WebSocket transport in GraphQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is WebSocket transport in GraphQL?

WebSocket transport is a way to keep a live connection open between client and server, allowing real-time data exchange for GraphQL subscriptions.

beginner

Why use WebSocket transport instead of HTTP for GraphQL subscriptions?

WebSocket keeps the connection open, so the server can send updates instantly without the client asking repeatedly, unlike HTTP which opens a new connection for each request.

intermediate

How does a GraphQL subscription work over WebSocket transport?

The client sends a subscription request over WebSocket. The server listens and sends data updates back through the same connection whenever data changes.

intermediate

Name one common protocol used for GraphQL subscriptions over WebSocket.

The 'graphql-ws' protocol is commonly used to handle GraphQL subscriptions over WebSocket connections.

intermediate

What happens if the WebSocket connection drops during a GraphQL subscription?

The client usually tries to reconnect automatically to keep receiving updates without losing data.

What is the main benefit of using WebSocket transport for GraphQL subscriptions?

A Real-time data updates without repeated requests

B Faster HTTP requests

C Better database storage

D Simpler query syntax

WebSocket transport keeps the connection open, allowing the server to push real-time updates without the client needing to ask repeatedly.

Which GraphQL operation commonly uses WebSocket transport?

A Introspection

B Query

C Subscription

D Mutation

Subscriptions use WebSocket transport to receive live updates from the server.

What does the client send first to start a GraphQL subscription over WebSocket?

A Query request

B Subscription request

C Mutation request

D Disconnect message

The client sends a subscription request to start receiving live data updates.

If the WebSocket connection drops, what usually happens?

A Server closes the database

B Subscription ends permanently

C Client sends a new HTTP request

D Client tries to reconnect automatically

Clients often try to reconnect automatically to keep the subscription active.

Which protocol is commonly used for GraphQL subscriptions over WebSocket?

A graphql-ws

B HTTP/2

C FTP

D SMTP

The 'graphql-ws' protocol is designed for GraphQL subscriptions over WebSocket.

Explain how WebSocket transport enables real-time updates in GraphQL subscriptions.

Think about how the connection stays open and data flows both ways.

Describe what happens when a WebSocket connection drops during a GraphQL subscription and how it is handled.

Consider how the client keeps the subscription alive despite connection issues.

Practice

1. What is the main benefit of using WebSocket transport with GraphQL subscriptions?

easy

A. It converts GraphQL queries into SQL automatically.

B. It stores data permanently in the database.

C. It keeps a live connection for real-time data updates.

D. It compresses data to reduce storage size.

Solution

1. Step 1: Understand WebSocket transport purpose

   WebSocket transport maintains a continuous connection between client and server.

2. Step 2: Connect with GraphQL subscriptions

   This live connection allows instant data updates, which is the main benefit for subscriptions.

3. Final Answer:

   It keeps a live connection for real-time data updates. -> Option C

4. Quick Check:

   WebSocket = live real-time updates [OK]

Hint: WebSocket means live connection for instant updates [OK]

Common Mistakes:

• Confusing WebSocket with data storage
• Thinking it converts queries automatically
• Assuming it compresses data

2. Which of the following is the correct way to initiate a WebSocket connection for GraphQL subscriptions?

easy

A. const ws = new WebSocket('ws://example.com/graphql');

B. const ws = new WebSocket('http://example.com/graphql');

C. const ws = new WebSocket('ftp://example.com/graphql');

D. const ws = new WebSocket('file://example.com/graphql');

Solution

1. Step 1: Identify the correct WebSocket URL scheme

   WebSocket URLs start with 'ws://' or 'wss://' for secure connections.

2. Step 2: Check the options

   Only const ws = new WebSocket('ws://example.com/graphql'); uses 'ws://' which is the correct scheme for WebSocket connections.

3. Final Answer:

   const ws = new WebSocket('ws://example.com/graphql'); -> Option A

4. Quick Check:

   WebSocket URL starts with ws:// [OK]

Hint: WebSocket URLs start with ws:// or wss:// [OK]

Common Mistakes:

• Using http:// instead of ws://
• Using ftp:// or file:// which are invalid for WebSocket
• Confusing secure ws with http

3. Given this GraphQL subscription client code snippet, what will be the output when a new message arrives?

const subscription = client.subscribe({ query: MESSAGE_SUBSCRIPTION });
subscription.subscribe({
  next(data) { console.log(data); },
  error(err) { console.error(err); }
});

medium

A. Does nothing because subscription is not started.

B. Throws a syntax error because subscribe is not a function.

C. Logs an error immediately.

D. Logs the new message data to the console.

Solution

1. Step 1: Understand the subscription method

   The client.subscribe returns an observable that listens for new data.

2. Step 2: Analyze the subscribe callback

   The next callback logs incoming data, so when a new message arrives, it prints it.

3. Final Answer:

   Logs the new message data to the console. -> Option D

4. Quick Check:

   Subscription next callback logs data [OK]

Hint: Subscription next callback logs incoming data [OK]

Common Mistakes:

• Thinking subscribe is not a function
• Assuming subscription needs manual start
• Expecting immediate error without cause

4. Identify the error in this WebSocket transport setup for GraphQL subscriptions:

const ws = new WebSocket('http://example.com/graphql');
const subscription = client.subscribe({ query: SUBSCRIPTION_QUERY });
subscription.subscribe({
  next(data) { console.log(data); },
  error(err) { console.error(err); }
});

medium

A. The error callback is missing a return statement.

B. WebSocket URL uses 'http://' instead of 'ws://'.

C. The subscribe method is called twice incorrectly.

D. Subscription query is missing variables.

Solution

1. Step 1: Check the WebSocket URL scheme

   The URL uses 'http://' which is incorrect for WebSocket connections; it should be 'ws://'.

2. Step 2: Verify other parts

   Subscription query and callbacks are correctly used; no missing variables or double subscribe calls.

3. Final Answer:

   WebSocket URL uses 'http://' instead of 'ws://'. -> Option B

4. Quick Check:

   WebSocket URL must start with ws:// [OK]

Hint: Check WebSocket URL scheme for ws:// [OK]

Common Mistakes:

• Ignoring URL scheme errors
• Assuming variables are always required
• Confusing subscribe method usage

5. You want to build a live chat app using GraphQL subscriptions over WebSocket. Which approach best ensures efficient real-time updates and minimal server load?

hard

A. Use WebSocket transport with filtered subscriptions to send only relevant messages to each client.

B. Use HTTP polling every second to fetch new messages.

C. Send all chat messages to every client regardless of chat room.

D. Use WebSocket but reconnect on every new message.

Solution

1. Step 1: Understand real-time update needs

   Live chat requires instant updates with minimal delay and server load.

2. Step 2: Evaluate options for efficiency

   Using WebSocket with filtered subscriptions sends only needed messages, reducing unnecessary data and load.

3. Step 3: Compare alternatives

   Polling wastes resources; sending all messages to all clients is inefficient; reconnecting often adds overhead.

4. Final Answer:

   Use WebSocket transport with filtered subscriptions to send only relevant messages to each client. -> Option A

5. Quick Check:

   Filtered WebSocket subscriptions = efficient real-time updates [OK]

Hint: Filter subscriptions to send only needed data [OK]

Common Mistakes:

• Using polling instead of WebSocket
• Broadcasting all messages to all clients
• Reconnecting WebSocket too frequently