Apollo Federation Schema Authoring Apollo Federation enables composing multiple GraphQL APIs (subgraphs) into a unified supergraph. Federation 2 Schema Setup Every Federation 2 subgraph must opt-in via @link : extend schema @link ( url : "https://specs.apollo.dev/federation/v2.12" , import : [ "@key" , "@shareable" , "@external" , "@requires" , "@provides" ] ) Import only the directives your subgraph uses. Core Directives Quick Reference Directive Purpose Example @key Define entity with unique key type Product @key(fields: "id") @shareable Allow multiple subgraphs to resolve field type Position @shareable { x: Int! } @external Reference field from another subgraph weight: Int @external @requires Computed field depending on external fields shippingCost: Int @requires(fields: "weight") @provides Conditionally resolve external field @provides(fields: "name") @override Migrate field to this subgraph @override(from: "Products") @inaccessible Hide from API schema internalId: ID! @inaccessible @interfaceObject Add fields to entity interface type Media @interfaceObject Reference Files Detailed documentation for specific topics: Directives - All federation directives with syntax, examples, and rules Schema Patterns - Multi-subgraph patterns and recipes Composition - Composition rules, error codes, and debugging Key Patterns Entity Definition type Product @key ( fields : "id" ) { id : ID ! name : String ! price : Int } Entity Contributions Across Subgraphs

Products subgraph

type Product @key ( fields : "id" ) { id : ID ! name : String ! price : Int }

Reviews subgraph

type Product @key ( fields : "id" ) { id : ID ! reviews : [ Review ! ] ! averageRating : Float } Computed Fields with @requires type Product @key ( fields : "id" ) { id : ID ! size : Int @external weight : Int @external shippingEstimate : String @requires ( fields : "size weight" ) } Value Types with @shareable type Money @shareable { amount : Int ! currency : String ! } Entity Stub (Reference Without Contributing) type Product @key ( fields : "id" , resolvable : false ) { id : ID ! } Ground Rules ALWAYS use Federation 2.x syntax with @link directive ALWAYS import only the directives your subgraph uses NEVER use @shareable without ensuring all subgraphs return identical values for that field PREFER @key with single ID field for simple entity identification USE rover supergraph compose to validate composition locally USE rover subgraph check to validate against production supergraph