Tag: onConnect

GraphQL subscriptions use WebSockets to provide real-time data.
Two important lifecycle hooks in this process are onConnect and onDisconnect.

Interviewers often ask questions about these hooks to test your understanding of:

• WebSocket lifecycle
• Subscription security
• Real-time system design

Let’s cover the most important interview questions with clear answers.

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1. What Are onConnect and onDisconnect in GraphQL Subscriptions?

Answer:

• onConnect is executed when a client establishes a WebSocket connection.
• onDisconnect is executed when the client disconnects from the WebSocket.

They act as entry and exit points of a GraphQL subscription connection.

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2. Why Are onConnect and onDisconnect Important?

Answer:
They are important for:

• Authentication and authorization
• Connection initialization
• Resource cleanup
• Tracking active users

Without these hooks, real-time applications can become insecure and inefficient.

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3. When Is onConnect Triggered?

Answer:
onConnect is triggered:

• After the WebSocket connection is opened
• Before any subscription starts

It runs only once per WebSocket connection.

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4. When Is onDisconnect Triggered?

Answer:
onDisconnect is triggered when:

• The user closes the browser or app
• Network connection is lost
• The client explicitly disconnects

It ensures proper cleanup of server resources.

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5. What Is Commonly Done Inside onConnect?

Answer:
Typical operations include:

• Validating JWT or auth token
• Identifying the user
• Creating subscription context
• Rejecting unauthorized connections

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6. Can onConnect Reject a Connection?

Answer:
Yes.
If authentication fails, onConnect can:

• Throw an error
• Reject the WebSocket connection
• Prevent subscriptions from starting

This is a common security practice.

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7. What Is Commonly Done Inside onDisconnect?

Answer:
Typical tasks include:

• Cleaning up memory or listeners
• Removing user from active connections list
• Updating online/offline status
• Logging disconnection events

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8. Is onDisconnect Always Called?

Answer:
In most cases, yes.
However, in sudden network failures, it may not fire immediately.

That’s why servers often use:

• Heartbeats
• Timeout detection

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9. How Are onConnect and Subscriptions Related?

Answer:

• onConnect runs before any subscription starts
• It prepares the context used by subscription resolvers

If onConnect fails, subscriptions will not work.

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10. Can Multiple Subscriptions Share the Same onConnect?

Answer:
Yes.
One WebSocket connection can handle multiple subscriptions, and onConnect runs only once for that connection.

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11. Where Is Authentication Best Handled for Subscriptions?

Answer:
Authentication is best handled in onConnect, not in each subscription resolver.

Reason:

• Authentication happens once
• Reduces repeated validation
• Improves performance

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12. What Is the Difference Between HTTP Auth and Subscription Auth?

Answer:

Feature	HTTP	WebSocket
Auth timing	Every request	Once at connection
Auth hook	Middleware	onConnect
Connection	Stateless	Stateful

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13. Can onDisconnect Access User Information?

Answer:
Yes.
If user data was stored during onConnect, it can be accessed during onDisconnect to perform cleanup or logging.

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14. What Happens If onConnect Is Not Implemented?

Answer:

• Subscriptions may still work
• But security, context, and connection control will be missing

In production apps, onConnect is strongly recommended.

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15. Are onConnect and onDisconnect Client-Side or Server-Side?

Answer:
They are server-side lifecycle hooks and are part of the WebSocket subscription server configuration.

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16. How Do onConnect and onDisconnect Help in Scaling?

Answer:
They help by:

• Tracking active connections
• Cleaning unused resources
• Preventing memory leaks

This is critical for high-traffic real-time systems.

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Interview Tip (Very Important)

onConnect = authenticate & initialize
onDisconnect = cleanup & finalize

If you remember this, you can confidently answer most interview questions.

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Final Summary Table

Hook	Purpose
onConnect	Authentication & context setup
onDisconnect	Resource cleanup & status update