Shared types across subgraphs in GraphQL - Mini Project: Build & Apply

Shared Types Across Subgraphs in GraphQL

📖 Scenario: You are building a GraphQL API for an online bookstore. The API is split into two subgraphs: Books and Authors. Both subgraphs need to share a common type called Author to link books with their authors.This project will guide you through creating shared types across these subgraphs to enable smooth data fetching and type consistency.

🎯 Goal: Create two GraphQL subgraphs named Books and Authors. Define a shared Author type that both subgraphs use. Implement the necessary schema directives and type extensions to share the Author type properly.

📋 What You'll Learn

Create a Books subgraph schema with a Book type referencing Author

Create an Authors subgraph schema defining the Author type

Use the @key directive on the shared Author type for federation

Extend the Author type in the Books subgraph to reference it

Ensure the schemas are compatible for Apollo Federation

💡 Why This Matters

🌍 Real World

Large applications often split their GraphQL APIs into smaller subgraphs for better modularity and team collaboration. Sharing types across these subgraphs allows consistent data representation and smooth integration.

💼 Career

Understanding how to share types and use Apollo Federation directives is essential for backend developers working with GraphQL in microservices or modular API architectures.

1

Create the Authors subgraph schema with the shared Author type

Create a GraphQL schema string called authorsSchema that defines a type Author with fields id: ID! and name: String!. Add the @key(fields: "id") directive to the Author type to mark it as a federated entity.

GraphQL

# Define the authorsSchema string with the Author type and @key directive
# Your code here

Need a hint?

Use the @key directive on the Author type with fields: "id" to enable federation.

2

Create the Books subgraph schema with Book type and extend Author

Create a GraphQL schema string called booksSchema that defines a type Book with fields id: ID!, title: String!, and author: Author!. Extend the Author type with @key(fields: "id") directive, and add the id: ID! @external field inside the extension.

GraphQL

const authorsSchema = `
  type Author @key(fields: "id") {
    id: ID!
    name: String!
  }
`;

# Define the booksSchema string with Book type and extend Author type
# Your code here

Need a hint?

Remember to use extend type Author with @key and mark the id field as @external in the Books subgraph.

3

Add a resolver placeholder for the author field in Book type

Create a JavaScript object called booksResolvers with a Book field resolver for author. The resolver should accept book as the first argument and return an object with __typename: "Author" and id equal to book.authorId.

GraphQL

const authorsSchema = `
  type Author @key(fields: "id") {
    id: ID!
    name: String!
  }
`;

const booksSchema = `
  type Book {
    id: ID!
    title: String!
    author: Author!
  }

  extend type Author @key(fields: "id") {
    id: ID! @external
  }
`;

# Define booksResolvers object with Book.author resolver
# Your code here

Need a hint?

The resolver returns a reference to the shared Author type using __typename and the id field.

4

Complete the federation setup by exporting schemas and resolvers

Export the authorsSchema, booksSchema, and booksResolvers variables using export statements so they can be used to build the federated GraphQL gateway.

GraphQL

const authorsSchema = `
  type Author @key(fields: "id") {
    id: ID!
    name: String!
  }
`;

const booksSchema = `
  type Book {
    id: ID!
    title: String!
    author: Author!
  }

  extend type Author @key(fields: "id") {
    id: ID! @external
  }
`;

const booksResolvers = {
  Book: {
    author(book) {
      return { __typename: "Author", id: book.authorId };
    }
  }
};

# Export authorsSchema, booksSchema, and booksResolvers
# Your code here

Need a hint?

Use a single export statement listing all three variables.