Subscriptions in NestJS - Mini Project: Build & Apply

Building a GraphQL Subscription with NestJS

📖 Scenario: You are creating a real-time chat application using NestJS and GraphQL. You want users to receive new messages instantly without refreshing the page.

🎯 Goal: Build a GraphQL subscription in NestJS that streams new chat messages to subscribed clients in real time.

📋 What You'll Learn

Create a simple message array to hold chat messages

Add a configuration variable for the subscription event name

Implement a subscription resolver using NestJS GraphQL decorators

Complete the subscription setup to broadcast new messages

💡 Why This Matters

🌍 Real World

Real-time chat apps, live notifications, and instant updates use subscriptions to push data to clients without refreshing.

💼 Career

Understanding subscriptions in NestJS is key for backend developers building interactive, real-time web applications.

1

Create the initial message data array

Create a constant array called messages with these exact objects: { id: 1, text: 'Hello' } and { id: 2, text: 'Welcome' }.

NestJS

// Create the messages array with two message objects
// Your code here

Hint

Use const messages = [ ... ] with two objects inside the array.

2

Add a subscription event name constant

Add a constant string called NEW_MESSAGE_EVENT and set it to 'newMessage'.

NestJS

const messages = [
  { id: 1, text: 'Hello' },
  { id: 2, text: 'Welcome' }
];

// Define the subscription event name
// Your code here

Hint

Use const NEW_MESSAGE_EVENT = 'newMessage'; to define the event name.

3

Implement the subscription resolver method

Inside a NestJS resolver class, create a method called newMessage decorated with @Subscription(() => Message) that returns an async iterator from pubSub.asyncIterator(NEW_MESSAGE_EVENT). Use pubSub as the PubSub instance.

NestJS

const messages = [
  { id: 1, text: 'Hello' },
  { id: 2, text: 'Welcome' }
];

const NEW_MESSAGE_EVENT = 'newMessage';

import { Resolver, Subscription } from '@nestjs/graphql';
import { PubSub } from 'graphql-subscriptions';

const pubSub = new PubSub();

@Resolver()
export class MessageResolver {
  // Add the subscription method here
  // Your code here
}

Hint

Use @Subscription(() => Message) above the newMessage method and return pubSub.asyncIterator(NEW_MESSAGE_EVENT).

4

Complete the subscription by publishing new messages

Add a method called sendMessage that takes a text string, creates a new message object with an id one higher than the last message, pushes it to messages, and calls pubSub.publish(NEW_MESSAGE_EVENT, { newMessage: newMsg }).

NestJS

const messages = [
  { id: 1, text: 'Hello' },
  { id: 2, text: 'Welcome' }
];

const NEW_MESSAGE_EVENT = 'newMessage';

import { Resolver, Subscription } from '@nestjs/graphql';
import { PubSub } from 'graphql-subscriptions';

const pubSub = new PubSub();

@Resolver()
export class MessageResolver {
  @Subscription(() => Message)
  newMessage() {
    return pubSub.asyncIterator(NEW_MESSAGE_EVENT);
  }

  // Add sendMessage method here
  // Your code here
}

Hint

Create sendMessage that adds a new message and publishes it with pubSub.publish.

Practice

1. What is the main purpose of subscriptions in NestJS GraphQL?

easy

A. To perform database migrations

B. To send live updates automatically to clients

C. To handle HTTP GET requests

D. To serve static files

Solution

1. Step 1: Understand the role of subscriptions

   Subscriptions are designed to push live data updates to clients without the client needing to request repeatedly.

2. Step 2: Compare with other options

   HTTP GET requests, database migrations, and serving static files are unrelated to live data updates.

3. Final Answer:

   To send live updates automatically to clients -> Option B

4. Quick Check:

   Subscriptions = live updates [OK]

Hint: Subscriptions push live data updates automatically [OK]

Common Mistakes:

• Confusing subscriptions with HTTP request handling
• Thinking subscriptions manage database tasks
• Assuming subscriptions serve static content

2. Which decorator is used in NestJS to define a GraphQL subscription method?

easy

A. @Query

B. @Mutation

C. @Subscription

D. @Injectable

Solution

1. Step 1: Identify the decorator for subscriptions

   The @Subscription decorator marks a method as a GraphQL subscription resolver in NestJS.

2. Step 2: Differentiate from other decorators

   @Query is for queries, @Mutation for mutations, and @Injectable for services, not subscriptions.

3. Final Answer:

   @Subscription -> Option C

4. Quick Check:

   @Subscription decorator = subscriptions [OK]

Hint: Use @Subscription decorator for live data methods [OK]

Common Mistakes:

• Using @Query or @Mutation instead of @Subscription
• Confusing @Injectable with subscription decorators
• Omitting the decorator entirely

3. Given this subscription resolver snippet, what will be the output when a new message is published?

import { Resolver, Subscription } from '@nestjs/graphql';
import { PubSub } from 'graphql-subscriptions';

const pubSub = new PubSub();

@Resolver()
export class ChatResolver {
  @Subscription(() => String, {
    resolve: (payload) => `New message: ${payload.text}`,
  })
  messageAdded() {
    return pubSub.asyncIterator('messageAdded');
  }
}

// Somewhere else: pubSub.publish('messageAdded', { text: 'Hello!' });

medium

A. New message: Hello!

B. Hello!

C. messageAdded

D. Error: asyncIterator not found

Solution

1. Step 1: Understand the subscription resolver

   The resolver listens to 'messageAdded' events and uses the resolve function to format the payload text.

2. Step 2: Analyze the published payload

   The payload has { text: 'Hello!' }, so the resolve function returns 'New message: Hello!'.

3. Final Answer:

   New message: Hello! -> Option A

4. Quick Check:

   Resolve formats payload = 'New message: Hello!' [OK]

Hint: Resolve function formats payload text in subscription [OK]

Common Mistakes:

• Expecting raw payload without resolve formatting
• Confusing event name with output
• Assuming asyncIterator causes error

4. Identify the error in this subscription setup:

import { Resolver, Subscription } from '@nestjs/graphql';
import { PubSub } from 'graphql-subscriptions';

const pubSub = new PubSub();

@Resolver()
export class NotificationResolver {
  @Subscription(() => String)
  newNotification() {
    return pubSub.asyncIterator('newNotification');
  }
}

// Missing pubSub.publish call elsewhere

medium

A. pubSub.asyncIterator called with wrong event name

B. Subscription method missing return statement

C. Incorrect decorator used for subscription

D. No events are published, so no updates sent

Solution

1. Step 1: Check subscription method correctness

   The method correctly returns pubSub.asyncIterator with the event name.

2. Step 2: Identify missing event publishing

   No call to pubSub.publish('newNotification', ...) means no events are emitted, so clients receive no updates.

3. Final Answer:

   No events are published, so no updates sent -> Option D

4. Quick Check:

   Missing publish call = no updates sent [OK]

Hint: Always publish events to trigger subscription updates [OK]

Common Mistakes:

• Forgetting to call pubSub.publish
• Using wrong event name in asyncIterator
• Omitting return in subscription method

5. You want to create a subscription that only sends updates to clients when the message's author matches their user ID. Which approach correctly implements this filter in NestJS subscriptions? A) Use pubSub.asyncIterator('messageAdded') directly without filtering. B) Use the filter option in the @Subscription decorator to compare payload author and user ID. C) Filter messages inside the resolver method after returning the async iterator. D) Use a separate query instead of subscription to filter messages.

hard

A. Use filter option in @Subscription to send only matching messages

B. Subscribe directly without filtering; client filters messages

C. Filter messages inside resolver after asyncIterator returns

D. Use query instead of subscription for filtering

Solution

1. Step 1: Understand subscription filtering

   NestJS supports a filter option in the @Subscription decorator to send events selectively based on payload and context.

2. Step 2: Evaluate options

   The filter option in the @Subscription decorator compares the message author with the client's user ID from context, sending only relevant updates. Subscribing directly without filtering sends all events to clients, filtering after the asyncIterator returns is not the standard approach, and using a query provides no live updates.

3. Final Answer:

   Use filter option in @Subscription to send only matching messages -> Option A

4. Quick Check:

   Filter option = selective subscription updates [OK]

Hint: Use @Subscription filter option to send selective updates [OK]

Common Mistakes:

• Not using filter option and sending all events
• Trying to filter after asyncIterator returns
• Using queries instead of subscriptions for live updates