Entity Sets | Redfish Specification Forum

fish
Guppy

Posts: 65

Entity Sets Dec 19, 2017 13:29:55 GMT

Quote

Post by fish on Dec 19, 2017 13:29:55 GMT

Hi,

Text from chapter 6.2 in Swordfish specification below.

6.2 Entity Sets

The Swordfish model does not currently expose any explicitly defined entity sets. OData specifies that an entity set is defined for each
NavigationProperty that is defined as a collection and that has the ContainsTarget attribute set to true. In all other cases, Swordfish assumes that an
entity set is defined globally within the implementation for each entity type. This is effectively the same as if the entity sets were explicitly defined in
the ServiceRoot entity container.

I asume this does not mean that Swordfish does not defined "Resource Collections"? (as many such exist in the schemas..)

Could someone explain what this means in practice? Examples? And what is different from "original Redfish"? (And why?)

Richelle Ahlvers
Minnow

Posts: 44

Entity Sets Dec 21, 2017 21:08:33 GMT

Quote

Post by Richelle Ahlvers on Dec 21, 2017 21:08:33 GMT

Swordfish is an extension of Redfish. All of the requirements set forth in the Redfish specification also apply to Swordfish.

This language is simply clarifying that the schema is not explicitly defining OData Entity Sets; the Swordfish specification does not in any way imply that Swordfish does not define "Resource Collections", as that would be a direct contradiction of the Redfish specification and behaviors. The Swordfish schema contains many Resource Collections.

Richelle

fish
Guppy

Posts: 65

Entity Sets Dec 22, 2017 13:00:05 GMT

Quote

Post by fish on Dec 22, 2017 13:00:05 GMT

I think that this chapter 6.2 "Entity set" is something that may differ between Redfish and Swordfish. I cant find this kind of "disclaimer" in Redfish specification.
I think also that Redfish definition of collections means that collcetions should be regarded as entity set, modelling containment/child relations only, in same way as for
entity sets on OData. (But Im not an OData expert..) Text from from OData spec: "An entity can be a member of at most one entity set".
It seems that Swordfish for some reason choses to handle this differently, by stating that no entity sets are defined explicitely. I assume that this means that
collections are NOT (always) entity set in Swordfish. Meaning that a resource may not be member
of more that one collection in Redfish, but in Swordfish it can be OK to list one resource instance (entity) in mutiple collections. (This seems also to be used
in Swordfish schemas and mockup.) In Swordfish it will be difficult to know which collation that actually conatins the resource (child/containement),
but according to gericson in below thread this is intentional, and left to each implementation. Personally I prefer the Redfish way to handle this, even if this also has some disadvantages.

See also possibly relate thread:
redfishforum.com/thread/88/drives-volumes

gericson
Minnow

Posts: 44

Entity Sets Jun 25, 2018 21:17:22 GMT

Quote

Post by gericson on Jun 25, 2018 21:17:22 GMT

Dec 22, 2017 13:00:05 GMT fish said:

I think that this chapter 6.2 "Entity set" is something that may differ between Redfish and Swordfish. I cant find this kind of "disclaimer" in Redfish specification.
I think also that Redfish definition of collections means that collcetions should be regarded as entity set, modelling containment/child relations only, in same way as for
entity sets on OData. (But Im not an OData expert..) Text from from OData spec: "An entity can be a member of at most one entity set".
It seems that Swordfish for some reason choses to handle this differently, by stating that no entity sets are defined explicitely. I assume that this means that
collections are NOT (always) entity set in Swordfish. Meaning that a resource may not be member
of more that one collection in Redfish, but in Swordfish it can be OK to list one resource instance (entity) in mutiple collections. (This seems also to be used
in Swordfish schemas and mockup.) In Swordfish it will be difficult to know which collation that actually conatins the resource (child/containement),
but according to gericson in below thread this is intentional, and left to each implementation. Personally I prefer the Redfish way to handle this, even if this also has some disadvantages.

See also possibly relate thread:
redfishforum.com/thread/88/drives-volumes

See: odata-csdl-xml clause 13.2 EntitySet.

Entity sets are top-level collection-valued resources.
An entity set is identified by its name, a simple identifier that MUST be unique within its entity container.
An entity set MUST specify a type that MUST be an entity type in scope.
An entity set MUST contain only instances of its specified entity type or its subtypes. The entity type MAY be abstract but MUST have a key defined.
An entity set MAY indicate whether it is included in the service document. If not explicitly indicated, it is included.
Entity sets that cannot be queried without specifying additional query options SHOULD NOT be included in the service document.

Also see: odata-csdl-xml clause 13.3 Singleton.

Singletons are top-level single-valued resources.
A singleton is identified by its name, a simple identifier that MUST be unique within its entity container.
A singleton MUST specify a type that MUST be an entity type in scope. A singleton MUST reference an instance its entity type.

A Redfish ResourceCollection is NOT an OData EntitySet. Instead, a ResourceCollection is defined as an OData EntityType. Entity types based on ResourceCollection each contain a Navigation Property named Members that is defined as a collection of entities.

OData allows query parameters like $skip and $top on a GET against a URL that locates a collection of entities. (Like the Members property of a Redfish ResourceCollection.)

Redfish extends the semantics of a ResourceCollection to act as if it is the underlying Members collection.

OData further defines that a NavigationProperty that is qualified with the attribute ContainsTarget="true" implies either a Singleton or an EntitySet, depending on whether it is a scalar or a collection.

The clause 6.2 Entity Sets of the Swordfish specification is not a difference between Redfish and Swordfish. That clause is only meant to explain some of the above semantic nuances as defined in Redfish/Swordfish and as defined in OData.

Last Edit: Jun 25, 2018 21:19:58 GMT by gericson