Subgraph definition in GraphQL - Mini Project: Build & Apply

Create a GraphQL Subgraph Definition

📖 Scenario: You are building a modular GraphQL API for a bookstore. Each part of the API is a subgraph that handles specific data. Your task is to define a subgraph schema for the Books part of the API.

🎯 Goal: Define a GraphQL subgraph schema for books with types and directives that enable it to work as a subgraph in a federated GraphQL architecture.

📋 What You'll Learn

Create a type Book with fields id (ID!), title (String!), and author (String!)

Add the @key(fields: "id") directive to Book to specify the primary key

Add the @extends directive to Book if extending from another subgraph (optional for this project)

Include the @external directive on fields that are resolved by other subgraphs (optional for this project)

Define the Query type with a field books returning a list of Book

💡 Why This Matters

🌍 Real World

GraphQL subgraphs allow teams to build modular APIs that can be composed into a single federated graph, improving scalability and collaboration.

💼 Career

Understanding subgraph definitions is essential for backend developers working with GraphQL federation in modern microservice architectures.

1

Define the Book type with fields

Create a GraphQL type called Book with fields id of type ID!, title of type String!, and author of type String!.

GraphQL

# Define the Book type with id, title, and author fields
# Your code here

Need a hint?

Use the type keyword to define a GraphQL type. Fields have names and types separated by a colon.

2

Add the @key directive to Book

Add the @key(fields: "id") directive to the Book type to specify that id is the primary key for this subgraph.

GraphQL

type Book @key(fields: "id") {
  id: ID!
  title: String!
  author: String!
}
# Your code here

Need a hint?

The @key directive is used to mark the primary key field(s) for federation.

3

Define the Query type with books field

Create a Query type with a field called books that returns a list of Book objects. Use square brackets [] to indicate a list.

GraphQL

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

# Define the Query type with books field
# Your code here

Need a hint?

The Query type is the entry point for queries. Use square brackets to indicate a list of items.

4

Add the @extends and @external directives (optional)

Add the @extends directive to the Book type and the @external directive to the author field to indicate that author is resolved by another subgraph.

GraphQL

type Book @key(fields: "id") @extends {
  id: ID!
  title: String!
  author: String! @external
}

type Query {
  books: [Book]
}
# Your code here

Need a hint?

The @extends directive marks a type as extending another subgraph's type. The @external directive marks fields resolved elsewhere.