What are WebSockets and what problems do they solve?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Identify the need

   Recognise that the app needs instant, two-way updates that HTTP request-response cannot deliver efficiently.

2. Step 2

   Open with a handshake

   The client sends an HTTP request with an Upgrade header; the server agrees and switches the connection to the WebSocket protocol.

3. Step 3

   Keep the connection alive

   A single TCP connection stays open, avoiding repeated connection setup and teardown costs.

4. Step 4

   Exchange messages freely

   Either side sends framed messages at any time in full-duplex, with tiny per-message overhead.

5. Step 5

   Close gracefully

   Either party sends a close frame and the connection shuts down cleanly when real-time updates are no longer needed.

Code Example

const socket = new WebSocket('wss://example.com/live')

socket.addEventListener('open', () => {
  socket.send(JSON.stringify({ type: 'subscribe', channel: 'prices' }))
})

socket.addEventListener('message', (event) => {
  const data = JSON.parse(event.data)
  console.log('Live update:', data)
})

socket.addEventListener('close', () => {
  console.log('Connection closed')
})

Follow-up Questions

• How do WebSockets differ from HTTP?
• What is Server-Sent Events and when would you use it instead?
• How do you scale WebSocket connections across many servers?
• How do you handle reconnection and dropped connections?
• What security concerns apply to WebSocket connections?