Gateway composition in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this federated query?

Given two GraphQL services composed via a gateway, the Product type is extended in the Reviews service. Consider the following query executed against the gateway:

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

Assuming the Product service returns { id: "1", name: "Table" } and the Reviews service returns two reviews with authors, what is the expected shape of the response?

A {"data":{"product":{"id":"1","name":"Table"}}}

B {"data":{"product":{"id":"1","name":"Table","reviews":[{"body":"Great!","author":{"name":"Alice"}},{"body":"Good quality","author":{"name":"Bob"}}]}}}

C {"data":{"product":{"id":"1","reviews":[{"body":"Great!","author":{"name":"Alice"}},{"body":"Good quality","author":{"name":"Bob"}}]}}}

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

💡 Hint

Remember that the gateway merges data from both services, so extended fields appear in the final response.

The gateway composes the schema so that the Product type includes fields from both services. The reviews field is added by the Reviews service, so the response includes it with nested author data.

🧠 Conceptual

intermediate

Which directive is essential for gateway composition in Apollo Federation?

In Apollo Federation, to indicate that a type or field is extended from another service, which directive must be used?

A @external

B @key

C @extends

D @provides

💡 Hint

Think about how a service declares it is adding fields to a type defined elsewhere.

The @extends directive marks a type as an extension of a type defined in another service, enabling gateway composition.

📝 Syntax

advanced

Identify the syntax error in this federated schema snippet

Consider this schema snippet from a federated service:

extend type User @key(fields: "id") { id: ID! name: String! @external posts: [Post] }

What is the syntax error here?

A Missing closing brace '}' for the type definition

B Missing @external directive on 'posts' field

C The @key directive must be on the base type, not on an extension

D The 'posts' field must have a non-nullable type

💡 Hint

Check the braces carefully in the type definition.

The snippet is missing the closing brace '}' at the end of the type extension, causing a syntax error.

❓ optimization

advanced

How to reduce query latency in gateway composition?

When a gateway composes multiple services, queries can become slow due to multiple network calls. Which approach best reduces latency?

A Increase the timeout for each subservice request

B Remove @key directives to simplify schema

C Disable schema stitching and use separate endpoints

D Batch requests to subservices and cache results at the gateway

💡 Hint

Think about how to minimize the number of calls and reuse data.

Batching requests and caching results at the gateway reduces the number of network calls and speeds up query resolution.

🔧 Debug

expert

Why does this federated query return a null for an extended field?

A federated gateway query requests an extended field reviews on Product. The base service returns the product data correctly, but reviews is always null. The Reviews service schema has:

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

And the resolver for reviews uses the id from the parent. What is the most likely cause?

A The <code>id</code> field is not marked with @external in the extension

B The <code>Product</code> type is not marked with @extends in the Reviews service

C The <code>reviews</code> field is missing the @requires directive

D The gateway is not forwarding headers to the Reviews service

💡 Hint

Check how fields from the base service are referenced in extensions.

In Apollo Federation, fields from the base service used in an extension must be marked with @external to indicate they come from another service. Without it, the resolver may not receive the correct value, causing null results.

Practice

1. What is the main purpose of gateway composition in GraphQL?

easy

A. To combine multiple GraphQL services into a single API endpoint

B. To create multiple endpoints for each service

C. To replace GraphQL with REST APIs

D. To disable client requests

Solution

1. Step 1: Understand gateway composition

   Gateway composition merges several GraphQL services into one unified API.

2. Step 2: Identify the main benefit

   This allows clients to send requests to a single endpoint instead of multiple services.

3. Final Answer:

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

4. Quick Check:

   Gateway composition = single API endpoint [OK]

Hint: Gateway composition means one API endpoint for many services [OK]

Common Mistakes:

• Thinking it creates multiple endpoints
• Confusing with REST APIs
• Believing it disables client requests

2. Which of the following is the correct way to initialize ApolloGateway with two services named 'users' and 'products'?

easy

A. ApolloGateway({ services: ['users', 'products'] })

B. new ApolloGateway(services: ['users', 'products'])

C. new ApolloGateway({ services: [{ name: 'users', url: 'http://users' }, { name: 'products', url: 'http://products' }] })

D. new ApolloGateway({ endpoints: ['users', 'products'] })

Solution

1. Step 1: Recall ApolloGateway initialization syntax

   ApolloGateway expects an object with a 'services' array containing objects with 'name' and 'url'.

2. Step 2: Match the correct syntax

   new ApolloGateway({ services: [{ name: 'users', url: 'http://users' }, { name: 'products', url: 'http://products' }] }) correctly uses new ApolloGateway with services array of objects including name and url.

3. Final Answer:

   new ApolloGateway({ services: [{ name: 'users', url: 'http://users' }, { name: 'products', url: 'http://products' }] }) -> Option C

4. Quick Check:

   ApolloGateway needs services array with name and url [OK]

Hint: Use services array with name and url objects in ApolloGateway [OK]

Common Mistakes:

• Passing services as simple string array
• Using 'endpoints' instead of 'services'
• Omitting 'new' keyword

3. Given this ApolloGateway setup:

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

What will be the result if a client queries for a product's name and its owner's username?

medium

A. The gateway throws an error because __exposeQueryPlanExperimental is false

B. The gateway returns data only from 'products' service ignoring 'users'

C. The gateway returns empty data because services are unreachable

D. The gateway fetches data from both 'products' and 'users' services and returns combined result

Solution

1. Step 1: Understand gateway data fetching

   The gateway composes schemas and fetches data from all relevant services for the query.

2. Step 2: Analyze the query involving product and user data

   Since the query asks for product and owner username, gateway calls both 'products' and 'users' services.

3. Final Answer:

   The gateway fetches data from both 'products' and 'users' services and returns combined result -> Option D

4. Quick Check:

   Gateway combines data from multiple services [OK]

Hint: Gateway merges data from all services needed by query [OK]

Common Mistakes:

• Assuming gateway fetches from only one service
• Confusing __exposeQueryPlanExperimental with data fetching
• Assuming error without service failure

4. You wrote this ApolloGateway code but get an error:

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

What is the likely cause of the error?

medium

A. Missing 'url' property for the 'products' service

B. Using 'services' instead of 'endpoints'

C. ApolloGateway requires only one service

D. The 'name' property should be 'serviceName'

Solution

1. Step 1: Check service definitions

   Each service object must have both 'name' and 'url' properties.

2. Step 2: Identify missing property

   The 'products' service lacks the 'url' property, causing the error.

3. Final Answer:

   Missing 'url' property for the 'products' service -> Option A

4. Quick Check:

   Each service needs name and url [OK]

Hint: Always include 'url' for each service in ApolloGateway [OK]

Common Mistakes:

• Confusing 'services' with 'endpoints'
• Thinking ApolloGateway supports single service only
• Using wrong property names

5. You want to disable subscriptions in your ApolloGateway setup while composing three services. Which is the correct way to do this?

hard

A. Set subscriptions: false inside each service definition in ApolloGateway

B. Pass { subscriptions: false } in ApolloServer constructor, not in ApolloGateway

C. Add subscriptions: false in ApolloGateway constructor options

D. Disable subscriptions by removing the 'subscriptions' field from the schema

Solution

1. Step 1: Understand where to disable subscriptions

   Subscriptions are disabled in ApolloServer, not ApolloGateway.

2. Step 2: Apply correct configuration

   Pass { subscriptions: false } as an option to ApolloServer constructor wrapping the gateway.

3. Final Answer:

   Pass { subscriptions: false } in ApolloServer constructor, not in ApolloGateway -> Option B

4. Quick Check:

   Disable subscriptions in ApolloServer, not ApolloGateway [OK]

Hint: Disable subscriptions in ApolloServer, not ApolloGateway [OK]

Common Mistakes:

• Trying to disable subscriptions inside ApolloGateway
• Removing schema fields instead of config
• Setting subscriptions false per service