apollo-federation

The full skill.

Original instructions from the publisher’s SKILL.md

# 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`:

```graphql
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](https://www.apollographql.com/docs/graphos/schema-design/federated-schemas/reference/versions)
for currently supported versions before copying it verbatim.

> A subgraph's `@link` version is a floor, not the composition version — see
> [Federation versions](references/composition.md#federation-versions-floor-vs-composition)
> 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](references/directives.md) - All federation directives with syntax, examples, and rules
- [Schema Patterns](references/schema-patterns.md) - Multi-subgraph patterns and recipes
- [Composition](references/composition.md) - Composition rules, error codes, and debugging

## Key Patterns

### Entity Definition

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

### Entity Contributions Across Subgraphs

```graphql
# 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

```graphql
type Product @key(fields: "id") {
  id: ID!
  size: Int @external
  weight: Int @external
  shippingEstimate: String @requires(fields: "size weight")
}
```

### Value Types with @shareable

```graphql
type Money @shareable {
  amount: Int!
  currency: String!
}
```

### Entity Stub (Reference Without Contributing)

```graphql
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