apollo-federation

To define entities in an Apollo Federation subgraph schema, use the @key directive to specify a type's unique identifier fields, enabling it to be referenced and resolved across multiple subgraphs within a supergraph.

The @shareable directive in GraphQL federation indicates that a field can be resolved by multiple subgraphs, allowing you to safely share type fields across different services without causing composition conflicts.

Use the @requires directive to compute a federated field based on @external fields from another subgraph, and use @provides to indicate a subgraph can resolve a field locally when querying an entity reference.

GraphQL schema composition fails with @external field errors when a subgraph marks a field as external but does not properly reference it using directives like @requires, or when the field is not actually defined in the owning subgraph.

The best way to troubleshoot Apollo Federation composition errors is to validate subgraph schemas for correct directive usage, ensuring entities, shared fields, and external references are properly defined before composing the supergraph.