Long polling simulates server push using HTTP:
Client sends request
Server holds connection until data available (or timeout)
Server responds with data
Client immediately sends new request
Advantages:
- Works everywhere (just HTTP)
- No special infrastructure
- Firewall-friendly
Disadvantages:
- Higher latency than WebSockets
- More HTTP overhead
- Server holds many connections
Use long polling as fallback when WebSockets aren't available, or for simple use cases.