
>> by marking that something as hydra:Class.
>> Only a mechanism that says something is *not* dereferenceable can have
>> such an influence.
> It's not about the "need". Think of something simpler, something like a
> crawler. It just follows hyperlinks. It shouldn't have to try to dereference
> every identifier it finds..

We shouldn't forget that an absence of hydra:Resource does not mean anything.
To clarify matters, I will discuss such a crawler in two situations.


The crawler receives a document with 35 resources with an HTTP(S) URL.
30 of them are explicitly labeled with type hydra:Resource.

In order to find out whether the other 5 resources are dereferenceable,
the crawler has to perform 5 GET requests.
3 of them dereference, 2 do not.

In order to dereference the 30 remaining resources,
the crawler has to perform 30 GET requests.
The 5 others have been dereferenced already.

Total GET requests: 35. Dereferenced: 33.


The crawler receives a document with 35 resources with an HTTP(S) URL.
None of them have type hydra:Resource.

In order to find out whether the 35 resources are dereferenceable,
the crawler has to perform 35 GET requests.
By the same action, 33 resources are dereferenced.

Total GET requests: 35. Dereferenced: 33.

Same result in both cases. Note in particular how hydra:Resource
did not prevent us from having to check dereferenceability.

>> But then again, what tangible benefit does this give?
>> To dereference it, you must GET it.
>> To check whether it is dereferenceable, you must GET it.
> To know whether it's worth (or expected) to being checked.

> Ever tried to dereference xsd:integer to get its definition?

I cannot tell whether I should by the absence of hydra:Resource.
hydra:Resource does _not_ solve non-dereferenceable resources,
and, perhaps surprisingly, thus also not even dereferenceable resources.
Indeed, every resource that is _not_ labeled with hydra:Resource,
but still dereferences, is by definition a hydra:Resource.

So “whether it's worth being checked” cannot be derived
from the presence of hydra:Resource,
as all (HTTP[S]) resources are worth being checked
until indicated otherwise—which hydra:Resource cannot.

Furthermore, “expected” to be checked is not part of its definition.
Again, everything that is dereferenceable is by definition a hydra:Resource,
even though it's not indicated. Should we then check them all?

In other words: all cases where hydra:Resource is _not_ mentioned
is actually an omission in the response.
Whether or not a particular server thinks we should follow it
does not affect hydra:Resource-ness in any way,
but only our immediate knowledge of it (in the positive case).
If we want a stronger contract (“expected”), we should create one.
The hydra:Resource notion does not provide meaningful info.

I do understand the purpose of hydra:Resource and hydra:Class,
but I strongly feel we have chosen the wrong way of addressing that purpose.
What happens now is that we are defending hydra:Resource and hydra:Class
based on (sometimes incorrectly implied) features we'd miss if they were removed.
It should be the other way round: what are the features we need,
and are hydra:Resource and hydra:Class really the best way to bring them?


Received on Tuesday, 13 January 2015 08:52:04 UTC

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