Subscription syntax in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this GraphQL subscription?

Consider this GraphQL subscription that listens for new messages:

subscription {
  newMessage {
    id
    content
    sender
  }
}

If a new message with id: 101, content: "Hello!", and sender: "Alice" is sent, what will the subscription receive?

GraphQL

subscription {
  newMessage {
    id
    content
    sender
  }
}

A {"data": {"newMessage": {"id": 101, "content": "Hello!", "sender": "Alice"}}}

B {"data": {"newMessage": {"id": "101", "content": "Hello!", "sender": "Alice"}}}

C {"data": {"message": {"id": 101, "content": "Hello!", "sender": "Alice"}}}

D {"error": "Subscription not found"}

💡 Hint

Check the exact field names in the subscription response.

The subscription listens to newMessage field, so the response wraps the message data inside newMessage. The id is a number, not a string.

📝 Syntax

intermediate

Which subscription syntax is valid in GraphQL?

Identify the valid GraphQL subscription syntax from the options below:

A

subscription {
  newMessage {
    id
    content
  }
  extraField
}

B

subscription NewMessages {
  newMessage {
    id
    content
  }
}

C

subscription newMessage {
  newMessage {
    id
    content
  }
}

D

subscription {
  newMessage(id: 5) {
    id
    content
  }
}

💡 Hint

Subscriptions can have operation names and must have a single root field.

Option B uses a valid operation name NewMessages and a single root field newMessage. Option B incorrectly uses arguments in subscription root field which is allowed but depends on schema; Option B uses lowercase operation name which is invalid; Option B has multiple root fields which is invalid.

❓ optimization

advanced

How to optimize a subscription to receive only necessary data?

You want to subscribe to user status updates but only need the userId and status fields. Which subscription query is optimized?

A

subscription {
  userStatus {
    userId
  }
}

B

subscription {
  userStatus {
    userId
    status
    lastLogin
    email
  }
}

C

subscription {
  userStatus {
    userId
    status
  }
}

D

subscription {
  userStatus {
    *
  }
}

💡 Hint

Request only the fields you need to reduce data transfer.

Option C requests only userId and status, which is optimal. Option C requests extra unnecessary fields. Option C uses invalid syntax. Option C misses the status field.

🔧 Debug

advanced

Why does this subscription cause an error?

Given this subscription:

subscription {
  newMessage {
    id
    content
    sender
  }
  userStatus {
    userId
    status
  }
}

Why does it cause an error?

A GraphQL subscriptions must have exactly one root field; multiple root fields cause an error.

B The subscription is missing an operation name.

C The fields inside newMessage are invalid.

D Subscriptions cannot request nested fields.

💡 Hint

Check the number of root fields in the subscription.

GraphQL subscriptions require exactly one root field. Having both newMessage and userStatus as root fields causes a syntax error.

🧠 Conceptual

expert

What is the main difference between a GraphQL subscription and a query?

Choose the best explanation of how subscriptions differ from queries in GraphQL.

A Queries are used for mutations, subscriptions are used for fetching data.

B Subscriptions can only fetch data once, queries keep the connection open for updates.

C Subscriptions and queries are identical except subscriptions require an operation name.

D Subscriptions maintain a continuous connection to receive real-time data updates, while queries fetch data once.

💡 Hint

Think about how data is delivered over time.

Subscriptions keep a live connection to get real-time updates. Queries run once and return data immediately.