Module composition_validator 

Multi-subgraph federation composition validation and schema merging

This module implements comprehensive validation for composing multiple federated subgraphs into a single Apollo Federation supergraph. It validates:

• @key Consistency: All @extends types must use the same @key as the primary type
• @external Ownership: Each @external field must have exactly one owning subgraph
• @shareable Consistency: If a field is @shareable in one subgraph, it must be in all
• Type Merging: Proper composition of type definitions across subgraphs

Example

let subgraphs = vec![
    ("users".to_string(), users_metadata),
    ("orders".to_string(), orders_metadata),
];

let validator = CompositionValidator::new();
let composed = validator.validate_composition(subgraphs)?;

Architecture

The composition process works in two phases:

1. Consistency Validation (CrossSubgraphValidator)

   • Validates across all subgraphs simultaneously
   • Checks federation directives for conflicts
   • Collects all errors before returning

2. Schema Composition (CompositionValidator)

   • Merges type definitions from all subgraphs
   • Errors on type conflicts (safest default)
   • Produces final supergraph schema

Structs

ComposedSchema

Composed supergraph schema

ComposedType

A type in the composed supergraph

CompositionValidator

Composes multiple subgraph schemas into a supergraph

CrossSubgraphValidator

Validator for cross-subgraph federation consistency

Enums

CompositionError

Errors during schema composition

ConflictResolutionStrategy

Configuration for schema composition