WebSockets

WebSockets provide full-duplex, persistent communication between client and server over a single TCP connection. Unlike HTTP (request-response), WebSockets allow the server to push data to clients in real-time. Use cases: chat, live dashboards, notifications, collaborative editing, gaming. Spring supports WebSockets via @MessageMapping with STOMP protocol over SockJS.

Key Concepts

Deep Dive: HTTP vs WebSocket

Feature	HTTP	WebSocket
Communication	Request-response	Full-duplex
Connection	New per request (or keep-alive)	Persistent
Direction	Client → Server	Bidirectional
Overhead	Headers on every request	Minimal after handshake
Use case	REST APIs, web pages	Real-time data

WebSocket handshake (HTTP upgrade):

Client: GET /chat HTTP/1.1
        Upgrade: websocket
        Connection: Upgrade

Server: HTTP/1.1 101 Switching Protocols
        Upgrade: websocket
        Connection: Upgrade

Deep Dive: Spring WebSocket with STOMP

@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {
    @Override
    public void registerStompEndpoints(StompEndpointRegistry registry) {
        registry.addEndpoint("/ws").withSockJS();
    }
    @Override
    public void configureMessageBroker(MessageBrokerRegistry registry) {
        registry.setApplicationDestinationPrefixes("/app");
        registry.enableSimpleBroker("/topic");
    }
}

@Controller
public class ChatController {
    @MessageMapping("/chat.send")
    @SendTo("/topic/messages")
    public ChatMessage send(ChatMessage message) {
        return message;
    }
}

Deep Dive: Alternatives

Technique	Description	Use When
Polling	Client requests periodically	Simple, low frequency
Long Polling	Server holds request until data	Moderate real-time
SSE (Server-Sent Events)	Server → Client stream only	One-way updates
WebSocket	Full-duplex	True real-time, bidirectional

Common Interview Questions

• What are WebSockets? How do they differ from HTTP?
• How does the WebSocket handshake work?
• What is STOMP? SockJS?
• What is the difference between WebSocket and SSE?
• When would you use polling vs WebSockets?