Composer directives for omitting elements

[gmac]

Similar to other composer libraries, there should be some controls for prioritizing and hiding elements in a schema. Specifically:

• @override: prioritize a field from a given location. This can replace the current root_field_location_selector proc. This might make more sense as a @priority directive that can assign a numeric priority to fields. Priority determines the location’s rank in the field’s delegation set (the planner searches field locations from first-to-last, so the first location has priority). It might also be nice to prioritize fields as “-1” to omit them from the delegation set entirely so they are never considered (the planner will still pick non-priority locations when it allows making fewer requests).

• @inaccessible: eliminates a field or argument from the combined schema. The element is omitted from the supergraph when any subschema marks it as inaccessible. This feature has a few implications:

  • This can lead to empty object scopes (object types with zero fields), which should not be allowed. Raise composer error if a type resolves with zero fields.
  • This can also lead to unreachable types left in the schema. We’d need to traverse from root query and mutation types before building the supergraph and prune any types without field/argument usage.
  • Inaccessible elements should still be known to resolver queries, which may operate with subgraph information beyond the scope of the public supergraph. This extended criteria should only ever be selected through exports, so should be compatible with the current Shaper, but tests should confirm that.