Subscription syntax in GraphQL - Mini Project: Build & Apply

GraphQL Subscription Syntax Basics

📖 Scenario: You are building a real-time chat application where users can see new messages instantly without refreshing the page.

🎯 Goal: Create a GraphQL subscription to listen for new chat messages as they arrive.

📋 What You'll Learn

Define a subscription called messageAdded

The subscription should return id, content, and author fields of the new message

Use the correct GraphQL subscription syntax

💡 Why This Matters

🌍 Real World

Real-time updates are essential in chat apps, live sports scores, and notifications.

💼 Career

Understanding GraphQL subscriptions is valuable for building interactive, real-time web applications.

1

Define the Subscription Type

Write the GraphQL schema definition for a subscription type named Subscription that includes a field called messageAdded which returns a Message type.

GraphQL

# Write the subscription type definition here

Hint

Use type Subscription { messageAdded: Message } to define the subscription.

2

Define the Message Type

Add a GraphQL type called Message with fields id (ID!), content (String!), and author (String!).

GraphQL

type Subscription {
  messageAdded: Message
}

# Define the Message type here

Hint

Define Message with the required fields and types.

3

Write the Subscription Query

Write a GraphQL subscription query named SubscribeToMessages that listens to messageAdded and requests the id, content, and author fields.

GraphQL

type Subscription {
  messageAdded: Message
}

type Message {
  id: ID!
  content: String!
  author: String!
}

# Write the subscription query here

Hint

Use subscription SubscribeToMessages { messageAdded { id content author } } syntax.

4

Complete the Subscription Setup

Add a comment above the subscription query explaining that this subscription listens for new messages in real-time.

GraphQL

type Subscription {
  messageAdded: Message
}

type Message {
  id: ID!
  content: String!
  author: String!
}

# Add a comment explaining the subscription
subscription SubscribeToMessages {
  messageAdded {
    id
    content
    author
  }
}

Hint

Add a clear comment starting with # above the subscription query.

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