What is a WebSocket?

0-2 years experience

1. 1How would you set up a simple WebSocket server in Node.js that broadcasts any incoming message to all connected clients?
2. 2If a client tries to connect to your WebSocket endpoint but the server is down, what error does the client see and how would you handle it in the front‑end code?
3. 3What happens during the WebSocket handshake compared to a normal HTTP request?

2-5 years experience

1. 1You added a feature that pushes live updates via WebSocket, but some users report they never receive updates after a network interruption. Walk me through how you'd debug this.
2. 2Explain why you might fall back to long polling for browsers that don’t support WebSockets, and how you’d implement that fallback in a Node.js service.
3. 3During a load test, the WebSocket server starts rejecting connections after 10k concurrent clients. What are the likely bottlenecks and how would you address them?

5-8 years experience

1. 1Design a scalable architecture for a chat application using Node.js and WebSockets that must support millions of concurrent users across multiple data centers.
2. 2How would you ensure message ordering and exactly‑once delivery when using WebSockets behind a load balancer that performs sticky sessions?
3. 3Discuss the security considerations—authentication, rate limiting, DoS protection—you’d put in place for a public WebSocket API.

8+ years experience

1. 1Your organization is moving from a legacy HTTP‑polling real‑time system to a WebSocket‑based platform. What migration strategy would you propose to minimize downtime and client impact?
2. 2When multiple teams need to publish events over WebSockets, how would you design a shared infrastructure that balances autonomy, versioning, and operational overhead?
3. 3Evaluate the trade‑offs of using a managed WebSocket service (e.g., AWS API Gateway WebSockets) versus a self‑hosted Node.js cluster for a global SaaS product.