[occi-wg] Query uniqueness

[Ralf Nyren]

please see below:

On Sun, 10 Apr 2011 19:08:44 +0200, Gary Mazz <garymazzaferro at gmail.com>  
wrote:

> Maybe I'm looking that the specifications too long, but it seems you can
> place more than one term in a query named space... for example, a query
> named space '/network/'  could refer to both the occi scheme and a
> private scheme.

The namespace for different Category/Kind/Mixin instances is defined by  
the scheme, e.g. http://schemas.ogf.org/occi/core#. In this case we talk  
about the namespace for different types, i.e. Entity, Resource, Link,  
Compute, Storage, etc.

We also talk about the namespace used for Entity IDs, i.e. the IDs of  
resource instances in a system. In the case of the HTTP Rendering this  
namespace is mapped to the URL namespace of HTTP.

To further complicate things we have a mechanism of hooking a Kind/Mixin  
instances into this HTTP URL namespace by use of the "location" parameter.  
As you know a Kind or Mixin instance represents a collection of all  
resource instances (Entity sub-type instances) associated with the Kind or  
Mixin. This collection can be retrieved by querying the "location" URL.

So to answer your question on the mapping of e.g. the "/network/" HTTP  
path it depends if you have a Kind/Mixin hooked into that path or not. If  
there is such a hook (i.e. you have a Category: ... location=/network/  
somewhere in the query interface) you will find the collection of the  
Kind/Mixin. However if you do not have a Kind/Mixin location mapped at  
/network/ you must follow the rules in section 3.4.2 "Operation on Paths  
in the Name-space".

hmm... makes sense? :P

> There is no requirement that identical 'terms' used to describe a
> resource category to have a similar or the same functional
> characteristics as terms defined in the occi scheme.

You are free to define e.g. Category: compute;  
scheme=http://provider.com/blah#  which is represents a coffee machine. It  
might be slightly misleading but as long as you use your own scheme  
namespace you can essentially do whatever you want.

> There is a mention of an "entity_type" to "Entity type uniquely
> identi%0Cfied by the Kind instance." However it doesn't say "entity_type"
> uniquely "identifies" the Kind Instance. The only way to uniquely
> identify a Kind is by scheme:term.  And this leads back to the issue
> with the query

The Kind.entity_type is intended to contain a reference to the type (a  
class in most OO langs) which is identified by the Kind. Example  
(python-ish):

# Compute is a class which inherits Resource which inherits Entity.

# Thus is the Kind instance which identifies the Compute type (class)  
defined:
compute_kind = Kind()
compute_kind.term = "compute"
compute_kind.scheme = "http://schemas.ogf.org/occi/core#"
compute_kind.entity_type = Compute

# If you want to create a new Compute instance you could do:
class_ref = compute_kind.entity_type
my_compute = class_ref()

So Kind.entity_type is a reference to the type which the Kind instance  
identifies. This is of course not applicable to all programming languages  
but the idea is that given an instance of Kind you should be able to find  
out the Entity type it identifies.

/Ralf