Difference Between Long-Polling, WebSockets, and Server-Sent Events

Long-Polling, WebSockets, and Server-Sent Events all solve the same problem: getting updates from the server to the client without refreshing the page. They just solve it in three different ways. Here is each one in a single sentence:

• Long-Polling: the client asks, and the server holds the request open until it has data to send. After every answer, the client asks again.
• WebSocket: one persistent connection where both the client and the server can send messages at any time.
• Server-Sent Events (SSE): one persistent HTTP connection where the server streams updates down to the client, one way only.

Key Differences

How to Choose

Ask three questions, in order:

1. Do the client and the server both need to send messages at any time? If yes, use WebSocket. Nothing else gives you a true two-way channel.
2. Do updates only flow from the server to the client? If yes, use Server-Sent Events. It is simpler than WebSocket, it is plain HTTP, and the browser handles reconnection for you.
3. Do you need something that works through old proxies and strict corporate networks, or a fallback when the other two fail? Use Long-Polling. It is the least efficient, but it works almost everywhere.

💡 In a system design interview, do not just name a technique. Say why it fits. "Chat needs messages in both directions with low latency, so I would use WebSockets, with long-polling as a fallback for restrictive networks." For a notifications feed: "Updates only flow to the client, so SSE gives me streaming over plain HTTP, and the browser reconnects on its own." Naming the trade-off is what earns the credit.

Quick Reference

• Long-Polling

  • Connection: a new HTTP request after every message
  • Direction: client asks, server answers when it has data
  • Reconnection: the client re-requests every time
  • Use it for: fallbacks and simple notification delivery

• WebSocket

  • Connection: one persistent, upgraded connection
  • Direction: both ways, at any time
  • Reconnection: your application handles it
  • Use it for: chat, multiplayer games, collaborative editing, trading

• Server-Sent Events

  • Connection: one persistent HTTP connection
  • Direction: server to client only
  • Reconnection: automatic, built into the browser
  • Use it for: live feeds, scores, prices, progress, streaming AI answers

Conclusion

There is no single winner. Long-Polling is the most compatible, WebSocket is the most capable, and Server-Sent Events is the simplest for one-way streams. Match the technique to the direction of your data, and you will get the right answer in both production and interviews.