Apollo Federation concepts in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary purpose of Apollo Federation?

Apollo Federation allows multiple GraphQL services to work together as one. What is its main goal?

A To combine multiple GraphQL services into a single graph for clients to query seamlessly

B To convert SQL databases into GraphQL schemas

C To cache GraphQL queries on the client side automatically

D To replace REST APIs with a single GraphQL service

💡 Hint

Think about how multiple teams can build parts of a graph independently.

Apollo Federation's main purpose is to let different teams build separate GraphQL services that combine into one unified graph for clients.

❓ query_result

intermediate

What is the output of this federated query?

Given two services: Product service with type Product { id, name } and Review service extending Product with { reviews }, what is the result of querying { product(id: "1") { id name reviews { body } } }?

GraphQL

query {
  product(id: "1") {
    id
    name
    reviews {
      body
    }
  }
}

A { "data": { "product": { "id": "1", "name": null, "reviews": [] } } }

B { "data": { "product": { "id": "1", "name": "Table", "reviews": [{ "body": "Great!" }] } } }

C { "errors": [{ "message": "Field 'reviews' not found on type 'Product'" }] }

D { "data": { "product": null } }

💡 Hint

Remember that Review service extends Product to add reviews.

The Review service extends Product to add reviews, so querying reviews on product returns the reviews data.

📝 Syntax

advanced

Which SDL snippet correctly defines an entity in Apollo Federation?

Choose the correct way to define a Product entity with a key field 'id' in Apollo Federation SDL.

GraphQL

type Product {
  id: ID!
  name: String
}

A

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

B

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

C

type Product @entity(fields: "id") {
  id: ID!
  name: String
}

D

type Product @key(id) {
  id: ID!
  name: String
}

💡 Hint

The @key directive marks the primary key field for the entity.

In Apollo Federation, @key(fields: "id") on the type defines the entity's primary key. The syntax must be exact.

❓ optimization

advanced

How does Apollo Gateway optimize resolving entities across services?

Apollo Gateway receives a query that requires data from multiple services. How does it optimize fetching entities?

A It caches all data indefinitely to avoid any network calls

B It sends separate requests for each field to each service

C It batches entity requests by keys to minimize network calls to subgraphs

D It merges all schemas into one and queries only one service

💡 Hint

Think about how to reduce repeated calls for the same entity.

Apollo Gateway batches entity requests by keys so it can fetch multiple entities in one call to each subgraph, reducing network overhead.

🔧 Debug

expert

Why does this federated query fail with an error?

Given two services: User service defines type User @key(fields: "id") { id: ID! name: String }, and Order service extends User with extend type User @key(fields: "id") { id: ID! orders: [Order] }, a query requests user { id name orders { id } }. The error says: "Cannot query field 'orders' on type 'User'". What is the cause?

A The User service must define the 'orders' field, not the Order service

B The query is missing the @external directive on the 'id' field in the Order service

C The 'orders' field must be marked as @requires(fields: "id") in the Order service

D The User type in the gateway schema does not include the 'orders' field because the Order service did not properly extend the User entity

💡 Hint

Check how the User type is extended and if the gateway schema includes the extension.

If the Order service does not properly extend User with the @key directive and matching fields, the gateway won't merge the 'orders' field into the User type, causing the query error.