Subscription resolvers in GraphQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a subscription resolver in GraphQL?

A subscription resolver is a special function in GraphQL that listens for real-time events and sends updates to clients when data changes.

beginner

How does a subscription resolver differ from a query resolver?

A query resolver fetches data once when requested, while a subscription resolver keeps a connection open to send data updates continuously.

intermediate

What role does PubSub play in subscription resolvers?

PubSub is a system that helps subscription resolvers by publishing events and allowing clients to subscribe to those events for real-time updates.

intermediate

Why do subscription resolvers use WebSockets?

WebSockets keep a live connection between client and server, enabling subscription resolvers to push data instantly without repeated requests.

advanced

What is the typical structure of a subscription resolver function?

It usually has a subscribe method that returns an AsyncIterator to listen for events, and optionally a resolve method to process the event data before sending.

What does a subscription resolver primarily do?

A Listens for real-time data changes and sends updates

B Fetches data once on request

C Deletes data from the database

D Updates data in the database

Subscription resolvers listen for events and send real-time updates to clients.

Which protocol is commonly used to maintain live connections for subscriptions?

A WebSocket

B FTP

C HTTP

D SMTP

WebSocket keeps a persistent connection needed for real-time subscriptions.

In GraphQL subscriptions, what does PubSub help with?

A Query optimization

B Schema validation

C Publishing and subscribing to events

D Authentication

PubSub manages event publishing and subscription for real-time updates.

What does the subscribe method in a subscription resolver return?

A A Promise

B An AsyncIterator

C A Boolean

D A String

The subscribe method returns an AsyncIterator to listen for events.

Which of these is NOT a feature of subscription resolvers?

A Event listening

B Continuous data updates

C Real-time communication

D Single data fetch

Subscription resolvers do not fetch data just once; they provide continuous updates.

Explain how subscription resolvers work in GraphQL and why they are useful.

Think about how apps get live data like chat messages or notifications.

Describe the difference between query resolvers and subscription resolvers in GraphQL.

Compare a snapshot photo to a live video stream.

Practice

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

easy

A. To modify data on the server

B. To fetch data only once when a query is made

C. To provide real-time updates to clients when data changes

D. To define the schema types

Solution

1. Step 1: Understand the role of subscription resolvers

   Subscription resolvers are designed to send data updates automatically as events happen, unlike queries which fetch data once.

2. Step 2: Compare with other resolver types

   Queries fetch data once, mutations modify data, and schema defines types. Only subscriptions provide live updates.

3. Final Answer:

   To provide real-time updates to clients when data changes -> Option C

4. Quick Check:

   Subscription = real-time updates [OK]

Hint: Subscriptions push live data updates automatically [OK]

Common Mistakes:

• Confusing subscriptions with queries
• Thinking subscriptions modify data
• Mixing schema definition with resolver purpose

2. Which of the following is the correct way to declare a subscription resolver in GraphQL SDL?

easy

A. type Subscription { newMessage: Message }

B. type Query { newMessage: Message }

C. type Mutation { newMessage: Message }

D. type Resolver { newMessage: Message }

Solution

1. Step 1: Identify the correct GraphQL operation keyword

   Subscriptions use the type Subscription keyword to declare real-time data streams in the schema.

2. Step 2: Check other operation keywords

   Queries use type Query, mutations use type Mutation, and type Resolver is not a valid keyword.

3. Final Answer:

   type Subscription { newMessage: Message } -> Option A

4. Quick Check:

   Subscription keyword = subscription declaration [OK]

Hint: Use 'type Subscription' keyword to declare live data streams [OK]

Common Mistakes:

• Using 'type Query' or 'type Mutation' instead of 'type Subscription'
• Trying to use 'type Resolver' keyword
• Misplacing subscription declaration outside schema

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

const resolvers = {
  Subscription: {
    newMessage: {
      subscribe: () => pubsub.asyncIterator(['MESSAGE_ADDED'])
    }
  }
};

medium

A. The client receives updates only when a message is added

B. The client receives all past messages immediately

C. The client receives no updates until a query is made

D. The client receives an error because subscribe is missing

Solution

1. Step 1: Understand the subscribe function

   The subscribe function returns an async iterator for the event 'MESSAGE_ADDED', so it listens for new messages.

2. Step 2: Determine what triggers updates

   Updates are sent only when the 'MESSAGE_ADDED' event occurs, so clients get notified only on new messages.

3. Final Answer:

   The client receives updates only when a message is added -> Option A

4. Quick Check:

   Subscription triggers on event = client updates [OK]

Hint: Subscribe returns async iterator for live event updates [OK]

Common Mistakes:

• Assuming all past data is sent immediately
• Thinking updates require queries first
• Missing that subscribe returns async iterator

4. Identify the error in this subscription resolver code:

const resolvers = {
  Subscription: {
    newNotification: {
      subscribe: pubsub.asyncIterator('NOTIFICATION_ADDED')
    }
  }
};

medium

A. The resolver key should be 'Query' instead of 'Subscription'

B. The event name should be an array, not a string

C. Subscription resolvers cannot use pubsub

D. The subscribe function should be a function returning asyncIterator, not the asyncIterator itself

Solution

1. Step 1: Check the subscribe property type

   The subscribe property must be a function that returns the async iterator, not the async iterator itself.

2. Step 2: Verify correct usage of asyncIterator

   It should be subscribe: () => pubsub.asyncIterator('NOTIFICATION_ADDED') to work properly.

3. Final Answer:

   The subscribe function should be a function returning asyncIterator, not the asyncIterator itself -> Option D

4. Quick Check:

   Subscribe must be a function returning asyncIterator [OK]

Hint: Subscribe must be a function returning asyncIterator [OK]

Common Mistakes:

• Assigning asyncIterator directly instead of a function
• Using string instead of array for event names (optional)
• Confusing subscription with query resolvers

5. You want to create a subscription that only sends updates to clients when a new comment is added to a specific post ID. Which resolver implementation correctly filters events by post ID?

const resolvers = {
  Subscription: {
    commentAdded: {
      subscribe: (parent, args, { pubsub }) => {
        return pubsub.asyncIterator('COMMENT_ADDED');
      },
      resolve: (payload, args) => {
        if (payload.postId === args.postId) {
          return payload;
        }
        return null;
      }
    }
  }
};

hard

A. The filtering should be done inside the subscribe function, not resolve

B. This implementation correctly filters comments by postId in the resolve function

C. The asyncIterator should listen to an array of post IDs, not a single event

D. Subscriptions cannot filter events by arguments

Solution

1. Step 1: Understand event filtering in subscriptions

   The resolve function can filter events by checking if the payload matches the subscription arguments.

2. Step 2: Check the given code logic

   The code returns the payload only if payload.postId === args.postId, otherwise returns null to skip irrelevant events.

3. Final Answer:

   This implementation correctly filters comments by postId in the resolve function -> Option B

4. Quick Check:

   Filtering in resolve function = correct approach [OK]

Hint: Filter events in resolve by comparing payload and args [OK]

Common Mistakes:

• Trying to filter inside subscribe instead of resolve
• Expecting asyncIterator to filter by arguments automatically
• Believing subscriptions cannot filter events