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.

Practice

1. What is the main purpose of a subscription in GraphQL?

easy

A. To fetch data only once from the server

B. To update data on the server

C. To delete data from the database

D. To receive real-time updates when data changes

Solution

1. Step 1: Understand the role of subscriptions

   Subscriptions are designed to keep the client updated automatically when data changes on the server.

2. Step 2: Compare with other GraphQL operations

   Queries fetch data once, mutations change data, but subscriptions push live updates.

3. Final Answer:

   To receive real-time updates when data changes -> Option D

4. Quick Check:

   Subscriptions = real-time updates [OK]

Hint: Subscriptions push live data updates automatically [OK]

Common Mistakes:

• Confusing subscriptions with queries
• Thinking subscriptions modify data
• Believing subscriptions fetch data only once

2. Which of the following is the correct syntax to start a GraphQL subscription named newMessage that listens for content and sender fields?

easy

A. subscription { newMessage { content sender } }

B. query { newMessage { content sender } }

C. mutation { newMessage { content sender } }

D. subscribe { newMessage { content sender } }

Solution

1. Step 1: Identify the correct keyword for subscriptions

   The keyword to start a subscription is subscription, not query, mutation, or subscribe.

2. Step 2: Check the structure of the subscription

   The subscription should specify the event name and requested fields inside curly braces.

3. Final Answer:

   subscription { newMessage { content sender } } -> Option A

4. Quick Check:

   Subscription syntax = subscription keyword + event + fields [OK]

Hint: Use 'subscription' keyword, not 'query' or 'mutation' [OK]

Common Mistakes:

• Using 'query' or 'mutation' instead of 'subscription'
• Using 'subscribe' keyword which is invalid
• Missing curly braces around fields

3. Given this subscription:

subscription { newNotification { id message } }

What will the client receive when a new notification with id: 5 and message: "Hello" is sent?

medium

A. { "data": { "newNotification": { "id": 5, "message": "Hello" } } }

B. { "data": { "newNotification": { "id": "5", "message": "Hello" } } }

C. { "error": "Subscription not found" }

D. { "data": { "newNotification": { "message": "Hello" } } }

Solution

1. Step 1: Understand subscription response format

   Subscriptions return data wrapped in a data object with the event name as key and requested fields as nested data.

2. Step 2: Match the expected data types and fields

   The id is a number 5, and message is "Hello". Both fields are included as requested.

3. Final Answer:

   { "data": { "newNotification": { "id": 5, "message": "Hello" } } } -> Option A

4. Quick Check:

   Subscription data = event with requested fields [OK]

Hint: Subscription returns data object with event and fields [OK]

Common Mistakes:

• Returning id as string instead of number
• Missing fields in response
• Expecting error when subscription is valid

4. You wrote this subscription:

subscription { newMessage { content sender } }

But your client never receives updates. Which is the most likely cause?

medium

A. The client is using a query instead of a subscription

B. The subscription syntax is invalid

C. The server does not publish events for newMessage

D. The fields content and sender do not exist

Solution

1. Step 1: Check subscription syntax

   The syntax is correct, so syntax errors are unlikely.

2. Step 2: Consider server event publishing

   If the server never sends events for newMessage, the client won't get updates.

3. Step 3: Verify client usage

   The client uses subscription syntax, so it's not using a query.

4. Step 4: Confirm fields existence

   Assuming fields exist, the main issue is missing server events.

5. Final Answer:

   The server does not publish events for newMessage -> Option C

6. Quick Check:

   No server events = no subscription updates [OK]

Hint: Check if server sends events for the subscription [OK]

Common Mistakes:

• Assuming syntax error without checking server
• Confusing query with subscription on client
• Ignoring server-side event publishing

5. You want to create a subscription that only sends updates when a new chat message is sent by user "Alice". Which subscription definition correctly filters messages by sender?

hard

A. subscription { newMessage { content sender } where sender = "Alice" }

B. subscription { newMessage(sender: "Alice") { content sender } }

C. subscription { newMessage { content sender } filter: sender == "Alice" }

D. subscription { newMessage { content sender } if sender = "Alice" }

Solution

1. Step 1: Understand argument usage in subscriptions

   GraphQL subscriptions accept arguments like queries to filter events, e.g., newMessage(sender: "Alice").

2. Step 2: Identify invalid syntax options

   Options B, C, and D use invalid keywords or syntax (where, filter, if) which are not part of GraphQL subscription syntax.

3. Final Answer:

   subscription { newMessage(sender: "Alice") { content sender } } -> Option B

4. Quick Check:

   Use arguments in subscription for filtering [OK]

Hint: Filter subscriptions using arguments inside parentheses [OK]

Common Mistakes:

• Using SQL-like WHERE or IF clauses in subscription
• Trying to filter inside selection set
• Not passing filter as argument