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.

Practice

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

easy

A. To delete data from the database

B. To receive real-time updates automatically when data changes

C. To fetch data only once from the server

D. To update data on the server

Solution

1. Step 1: Understand the role of subscription

   Subscriptions are designed to keep the client updated automatically when data changes, without asking repeatedly.

2. Step 2: Compare with other operations

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

3. Final Answer:

   To receive real-time updates automatically when data changes -> Option B

4. Quick Check:

   Subscription = live updates [OK]

Hint: Subscriptions push updates; queries pull once [OK]

Common Mistakes:

• Confusing subscription with query or mutation
• Thinking subscription fetches data only once
• Assuming subscription modifies data

2. Which of the following is the correct syntax to start a GraphQL subscription named newMessage requesting id and content fields?

easy

A. query { newMessage { id content } }

B. mutation { newMessage { id content } }

C. subscription { newMessage { id content } }

D. subscribe { newMessage { id content } }

Solution

1. Step 1: Identify the correct keyword for subscription

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

2. Step 2: Check the structure

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

3. Final Answer:

   subscription { newMessage { id content } } -> Option C

4. Quick Check:

   Keyword subscription + fields = subscription { newMessage { id content } } [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 the subscription below, what fields will the client receive when a new comment is added?

subscription { newComment { id text author { name } } }

medium

A. id, text, and author name of the new comment

B. Only id and text of the new comment

C. Only author name of the new comment

D. All comments in the database

Solution

1. Step 1: Analyze the subscription fields

   The subscription requests id, text, and nested author { name } fields for the newComment event.

2. Step 2: Understand what is sent on event

   When a new comment is added, the client receives exactly these fields for that comment, not all comments.

3. Final Answer:

   id, text, and author name of the new comment -> Option A

4. Quick Check:

   Requested fields = id, text, author name [OK]

Hint: Nested fields inside subscription are included in output [OK]

Common Mistakes:

• Assuming all comments are sent, not just new ones
• Ignoring nested author name field
• Thinking only scalar fields are sent

4. Identify the error in this subscription syntax:

subscription newMessage { id content }

medium

A. Missing curly braces around the subscription fields

B. Subscription name cannot be 'newMessage'

C. Subscription keyword should be 'query'

D. Fields 'id' and 'content' should be inside the event name braces

Solution

1. Step 1: Check subscription structure

   The subscription must specify the event name followed by its fields inside braces, e.g., subscription { newMessage { id content } }.

2. Step 2: Identify the error in given syntax

   The given syntax places fields directly after subscription name without nesting inside event braces.

3. Final Answer:

   Fields 'id' and 'content' should be inside the event name braces -> Option D

4. Quick Check:

   Fields must be nested inside event braces [OK]

Hint: Fields must be inside event braces, not directly after subscription name [OK]

Common Mistakes:

• Placing fields outside event braces
• Using 'query' instead of 'subscription'
• Omitting braces entirely

5. You want to subscribe to live updates of stock prices but only for stocks with price above 100. Which subscription syntax correctly applies this filter?

hard

A. subscription { stockPrice(price_gt: 100) { symbol price } }

B. subscription { stockPrice { symbol price where price > 100 } }

C. subscription { stockPrice { symbol price } filter: { price: { gt: 100 } } }

D. subscription { stockPrice { symbol price } if price > 100 }

Solution

1. Step 1: Understand argument filtering in subscriptions

   GraphQL subscriptions accept arguments like queries, so filters are passed as arguments inside parentheses after the event name.

2. Step 2: Identify correct filter syntax

   subscription { stockPrice(price_gt: 100) { symbol price } } uses price_gt: 100 as an argument, which is the standard way to filter for price greater than 100.

3. Final Answer:

   subscription { stockPrice(price_gt: 100) { symbol price } } -> Option A

4. Quick Check:

   Filters go as arguments inside parentheses [OK]

Hint: Use arguments in parentheses for filtering subscriptions [OK]

Common Mistakes:

• Trying to filter inside field braces
• Using invalid keywords like 'where' or 'if'
• Placing filters outside subscription block