Subscription syntax in GraphQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a GraphQL subscription?

A GraphQL subscription is a way to get real-time updates from the server. It keeps a connection open and sends data when something changes.

beginner

Basic syntax to start a subscription in GraphQL?

Use the subscription keyword followed by the subscription name and the fields you want to receive updates for.

intermediate

How do you specify which events to listen to in a subscription?

Inside the subscription, you specify the event or data field you want updates for, similar to a query but it listens continuously.

beginner

Example of a simple subscription for new messages?

subscription { newMessage { id content sender } } This listens for new messages and returns their id, content, and sender.

intermediate

Why use subscriptions instead of queries for real-time data?

Queries fetch data once. Subscriptions keep the connection open and send updates automatically when data changes, saving time and resources.

Which keyword starts a GraphQL subscription?

A query

B subscription

C mutation

D listen

The subscription keyword is used to start a subscription operation in GraphQL.

What does a subscription do?

A Fetches data once

B Updates data on the server

C Keeps connection open for real-time updates

D Deletes data

Subscriptions keep the connection open and send data updates in real-time.

Which of these is a valid subscription field selection?

A subscription { newMessage { id content } }

B query { newMessage { id content } }

C mutation { newMessage { id content } }

D listen { newMessage { id content } }

Subscriptions use the subscription keyword and specify fields to receive updates.

Subscriptions are best used for:

A One-time data fetch

B Batch processing

C Data deletion

D Real-time data updates

Subscriptions provide real-time updates by keeping the connection open.

Which statement is true about GraphQL subscriptions?

A They listen for events and send data continuously

B They use the same syntax as mutations

C They close the connection after one response

D They cannot specify fields to return

Subscriptions listen for events and send data continuously as changes happen.

Explain how to write a basic GraphQL subscription and what it does.

Think about how subscriptions differ from queries.

Describe why and when you would use a subscription instead of a query in GraphQL.

Consider the difference between one-time fetch and live updates.

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