Apollo Federation concepts in GraphQL - Mini Project: Build & Apply

Building a Simple Apollo Federation Setup

📖 Scenario: You are working on a project where multiple teams manage different parts of a product catalog. Each team has its own GraphQL service. You want to combine these services into one unified graph so clients can query all product information easily.

🎯 Goal: Build a basic Apollo Federation setup with two services: products and reviews. Each service defines its own GraphQL schema. Then create a gateway schema that composes these services into one federated graph.

📋 What You'll Learn

Create a products service schema with a Product type having id and name fields

Create a reviews service schema with a Review type and extend the Product type to add reviews field

Use Apollo Federation directives like @key and @extends correctly

Create a gateway schema that composes the two services into one federated graph

💡 Why This Matters

🌍 Real World

Large companies often split their GraphQL APIs into multiple services owned by different teams. Apollo Federation helps combine these into one seamless API for clients.

💼 Career

Understanding Apollo Federation is valuable for backend developers working with GraphQL in microservice architectures.

1

Create the Products Service Schema

Create a GraphQL schema string called productsTypeDefs that defines a Product type with fields id (ID!) and name (String!). Add the Apollo Federation directive @key(fields: "id") to the Product type.

GraphQL

# Your code here to create productsTypeDefs schema string

Hint

Use backticks to create a multi-line string. The @key directive marks the primary key field for federation.

2

Create the Reviews Service Schema

Create a GraphQL schema string called reviewsTypeDefs that defines a Review type with fields id (ID!) and content (String!). Extend the Product type with @extends and @key(fields: "id") directives, adding a reviews field that returns a list of Review.

GraphQL

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

# Your code here to create reviewsTypeDefs schema string

Hint

Use extend type to add fields to Product in the reviews service. Mark id as @external because it is owned by the products service.

3

Create the Apollo Gateway Setup

Create a variable called gateway that initializes an ApolloGateway instance with a serviceList containing two services: one named products with URL http://localhost:4001/graphql and one named reviews with URL http://localhost:4002/graphql.

GraphQL

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

const reviewsTypeDefs = `
  type Review {
    id: ID!
    content: String!
  }

  extend type Product @key(fields: "id") {
    id: ID! @external
    reviews: [Review]
  }
`;

# Your code here to create ApolloGateway instance called gateway

Hint

Use new ApolloGateway({ serviceList: [...] }) to configure the gateway with the two services.

4

Complete the Apollo Server Setup with Gateway

Create a variable called server that initializes an ApolloServer instance using the gateway variable. Set subscriptions to false in the server options.

GraphQL

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

const reviewsTypeDefs = `
  type Review {
    id: ID!
    content: String!
  }

  extend type Product @key(fields: "id") {
    id: ID! @external
    reviews: [Review]
  }
`;

const gateway = new ApolloGateway({
  serviceList: [
    { name: 'products', url: 'http://localhost:4001/graphql' },
    { name: 'reviews', url: 'http://localhost:4002/graphql' }
  ]
});

# Your code here to create ApolloServer instance called server with gateway and subscriptions false

Hint

Pass the gateway variable to the ApolloServer constructor and disable subscriptions.

Practice

1. What is the primary purpose of Apollo Federation in GraphQL?

easy

A. To replace REST APIs with GraphQL

B. To create a new database schema

C. To combine multiple GraphQL services into a single API

D. To optimize SQL queries automatically

Solution

1. Step 1: Understand Apollo Federation's role

   Apollo Federation is designed to combine multiple GraphQL services into one unified API.

2. Step 2: Compare options

   Options B, C, and D describe unrelated tasks. Only To combine multiple GraphQL services into a single API correctly describes Apollo Federation's purpose.

3. Final Answer:

   To combine multiple GraphQL services into a single API -> Option C

4. Quick Check:

   Apollo Federation = combine services [OK]

Hint: Federation = combining services into one API [OK]

Common Mistakes:

• Confusing Federation with database schema design
• Thinking Federation replaces REST APIs directly
• Assuming Federation optimizes SQL queries

2. Which of the following is the correct syntax to mark a unique identifier for an entity in Apollo Federation?

easy

A. type User @unique(fields: "id") { id: ID! name: String }

B. type User @key(fields: "id") { id: ID! name: String }

C. type User @id(fields: "id") { id: ID! name: String }

D. type User @primary(fields: "id") { id: ID! name: String }

Solution

1. Step 1: Recall the directive for unique identifiers

   In Apollo Federation, the @key directive marks the unique identifier fields for an entity.

2. Step 2: Check syntax correctness

   type User @key(fields: "id") { id: ID! name: String } uses @key(fields: "id"), which is the correct syntax. Other options use incorrect directives.

3. Final Answer:

   type User @key(fields: "id") { id: ID! name: String } -> Option B

4. Quick Check:

   @key directive = unique ID marker [OK]

Hint: Use @key to mark unique entity IDs [OK]

Common Mistakes:

• Using @unique or @id instead of @key
• Missing quotes around field names
• Confusing @key with database primary key syntax

3. Given the following schema in a federated service:

extend type Product @key(fields: "upc") { upc: String @external price: Int }

What does the extend type keyword do here?

medium

A. Adds the price field to an existing Product type from another service

B. Removes the upc field from Product type

C. Defines a new Product type with fields upc and price

D. Creates a local copy of Product type without federation

Solution

1. Step 1: Understand 'extend type' in Apollo Federation

   The extend type keyword adds fields to a type defined in another service.

2. Step 2: Analyze the example

   The Product type is extended to add the price field, while upc is marked as external, meaning it comes from the original service.

3. Final Answer:

   Adds the price field to an existing Product type from another service -> Option A

4. Quick Check:

   extend type = add fields to existing type [OK]

Hint: extend type adds fields to existing types [OK]

Common Mistakes:

• Thinking extend type creates a new type
• Confusing @external with local fields
• Assuming extend type removes fields

4. Identify the error in this federated schema snippet:

type Review @key(fields: "id") { id: ID! body: String author: User }

Assuming User is defined in another service, what is missing?

medium

A. The User type should be imported explicitly

B. The author field should be marked with @external

C. The author field should be marked with @provides or @requires

D. The Review type should use extend keyword

Solution

1. Step 1: Understand referencing external entities

   In Apollo Federation, to reference an entity type like User from another service, the schema must explicitly extend that type with its @key directive: extend type User @key(fields: "id") { id: ID! }.

2. Step 2: Analyze the snippet

   The Review type references User via author: User but lacks the required extend declaration for User, which is necessary for the gateway to know this service can resolve User.

3. Final Answer:

   The User type should be imported explicitly -> Option A

4. Quick Check:

   Reference entities -> extend type @key [OK]

Hint: Extend referenced entities with @key [OK]

Common Mistakes:

• Marking author field with @provides or @requires
• Marking author field as @external
• Using extend keyword for Review type

5. You have two services: Accounts defining type User @key(fields: "id") { id: ID! name: String } and Reviews extending User with extend type User @key(fields: "id") { id: ID! reviews: [Review] }. How does Apollo Federation resolve the User entity across these services?

hard

A. It requires manual merging of User data in the gateway

B. It duplicates User types separately in each service

C. It ignores the extend type and uses only the Accounts User type

D. It merges User types by matching the @key field 'id' across services

Solution

1. Step 1: Understand @key usage in federation

   The @key directive identifies the unique field used to match entities across services.

2. Step 2: Analyze entity resolution

   Apollo Federation merges types with the same @key field by matching their unique identifiers, combining fields from both services.

3. Final Answer:

   It merges User types by matching the @key field 'id' across services -> Option D

4. Quick Check:

   @key fields unify entities across services [OK]

Hint: Entities merge by matching @key fields [OK]

Common Mistakes:

• Thinking types are duplicated instead of merged
• Assuming extend type is ignored
• Believing manual merging is required