Apollo Federation concepts in GraphQL

Introduction

Apollo Federation helps combine many small GraphQL services into one big service. It makes it easy to work together on data from different places.

When you have multiple teams working on different parts of your data and want to combine their work.

When you want to split a big GraphQL API into smaller, manageable parts.

When you want to share data between different GraphQL services without merging them manually.

When you want to add new features or data sources without changing the whole API.

When you want to keep your GraphQL services independent but still provide a single API to users.

Syntax

GraphQL

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

extend type Query {
  product(id: ID!): Product
}

@key marks the primary field to identify an entity across services.

extend type is used to add fields to types defined in other services.

Examples

This example shows a User entity with an @key and extending another type Review to link to User.

GraphQL

type User @key(fields: "id") {
  id: ID!
  username: String
}

extend type Review {
  author: User
}

Defines a Product entity identified by upc and extends the Query type to add a new field.

GraphQL

type Product @key(fields: "upc") {
  upc: String!
  name: String
}

extend type Query {
  topProducts(first: Int): [Product]
}

Sample Program

This schema shows two entities, Book and Author, each with a unique key. It also extends the Query type to fetch books and authors by their keys.

GraphQL

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

type Author @key(fields: "id") {
  id: ID!
  name: String
}

extend type Query {
  book(isbn: String!): Book
  author(id: ID!): Author
}

Important Notes

Each service defines its own part of the schema and uses @key to tell Apollo Federation how to identify entities.

Federation allows services to share and extend types without conflicts.

Use extend type to add fields to types owned by other services.

Summary

Apollo Federation helps combine multiple GraphQL services into one API.

Use @key to mark unique identifiers for entities.

Use extend type to add fields to types from other services.

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