Gateway composition in GraphQL - Time & Space Complexity

Time Complexity: Gateway composition

O(n)

Understanding Time Complexity

When using gateway composition in GraphQL, we combine data from multiple services into one response.

We want to understand how the time to get data grows as the number of services or data size increases.

Scenario Under Consideration

Analyze the time complexity of the following GraphQL gateway composition query.

query GetUserAndOrders($userId: ID!) {
  user(id: $userId) {
    id
    name
    orders {
      id
      total
    }
  }
}

This query fetches a user and their orders by combining data from a user service and an order service.

Identify Repeating Operations

Look for repeated data fetching or processing steps.

• Primary operation: Fetching each order for the user from the order service.
• How many times: Once per order linked to the user.

How Execution Grows With Input

As the number of orders grows, the time to fetch all orders grows too.

Input Size (n orders)	Approx. Operations
10	10 fetches for orders + 1 fetch for user
100	100 fetches for orders + 1 fetch for user
1000	1000 fetches for orders + 1 fetch for user

Pattern observation: The total work grows roughly in direct proportion to the number of orders.

Final Time Complexity

Time Complexity: O(n)

This means the time to get the full response grows linearly with the number of orders.

Common Mistake

[X] Wrong: "Fetching data from multiple services happens all at once, so time stays the same no matter how many orders there are."

[OK] Correct: Even if requests are parallel, each order still requires processing, so total work grows with the number of orders.

Interview Connect

Understanding how gateway composition scales helps you design efficient APIs and explain performance trade-offs clearly.

Self-Check

What if the gateway batches order requests instead of fetching each order separately? How would the time complexity change?

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