[occi-wg] Another proposal on Linking

[Ralf Nyren]

Alexander,

Thanks for clarifying the Thin Link proposal. I would like to let the  
discussion of "Link as a resource or not" rest for a while and only focus  
on Thin vs Fat Links for now.

You state simplicity and non-extensibility as design goals of Thin Links.  
I understand your motivation but must object against the "no  
extensibility" part. I cannot see how non-extensible Thin Links could meet  
the real world use cases OCCI has to cope with for wider adoption.

 From my point of view there will always be use cases where it is necessary  
to express attributes of the relation between two resources, attributes  
which have no meaning without the relation. The alternative would be some  
sort of pseudo-resources existing only for the purpose of containing  
relation attributes but that does not seem like a clean approach to me.

Let me take my favorite example again. We have:
  - A Compute resource - in this case it is a Virtual Machine
  - A Storage resource - in this case it is a shared disk for use with a  
cluster filesystem (e.g. GFS2)
Now we need to attach the disk to the virtual machine and in order to do  
so we must specify what disk controller interface/address the disk should  
appear on in the virtual machine. Let's say we want it to appear as an IDE  
drive on bus 1, unit 0 (hdc for Linux people).

The disk controller attribute cannot be stored as a Compute attribute  
since it is irrelevant without the associated disk and a Compute resource  
can be Linked to many Storage resources.

The disk controller attribute cannot be stored as a Storage attribute  
since the Storage resource can be Linked to many Compute resources.

How do we solve this scenario with Thin Links?

regards, Ralf

On Tue, 17 Aug 2010 11:48:33 +0200, Alexander Papaspyrou  
<alexander.papaspyrou at tu-dortmund.de> wrote:

> Ok, to further confuse the situation, I decided to write down the "thin  
> link" proposal [1] we have been discussing as one alternative during the  
> last telco.
>
> Wearing my fire-proof pants...
>
> -Alexander
>
> [1] http://forge.ogf.org/sf/wiki/do/viewPage/projects.occi-wg/wiki/Link
>
> Am 17.08.2010 um 01:04 schrieb Edmonds, AndrewX:
>
>> Nice work Ralf! I like this, is KIS and covers the need for  
>> specialized/extended Link types. The only real difference with this and  
>> the previous is that the lifecycle of the link is managed by the source  
>> Resource and as such makes the association a composition. As it is a  
>> composite association there is no need to explicitly interact with the  
>> link via HTTP verbs but rather indirectly via its owning source  
>> Resource.
>>
>> To your question "Is it an interface or is it a representation model?"  
>> - It's both. The interface is the means by which we manipulate the  
>> model.
>>
>> Andy
>>
>> -----Original Message-----
>> From: occi-wg-bounces at ogf.org [mailto:occi-wg-bounces at ogf.org] On  
>> Behalf Of Ralf Nyren
>> Sent: Monday, August 16, 2010 7:09 PM
>> To: occi-wg at ogf.org
>> Subject: [occi-wg] Another proposal on Linking
>>
>> While analyzing the new Link & Linking proposal I went back to where we
>> started and tried to see what benefits the proposal would have. Except  
>> for
>> link attributes through link specialisation I failed to see any real
>> benefits. I mean it is indeed tempting to say that a Link is a REST
>> resource and therefore we can apply CRUD (POST/GET/PUT/DELETE)  
>> operations
>> directly on links. But why would we want to do that? A link is not very
>> useful on its own and what is it we cannot do with the simpler approach?
>>
>> Based on this reasoning I wrote up "another proposal" and added to the
>> Link & Linking wiki page [1]. It is indeed very simple (but I like
>> simple!) and I believe it can do all things we realistically would want  
>> to
>> do with the Fat Link approach.
>>
>> I am very much looking forward to your comments.
>>
>> regards, Ralf
>>
>> [1] http://forge.ogf.org/sf/wiki/do/viewPage/projects.occi-wg/wiki/Link
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