Why federation scales GraphQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a federated service with a key directive.

GraphQL

type Product @key(fields: "[1]") { id: ID! name: String }

Drag options to blanks, or click blank then click option'

Aprice

Bsku

Cid

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-unique field like 'name' as the key.

Omitting the @key directive entirely.

The @key directive specifies the primary field used to identify the entity across services. Here, id is the correct key.

2fill in blank

medium

Complete the code to extend a type from another service in federation.

GraphQL

extend type User @key(fields: "[1]") { id: ID! username: String }

Drag options to blanks, or click blank then click option'

Aid

Bprofile

Cemail

Dusername

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-key field like 'username' in the @key directive.

Forgetting to use the extend keyword.

When extending a type, the @key directive must use the field that uniquely identifies the entity, typically id.

3fill in blank

hard

Fix the error in the federation SDL by completing the missing directive.

GraphQL

type Review { id: ID! body: String author: User [1] }

Drag options to blanks, or click blank then click option'

A@external

B@provides

C@requires

D@key

Attempts:

3 left

💡 Hint

Common Mistakes

Using @key on a field that is not a key.

Using @provides or @requires incorrectly.

The @external directive marks fields that are owned by another service but referenced here, which is necessary for federation to work correctly.

4fill in blank

hard

Fill both blanks to complete the resolver that fetches a federated entity by its key.

GraphQL

User: { __resolveReference([1]) { return fetchUserById([1].[2]); } }

Drag options to blanks, or click blank then click option'

Aid

Buser

Dusername

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong argument name in the resolver.

Using a non-key field to fetch the entity.

The resolver receives the reference object as user, and the key field to fetch the user is id.

5fill in blank

hard

Fill all three blanks to complete the federated schema with multiple services.

GraphQL

schema { query: [1] } type [2] { products: [Product] } type Product @key(fields: "[3]") { id: ID! name: String }

Drag options to blanks, or click blank then click option'

AQuery

BProduct

Cid

DMutation

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Mutation' instead of 'Query' as the root type.

Using 'Product' as the root query type name.

Using a non-key field in the @key directive.

The schema's query root is Query. The type containing the products field is also Query. The key field for Product is id.

Practice

1. What is the main benefit of using GraphQL federation in a large project?

easy

A. It removes the need for any backend services.

B. It makes the API slower by adding more layers.

C. It splits a big API into smaller parts for easier management.

D. It forces all teams to work on the same codebase.

Solution

1. Step 1: Understand federation purpose

   Federation breaks a large GraphQL API into smaller, manageable services.

2. Step 2: Identify the benefit

   This splitting helps teams work independently and manage parts easily.

3. Final Answer:

   It splits a big API into smaller parts for easier management. -> Option C

4. Quick Check:

   Federation = splits API for management [OK]

Hint: Federation means splitting big API into smaller parts [OK]

Common Mistakes:

• Thinking federation slows down the API
• Believing federation removes backend services
• Assuming all teams share one codebase

2. Which of the following is the correct way to define a federated service in GraphQL SDL?

easy

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

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

C. service Query { product(id: ID!): Product }

D. federation type Query { product(id: ID!): Product }

Solution

1. Step 1: Recall federation SDL syntax

   Federated services use extend type to add fields to shared types.

2. Step 2: Match correct syntax

   extend type Query { product(id: ID!): Product } uses extend type Query, which is correct for federation.

3. Final Answer:

   extend type Query { product(id: ID!): Product } -> Option B

4. Quick Check:

   Federation uses 'extend type' syntax [OK]

Hint: Federation adds fields with 'extend type' [OK]

Common Mistakes:

• Using 'type' instead of 'extend type' in federated services
• Using non-existent 'service' or 'federation' keywords
• Confusing base schema with extended schema

3. Given two federated services: Product service defines type Product { id: ID!, name: String } and Review service extends it with extend type Product { reviews: [Review] }. What will a query for { product(id: "1") { name reviews { body } } } return?

medium

A. Product name and list of reviews with their body fields.

B. Only product name, reviews field will be null.

C. Error because reviews field is not defined in Product service.

D. Empty result because federated services cannot combine fields.

Solution

1. Step 1: Understand federation field extension

   The Review service extends Product with reviews, so combined schema includes reviews.

2. Step 2: Query result combines data

   The query asks for product name and reviews body, which federation resolves from both services.

3. Final Answer:

   Product name and list of reviews with their body fields. -> Option A

4. Quick Check:

   Federation merges fields, query returns combined data [OK]

Hint: Federation merges extended fields into one response [OK]

Common Mistakes:

• Assuming extended fields are unavailable
• Expecting errors due to field extension
• Thinking federated services cannot combine data

4. A federated GraphQL setup has two services: User and Order. The User service defines type User { id: ID!, name: String }. The Order service tries to extend User with extend type User { orders: [Order] } but the gateway returns an error. What is the likely cause?

medium

A. User type is not marked with @key directive in User service.

B. Order service must define User type fully, not extend it.

C. Gateway does not support federation.

D. Orders field must be defined in User service, not Order service.

Solution

1. Step 1: Check federation key requirement

   Federation requires types extended across services to have a @key directive for identification.

2. Step 2: Identify missing @key

   User type lacks @key in User service, so gateway cannot resolve extensions.

3. Final Answer:

   User type is not marked with @key directive in User service. -> Option A

4. Quick Check:

   @key missing causes federation errors [OK]

Hint: Missing @key on base type breaks federation [OK]

Common Mistakes:

• Thinking extended types must be fully redefined
• Blaming gateway instead of schema directives
• Assuming fields must be in base service only

5. In a large company, multiple teams manage different parts of a GraphQL API using federation. Which of these practices best helps federation scale effectively?

hard

A. One team manages all services to ensure consistency.

B. All teams edit the same schema file to avoid conflicts.

C. Teams avoid using @key directives to keep schemas simple.

D. Each team owns a service with clear @key types and minimal overlap.

Solution

1. Step 1: Understand federation team ownership

   Federation scales by letting teams own services with clear boundaries and keys.

2. Step 2: Identify best practice

   Clear @key types and minimal overlap avoid conflicts and enable smooth composition.

3. Final Answer:

   Each team owns a service with clear @key types and minimal overlap. -> Option D

4. Quick Check:

   Team ownership + @key = scalable federation [OK]

Hint: Clear ownership and @key enable smooth federation scaling [OK]

Common Mistakes:

• Thinking one schema file for all teams scales well
• Avoiding @key directives breaks federation
• Centralizing all services under one team limits scaling