Why Gateway composition in GraphQL? - Purpose & Use Cases

The Big Idea

What if you could get all your app's data with just one simple question, no matter how many teams built it?

The Scenario

Imagine you have many different teams building separate parts of a big app, each with its own data and services. You try to ask each team separately for information and then combine it yourself.

The Problem

This manual way is slow and confusing. You have to remember many places to ask, write lots of code to join data, and if one team changes their data, your code breaks. It's like juggling many balls and dropping some.

The Solution

Gateway composition lets you create one smart gateway that talks to all teams' services behind the scenes. You ask the gateway once, and it gathers and combines data for you automatically, making your life much easier.

Before vs After

✗ Before

fetchUser(); fetchOrders(); combineDataManually();

✓ After

query { user { name orders { id } } }

What It Enables

It enables a smooth, single point of access to complex, distributed data, so you get exactly what you need with one simple request.

Real Life Example

A shopping app where user info, product details, and order history come from different teams but appear together instantly when you open your profile.

Key Takeaways

Manual data gathering from many sources is slow and error-prone.

Gateway composition creates one unified access point.

This simplifies data fetching and keeps apps fast and reliable.

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