Why subscriptions enable real-time data in GraphQL - Challenge Your Understanding

Challenge - 5 Problems

🧠 Conceptual

intermediate

How do GraphQL subscriptions work?

Which statement best explains how GraphQL subscriptions enable real-time data updates?

A Subscriptions store data locally and update only when the user refreshes the page.

B Subscriptions require the client to repeatedly ask the server for new data at fixed intervals.

C Subscriptions send data only once when the client first connects and then close the connection.

D Subscriptions keep an open connection to the server, allowing it to push updates instantly when data changes.

💡 Hint

Think about how the server communicates changes without waiting for the client to ask.

GraphQL subscriptions create a persistent connection, usually via WebSocket, so the server can send data updates immediately when they happen.

❓ query_result

intermediate

Subscription data update example

Given a subscription that listens for new messages, what will the client receive when a new message is sent?

GraphQL

subscription {
  newMessage {
    id
    content
    sender
  }
}

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

B {"data":{"newMessage":null}}

C {}

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

💡 Hint

The subscription sends data only when a new message arrives.

When a new message is sent, the subscription pushes the message data to the client in the specified format.

📝 Syntax

advanced

Identify the correct subscription syntax

Which of the following GraphQL subscription definitions is syntactically correct?

A

subscription NewComments {
  commentAdded(postId: 5) {
    id text
  }
}

B

subscription NewComments {
  commentAdded(postId: 5) {
    id,
    text
  }
}

C

subscription NewComments {
  commentAdded(postId: 5) {
    id
    text
  }
}

D

subscription NewComments {
  commentAdded(postId: 5) {
    id; text
  }
}

💡 Hint

Look for correct field separation and punctuation in GraphQL syntax.

GraphQL fields inside selection sets are separated by whitespace or new lines without commas or semicolons.

❓ optimization

advanced

Optimizing subscription data payload

To reduce bandwidth in a subscription that sends user status updates, which approach is best?

A Send only the changed fields (e.g., status) instead of the entire user object.

B Send updates with large nested objects to ensure completeness.

C Send updates only once per hour to limit data.

D Send the full user object every time, regardless of changes.

💡 Hint

Think about sending only what is necessary to keep data fresh.

Sending only changed fields reduces data size and improves performance for real-time updates.

🔧 Debug

expert

Why does this subscription fail to receive updates?

A client subscribes to new orders but never receives updates. The subscription query is correct. What is the most likely cause?

GraphQL

subscription {
  newOrder {
    id
    total
  }
}

A The client query has a syntax error preventing subscription.

B The server does not publish events to the subscription channel for new orders.

C The client is using HTTP instead of WebSocket for the subscription connection.

D The subscription query is missing required arguments.

💡 Hint

Check if the server triggers subscription events properly.

If the server does not emit events on the subscription channel, clients will not receive updates even if the query is correct.