Subscriptions in NestJS

Introduction

Subscriptions let your app send real-time updates to users. They keep users informed without needing to refresh the page.

You want to show live chat messages as they arrive.

You need to update a dashboard with live data like stock prices.

You want to notify users instantly when new content is available.

You want to build a live feed or notifications system.

You want to push updates to clients without them asking repeatedly.

Syntax

NestJS

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

const pubSub = new PubSub();

@Resolver()
export class MyResolver {
  @Subscription(() => String, {
    resolve: (payload) => payload,
  })
  messageSent() {
    return pubSub.asyncIterator('messageSent');
  }
}

Use @Subscription decorator to mark a method as a subscription.

PubSub helps send and listen to events for subscriptions.

Examples

This example listens for 'newMessage' events and sends the string message to subscribers.

NestJS

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

const pubSub = new PubSub();

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

This example filters notifications so only the user with matching userId receives them.

NestJS

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

const pubSub = new PubSub();

@Resolver()
export class NotificationResolver {
  @Subscription(() => String, {
    filter: (payload, variables) => payload.userId === variables.userId,
  })
  userNotification(userId: string) {
    return pubSub.asyncIterator('userNotification');
  }
}

Sample Program

This resolver stores messages and allows clients to send new messages via mutation. When a message is sent, it publishes an event. The subscription listens for these events and sends new messages to subscribers in real-time.

NestJS

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

const pubSub = new PubSub();

@Resolver()
export class MessageResolver {
  private messages: string[] = [];

  @Query(() => [String])
  getMessages() {
    return this.messages;
  }

  @Mutation(() => Boolean)
  sendMessage(@Args('message') message: string) {
    this.messages.push(message);
    pubSub.publish('messageSent', message);
    return true;
  }

  @Subscription(() => String)
  messageSent() {
    return pubSub.asyncIterator('messageSent');
  }
}

Important Notes

Subscriptions require a WebSocket connection to work properly.

Use PubSub or other event systems to manage events.

Remember to handle cleanup to avoid memory leaks in long-running subscriptions.

Summary

Subscriptions let your app send live updates to users.

Use @Subscription decorator and PubSub to create subscriptions.

They are great for chat, notifications, and live data feeds.

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