apollo-federation

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. The version shown (v2.12) is illustrative — check the Federation changelog for currently supported versions before copying it verbatim.

A subgraph's @link version is a floor, not the composition version — see Federation versions for the full explanation.

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