GraphQL Federation

Di microservices, setiap service punya schema sendiri. Federation menggabungkan multiple schema menjadi satu unified graph yang client akses via satu endpoint.

Architecture

Client → Gateway (Router)
           ├── User Service (User schema)
           ├── Post Service (Post schema)
           └── Comment Service (Comment schema)

# Client melihat satu schema unified
# Gateway routes queries ke service yang tepat

Subgraph Schemas

# User Service
type User @key(fields: "id") {
  id: ID!
  name: String!
  email: String!
}

type Query {
  user(id: ID!): User
  me: User
}

# Post Service
type Post @key(fields: "id") {
  id: ID!
  title: String!
  content: String!
  author: User! # Reference ke User dari service lain
}

# Extend User dari service lain
extend type User @key(fields: "id") {
  id: ID! @external
  posts: [Post!]!  # Field baru yang di-resolve di Post Service
}

@key Directive

Menandakan entity yang bisa di-reference antar subgraph. Gateway menggunakan key untuk "stitch" data.

Reference Resolver

// Post Service harus bisa resolve User reference
const resolvers = {
  User: {
    __resolveReference: (reference) => {
      // reference = { id: "123" }
      // Return user posts
      return { id: reference.id };
    },
    posts: (user) => {
      return db.posts.findMany({ where: { authorId: user.id } });
    },
  },
};

Tools

• Apollo Router — Production-ready gateway (Rust, high performance)
• Apollo Studio — Schema registry, composition checks
• Hive — Open-source schema registry alternative