Subscription lifecycle in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this subscription query?

Consider a GraphQL subscription that listens for new messages in a chat room. When a new message is sent, the subscription returns the message content and the sender's name.

Given the following subscription query:

subscription NewMessage {
  messageAdded {
    content
    sender {
      name
    }
  }
}

If a new message with content "Hello!" is sent by user "Alice", what will the subscription return?

GraphQL

subscription NewMessage {
  messageAdded {
    content
    sender {
      name
    }
  }
}

A {"messageAdded": {"content": "Hello!", "sender": {"name": "Alice"}}}

B {"data": {"messageAdded": {"content": "Hello!", "sender": {"name": "Alice"}}}}

C {"data": {"newMessage": {"content": "Hello!", "sender": {"name": "Alice"}}}}

D {"data": {"messageAdded": {"content": "Hello!", "sender": "Alice"}}}

💡 Hint

Remember the subscription response wraps data inside a data field matching the subscription field name.

The subscription response wraps the returned data inside a data object. The field name matches the subscription field (messageAdded), and nested fields are included as requested.

🧠 Conceptual

intermediate

Which event triggers a GraphQL subscription update?

In a GraphQL subscription lifecycle, what kind of event causes the server to send updated data to subscribed clients?

A A mutation changes data on the server.

B A client sends a new query request.

C The server restarts.

D A client disconnects from the subscription.

💡 Hint

Think about what changes data on the server that clients want to be notified about.

Subscriptions send updates when data changes, which happens through mutations. Queries only request data, server restarts or client disconnects do not trigger subscription updates.

📝 Syntax

advanced

Identify the syntax error in this subscription definition

Which option contains a syntax error in defining a GraphQL subscription?

subscription OnUserStatusChanged {
  userStatusChanged {
    id
    status
  }
}

A

subscription OnUserStatusChanged {
  userStatusChanged {
    id
    status
  }
}

B

subscription OnUserStatusChanged {
  userStatusChanged {
    id
    status
  }}

C

subscription OnUserStatusChanged {
  userStatusChanged {
    id
    status
  }

D

}  
sutats    
di    
{ degnahCsutatSresu  
{ degnahCsutatSresUnO noitpircsbus

💡 Hint

Check for matching braces and proper closing of the subscription block.

Option C is missing the closing brace for the subscription block, causing a syntax error. Options A and B have balanced braces. Option C is also missing a closing brace.

🔧 Debug

advanced

Why does this subscription not receive updates?

A client subscribes to newOrder events, but never receives any updates even though new orders are created. Which option explains the most likely cause?

subscription NewOrder {
  newOrder {
    id
    total
  }
}

A The client is not authorized to subscribe.

B The client query is missing required variables.

C The subscription query uses incorrect field names.

D The server does not publish <code>newOrder</code> events after mutations.

💡 Hint

Check if the server triggers events to notify subscribers after data changes.

If the server does not publish events for the subscription, clients will not receive updates. Missing variables or authorization would cause errors, and incorrect field names would cause errors or no data.

❓ optimization

expert

How to optimize subscription performance for many clients?

You have thousands of clients subscribed to a stockPriceUpdated subscription. What is the best way to optimize server performance when sending updates?

A Batch updates and send only relevant stock price changes to each client.

B Send every update to all clients regardless of their subscribed stocks.

C Disconnect clients frequently to reduce load.

D Use polling queries instead of subscriptions.

💡 Hint

Think about sending only necessary data to each client to reduce server work.

Batching updates and filtering them per client reduces unnecessary data sent and server load. Sending all updates to all clients wastes resources. Disconnecting clients or polling loses real-time benefits.