Concerns around the practicality of the requirement for an empty json object on present entity type identifiers

[MichaelFraser1999]

Section 3 includes a clause under metadata as follows

When an Entity participates in a federation or federations with one or more Entity Types, its Entity Configuration MUST contain a metadata claim with JSON object values for each of the corresponding Entity Type Identifiers, even if the values are the empty JSON object {} (when the Entity Type has no associated metadata or Immediate Superiors supply any needed metadata).

Having recently done a second implementation of this, I have concerns about the practicality of this. Take the following example:

graph TD;
    UkHealth-->Bar;
    UkHealth-->Baz;
    Baz-->Bin;
    Baz-->Bong;

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In a Federation containing and listing entity types:

• openid_provider
• openid_relying_party
• federation_entity

In this scenario, this is fine, but let's say Entity UkHealth joins another federation (maybe joining two national health federations together)

graph TD;
    HealthManager-->UkHealth;
    HealthManager-->IreHealth;
    UkHealth-->Bar;
    UkHealth-->Baz;
    Baz-->Bin;
    Baz-->Bong;
    IreHealth-->SomeEntity;
    IreHealth-->SomeOtherEntity

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Let's also say IreHealth contains oauth_client entity type identifiers too (or possibly some custom entity type identifier). As the spec is currently written, all participants in UkHealth will have to now list the oauth_client metadata as {} at the very least to be compliant. As global Federations grow (especially when custom entity identifiers are used), this will quickly become unsustainable and lead to interoperability issues.

I would like to understand why this was added and if we still feel it is needed.

If not, I'd be in favour of dropping this clause.

[MichaelFraser1999]

Having spoken with @selfissued, the intention here seems to be for the resolved metadata to contain this. As it reads right now, the text requires this in the Entity Configuration itself so this is possibly resolved just by changing where this is requirement is applied

[selfissued]

The requirement to contain an Entity Type Identifier for each of the roles than an Entity plays in a federation in its Entity Configuration metadata is there to declare what roles the Entity plays in the federation. Actions such as joining two federations won't change what roles each of its Entities plays. In particular, leaf nodes only play the roles that they're coded to perform and declare with their Entity Type Identifiers, no matter what kinds of other roles may be present elsewhere in the federation.

I propose to clarify this by changing

metadata
OPTIONAL. JSON object that represents the Entity's Types and the metadata for those Entity Types.

to

metadata
OPTIONAL. JSON object that declares roles that the Entity plays - its Entity Types - and that contains metadata for those Entity Types.