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

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

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

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

Hint

Use a single export statement listing all three variables.

Practice

1. What is the main purpose of using @key in shared types across GraphQL subgraphs?

easy

A. To mark fields that uniquely identify an entity across subgraphs

B. To define a field as optional in the schema

C. To specify the data type of a field

D. To mark a field as deprecated

Solution

1. Step 1: Understand the role of @key

   The @key directive marks fields that uniquely identify an entity across subgraphs, enabling them to share the same type.

2. Step 2: Differentiate from other directives

   Other directives like @external or @deprecated serve different purposes, not unique identification.

3. Final Answer:

   To mark fields that uniquely identify an entity across subgraphs -> Option A

4. Quick Check:

   @key marks unique identifiers [OK]

Hint: Remember: @key means unique ID for shared types [OK]

Common Mistakes:

• Confusing @key with @external
• Thinking @key marks optional fields
• Assuming @key defines data types

2. Which of the following is the correct way to mark a field as coming from another subgraph in a shared type?

easy

A. Use @key directive on the field

B. Use @provides directive on the field

C. Use @requires directive on the field

D. Use @external directive on the field

Solution

1. Step 1: Identify the directive for external fields

   The @external directive marks fields that are owned by another subgraph but referenced in the current one.

2. Step 2: Differentiate from other directives

   @key marks unique identifiers, @requires and @provides relate to field dependencies, not external ownership.

3. Final Answer:

   Use @external directive on the field -> Option D

4. Quick Check:

   @external marks fields from other subgraphs [OK]

Hint: External fields use @external directive [OK]

Common Mistakes:

• Using @key instead of @external
• Confusing @requires with @external
• Not marking external fields at all

3. Given the following subgraph schema snippet:

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

Which statement is true about the price field?

medium

A. It is defined and owned by this subgraph

B. It is a unique identifier for Product

C. It is defined in another subgraph and referenced here

D. It is deprecated and should not be used

Solution

1. Step 1: Analyze the @external directive on price

   The @external directive means price is not owned here but comes from another subgraph.

2. Step 2: Understand the role of @key on id

   The id field is the unique identifier, so price is not an ID.

3. Final Answer:

   It is defined in another subgraph and referenced here -> Option C

4. Quick Check:

   @external means field is from another subgraph [OK]

Hint: Fields with @external come from other subgraphs [OK]

Common Mistakes:

• Thinking @external means field is owned here
• Confusing @key with @external
• Assuming @external means deprecated

4. Consider this subgraph type definition:

type User @key(fields: "userId") {
  userId: ID!
  email: String @external
  name: String
}

Which statement is true about the email field?

medium

A. It is defined in another subgraph and referenced here

B. The @key directive must include email field

C. The userId field cannot be used as a key

D. The name field must be marked @external

Solution

1. Step 1: Analyze the @external directive on email

   The @external directive means email is defined in another subgraph and referenced here.

2. Step 2: Differentiate from other fields

   userId is the @key field provided locally, name is owned locally (no directive).

3. Final Answer:

   It is defined in another subgraph and referenced here -> Option A

4. Quick Check:

   @external means field from another subgraph [OK]

Hint: @external means field from another subgraph [OK]

Common Mistakes:

• Thinking @external means owned locally
• Believing @key must include all fields
• Assuming all fields need @external

5. You have two subgraphs sharing a Book type. Subgraph A defines:

type Book @key(fields: "isbn") {
  isbn: ID!
  title: String
}

Subgraph B defines:

extend type Book @key(fields: "isbn") {
  isbn: ID! @external
  author: String
}

Which statement best describes how these shared types work together?

hard

A. Both subgraphs own isbn, causing a conflict

B. Subgraph A owns isbn and title, Subgraph B extends Book using isbn as key and adds author

C. Subgraph B owns isbn and author, Subgraph A only references isbn

D. Subgraph B cannot extend Book without redefining title

Solution

1. Step 1: Identify ownership of fields

   Subgraph A defines Book with isbn and title, so it owns these fields.

2. Step 2: Understand extension in Subgraph B

   Subgraph B extends Book, marking isbn as @external (owned by A) and adds author.

3. Step 3: Confirm no conflicts

   Using @key with the same field isbn allows both subgraphs to share the type without conflict.

4. Final Answer:

   Subgraph A owns isbn and title, Subgraph B extends Book using isbn as key and adds author -> Option B

5. Quick Check:

   Extension uses @external keys to share types [OK]

Hint: Extension uses @external keys to share types [OK]

Common Mistakes:

• Thinking both subgraphs own the same key field
• Believing extension requires redefining all fields
• Assuming conflicts occur with shared keys